Acids, Bases and Salts
Have you ever wondered why lemon juice tastes sour, why soap feels slippery, or why an ant sting can cause a burning sensation? These everyday experiences are connected to three important groups of chemical substances: acids, bases and salts. They play a major role in our daily lives, laboratory experiments, agriculture, medicine and many industrial processes.
In Class 10 Science, this chapter helps you understand how acids and bases behave, why some substances change the colour of indicators, how chemical reactions produce salts, and how important compounds such as baking soda, washing soda, bleaching powder and Plaster of Paris are prepared or used.
These notes cover the fundamental concepts, balanced chemical equations, important differences, numerical and conceptual applications, common misconceptions, quick revision points and 20 practice MCQs for examination preparation.
1. Acids, Bases and Salts: Basic Concepts
What are acids?
Acids are substances that produce hydrogen ions in aqueous solution. More precisely, when an acid dissolves in water, it transfers a proton to a water molecule, producing hydronium ions (H₃O⁺).
For example, hydrochloric acid ionizes in water:
Hydrogen ions do not normally exist independently in water; they are associated with water molecules, forming hydronium ions.
Common examples of acids include:
- Hydrochloric acid (HCl): Present in gastric juice in the stomach.
- Sulphuric acid (H₂SO₄): Used in lead-acid batteries and industries.
- Nitric acid (HNO₃): Used in the manufacture of fertilizers and other chemicals.
- Acetic acid (CH₃COOH): Present in vinegar.
- Citric acid: Found in lemons and other citrus fruits.
- Lactic acid: Produced during certain biological processes and found in fermented foods.
Acids commonly taste sour, but never taste an unknown chemical in order to identify it. Laboratory identification should use appropriate indicators and safety procedures.
What are bases?
Bases are substances that neutralize acids. According to the Arrhenius definition, a base produces hydroxide ions (OH⁻) in aqueous solution.
For example, sodium hydroxide dissolves in water and separates into ions:
NaOH → Na⁺ + OH⁻
Examples of bases include:
- Sodium hydroxide (NaOH): Used in soap manufacture.
- Potassium hydroxide (KOH): Used in the manufacture of certain soaps.
- Calcium hydroxide : Used in whitewashing and the preparation of limewater.
- Magnesium hydroxide : Used in some antacid preparations.
Many bases feel slippery, but touching an unknown substance is unsafe because strong bases can cause severe chemical burns.
What are alkalis?
An alkali is a base that dissolves in water and produces hydroxide ions in solution.
Therefore:
- Every alkali is a base.
- Not every base is an alkali, because some bases are insoluble or only sparingly soluble in water.
For example, sodium hydroxide is an alkali, whereas copper(II) oxide (CuO) is a basic oxide but is insoluble in water.
What are salts?
Salts are ionic compounds formed when the replaceable hydrogen ion of an acid is replaced by a metal ion or another suitable positive ion, such as ammonium (NH₄⁺).
Many salts are formed by the reaction of an acid with a base. For example:
Here, sodium chloride (NaCl) is the salt and water is also produced.
Salts are not necessarily neutral in their effect on a solution. Depending on their chemical composition, some salt solutions are acidic, some are basic, and others are approximately neutral.
2. Indicators: How to Identify Acids and Bases
An indicator is a substance that changes colour or another observable property depending on whether it is in an acidic or basic medium. Indicators help identify the nature of a solution without tasting or touching it.
Types of indicators
Indicators can be classified into natural, synthetic and olfactory indicators.
A. Natural indicators
Natural indicators are obtained from natural sources such as plants or microorganisms.
1. Litmus
Litmus is a natural dye obtained from lichens. It is commonly available as red and blue litmus paper.
- An acid turns blue litmus red.
- A base turns red litmus blue.
- A neutral solution generally does not change either red or blue litmus.
2. Turmeric
Turmeric contains curcumin, a natural pigment.
- Turmeric remains yellow in an acidic or neutral medium.
- It turns reddish-brown in a basic medium.
This explains why turmeric stains can turn reddish-brown when they come into contact with certain alkaline cleaning substances.
3. China rose indicator
An extract of China rose petals can help distinguish acids from bases.
- Acidic solution: dark pink or magenta.
- Basic solution: green.
The precise shade may depend on the extract and the concentration of the solution.
B. Synthetic indicators
Synthetic indicators are prepared chemically and are widely used in laboratories.
| Indicator | In acidic solution | In basic solution |
|---|---|---|
| Blue litmus | Turns red | Remains blue |
| Red litmus | Remains red | Turns blue |
| Phenolphthalein | Colourless | Pink |
| Methyl orange | Red | Yellow |
Phenolphthalein and methyl orange are especially useful in laboratory experiments involving acid-base reactions.
C. Olfactory indicators
Olfactory indicators are substances whose smell changes or becomes less detectable under certain acidic or basic conditions.
Examples include onion, vanilla essence and clove oil in school-level demonstrations.
For example, the smell of onion may become difficult to detect when it is mixed with a basic solution such as sodium hydroxide solution. The observation depends on the conditions of the experiment.
Important examination point
An indicator identifies the nature of a solution, but it does not necessarily tell you its exact pH or concentration.
For example, blue litmus turning red establishes that a solution is acidic, but it does not tell you whether its pH is 2, 4 or 6.
3. Chemical Properties of Acids
Acids show several characteristic chemical reactions. Understanding these reactions, their observations and their balanced equations is essential for Class 10 examinations.
3.1 Reaction of acids with metals
Many metals react with dilute acids to produce a salt and hydrogen gas.
General reaction:
Metal + Dilute acid → Salt + Hydrogen gas
For example, zinc reacts with dilute hydrochloric acid:
Here:
- Zinc (Zn) is the metal.
- Hydrochloric acid (HCl) is the acid.
- Zinc chloride (ZnCl₂) is the salt.
- Hydrogen gas (H₂) is released.
Observations:
- Bubbles appear because a gas is released.
- Zinc gradually dissolves.
- The evolved hydrogen gas can produce a characteristic pop sound when tested using a safe laboratory procedure.
Another example is the reaction of magnesium with dilute sulphuric acid:
Magnesium sulphate and hydrogen gas are produced.
Important limitation: Not every metal reacts with dilute acids in the same way. Metals below hydrogen in the usual school-level reactivity series, such as copper, generally do not liberate hydrogen from dilute hydrochloric acid. Some acids, particularly nitric acid, may behave differently because they can act as oxidizing agents.
3.2 Reaction of acids with metal carbonates
Acids react with metal carbonates to form a salt, water and carbon dioxide gas.
General reaction:
Metal carbonate + Acid → Salt + Water + Carbon dioxide
For example:
Sodium carbonate reacts with hydrochloric acid to produce sodium chloride, water and carbon dioxide.
Another example is calcium carbonate reacting with hydrochloric acid:
Calcium carbonate is present in materials such as limestone, marble and chalk.
Observation: Effervescence occurs because carbon dioxide gas is released.
3.3 Reaction of acids with metal hydrogencarbonates
Metal hydrogencarbonates also react with acids to produce a salt, water and carbon dioxide.
For example, sodium hydrogencarbonate reacts with hydrochloric acid:
Sodium hydrogencarbonate is also called sodium bicarbonate or baking soda.
The reaction with both carbonates and hydrogencarbonates can be tested using limewater.
The carbon dioxide turns clear limewater milky because insoluble calcium carbonate is formed.
If excess carbon dioxide is passed through limewater, the milkiness can disappear as soluble calcium hydrogencarbonate forms:
This is a useful distinction in laboratory-based questions.
3.4 Reaction of acids with metal oxides
Many metal oxides are basic in nature. They react with acids to form salt and water.
General reaction:
Basic metal oxide + Acid → Salt + Water
For example, copper(II) oxide reacts with hydrochloric acid:
Copper(II) oxide is a black solid. During the reaction, it dissolves to form a copper(II) chloride solution.
Another example is magnesium oxide reacting with sulphuric acid:
These reactions demonstrate the basic nature of metal oxides.
3.5 Reaction of acids with bases
Acids react with bases to form salt and water. This reaction is called neutralization.
For example:
Hydrochloric acid reacts with sodium hydroxide to produce sodium chloride and water.
At the ionic level, the essential reaction is:
In aqueous solution, hydronium ions provide the hydrogen ions, so the more explicit representation is:
Neutralization is important in everyday life, agriculture and industrial chemistry.
4. Chemical Properties of Bases
Bases also undergo characteristic chemical reactions. The following are particularly important for school examinations.
4.1 Reaction of bases with certain metals
Certain metals, particularly zinc and aluminium, react with strong bases under suitable conditions to produce a salt and hydrogen gas.
For example:
Under suitable conditions, zinc reacts with sodium hydroxide to form sodium zincate and hydrogen.
This reaction demonstrates that the chemical behaviour of metals depends on the substances with which they react. It should not be assumed that every metal reacts with every base.
4.2 Reaction of bases with non-metal oxides
Many non-metal oxides are acidic in nature. They react with bases to form salt and water.
For example, carbon dioxide reacts with calcium hydroxide:
Carbon dioxide is an acidic oxide, whereas calcium hydroxide is a base.
Another example is the reaction of sulphur dioxide with sodium hydroxide:
This reaction forms sodium sulphite and water.
These examples establish an important relationship:
- Many metal oxides are basic.
- Many non-metal oxides are acidic.
However, these are general trends rather than universal rules. Some oxides are amphoteric, meaning that they can react with both acids and bases.
4.3 Reaction of bases with acids
This is the neutralization reaction already discussed.
For example:
One mole of sulphuric acid requires two moles of sodium hydroxide for complete neutralization under this reaction equation.
The coefficients in a balanced equation represent the relative numbers of reacting particles or moles, not necessarily equal masses or equal solution volumes.
5. Strong and Weak Acids and Bases
Students often confuse strength with concentration. These concepts are related to different chemical properties.
Strength of an acid
The strength of an acid refers to the extent to which it ionizes in water.
- A strong acid ionizes almost completely in water.
- A weak acid ionizes only partially in water.
Examples:
| Strong acids | Weak acids |
|---|---|
| Hydrochloric acid (HCl) | Acetic acid (CH₃COOH) |
| Nitric acid (HNO₃) | Carbonic acid (H₂CO₃) |
| Sulphuric acid (H₂SO₄) | Citric acid |
Sulphuric acid has more complex ionization behaviour than a simple one-step equation suggests. At Class 10 level, it is commonly classified as a strong acid.
Strength of a base
The strength of a base describes its ability to produce hydroxide ions or accept protons, depending on the definition used.
Examples:
- Strong bases: Sodium hydroxide and potassium hydroxide.
- Weak bases: Ammonia in water.
Strength versus concentration
Concentration describes how much solute is present in a given amount of solution.
Consider two hydrochloric acid solutions:
- Solution A is dilute hydrochloric acid.
- Solution B is concentrated hydrochloric acid.
Both contain the same strong acid, but they contain different amounts of acid per unit volume.
A dilute solution of a strong acid can contain fewer acid particles per unit volume than a concentrated solution of a weak acid. Therefore, strength alone does not determine concentration or pH.
| Basis of comparison | Strength | Concentration |
|---|---|---|
| Meaning | Extent of ionization or acid-base behaviour | Amount of solute per unit volume or mass of solution |
| Depends on | Chemical nature and ionization behaviour | Quantity of dissolved substance |
| Example | HCl is a strong acid | A particular HCl solution may be dilute or concentrated |
| Can they be used interchangeably? | No | No |
Exam tip: Remember that strong does not automatically mean concentrated, and weak does not automatically mean dilute.
6. Why Do Acids and Bases Show Their Properties in Water?
An acid or base may not show its familiar behaviour in the absence of water. The reason is connected to the formation of ions.
Acids in aqueous solution
Hydrochloric acid forms hydronium ions in water:
These hydronium ions are responsible for the characteristic acidic behaviour of the solution.
Dry hydrogen chloride gas does not turn dry blue litmus paper red because, without water, the ions needed for the usual acidic behaviour are not produced to the same extent.
Bases in aqueous solution
Sodium hydroxide separates into ions in water:
The hydroxide ions give the solution its basic properties.
Why is water important?
Water is a polar solvent, meaning its molecules have an uneven distribution of electrical charge. This helps many ionic substances dissolve and allows acids and bases to form or release ions.
The presence of water also enables ions to move through the solution, which is why aqueous acid and base solutions can conduct electricity.
Dilution of acids
Dilution means reducing the concentration of a solution by adding more solvent, usually water.
When an acid solution is diluted:
- The concentration of acid per unit volume decreases.
- Its pH generally increases if it is acidic.
- The solution may still remain acidic after dilution.
Dilution does not mean that the acid has been completely neutralized.
Safety rule: Always add acid slowly to water while stirring, never water to concentrated acid. Dilution can release substantial heat, causing splashing and burns.
7. The pH Scale
The pH scale is used to describe how acidic or basic an aqueous solution is. It is one of the most important concepts in this chapter because it connects chemical theory with biological processes and everyday life.
What is pH?
At Class 10 level, pH is used to indicate the acidic or basic nature of a solution.
For many ordinary dilute aqueous solutions at approximately room temperature:
- pH below 7 indicates an acidic solution.
- pH equal to 7 indicates a neutral solution.
- pH above 7 indicates a basic solution.
The pH scale is commonly shown from 0 to 14 in school-level examples. However, this is a useful conventional range, not an absolute limit for every possible solution.
Understanding the pH scale
0–3
Strongly acidic region
Examples: dilute solutions of strong acids at suitable concentrations.
4–6
Weakly acidic region
Examples: many fruit juices and mildly acidic solutions.
7
Neutral point
Pure water at about 25°C has a pH close to 7.
8–10
Weakly basic region
Examples: some mildly alkaline solutions.
11–14
Strongly basic region
Examples: sufficiently concentrated solutions of strong bases.
These ranges are illustrative rather than strict classifications. The actual pH depends on concentration, temperature and the chemical properties of the substance.
How does pH indicate acidity and basicity?
The pH of a solution is related to the activity of hydrogen ions in the solution. For dilute solutions, it is commonly explained in terms of hydrogen-ion concentration.
- Lower pH generally means greater acidity.
- Higher pH generally means greater basicity.
- A change of one pH unit represents a tenfold change in hydrogen-ion activity.
For example, a solution with pH 3 has approximately ten times the hydrogen-ion activity of a solution with pH 4 under comparable conditions.
Students do not generally need to calculate logarithms for the Class 10 CBSE syllabus; the important learning outcome is to interpret the pH scale correctly.
How is pH measured?
Common methods include:
- Litmus paper: Identifies whether a solution is acidic or basic, but does not give an exact pH.
- Universal indicator: Produces different colours over a range of pH values.
- pH paper: Gives an approximate pH by comparing its colour with a standard chart.
- pH meter: Provides a more precise instrumental measurement when properly calibrated.
Why is pH important in everyday life?
1. pH in the human digestive system
The stomach produces hydrochloric acid, which helps digestion and supports the action of digestive enzymes.
Excess stomach acidity may cause discomfort. Some antacids contain basic substances that neutralize excess acid.
For example, magnesium hydroxide reacts with hydrochloric acid:
Medicines should be used according to appropriate medical guidance rather than by attempting to neutralize stomach acid with arbitrary chemicals.
2. pH and tooth decay
Bacteria in dental plaque can produce acids when they break down sugars. These acids can lower the pH around the teeth and promote the loss of minerals from tooth enamel.
When the pH near the tooth surface falls sufficiently, demineralization becomes more likely.
Regular oral hygiene, limiting frequent sugar exposure and appropriate fluoride use help protect teeth.
3. pH of soil
Plants grow best within suitable soil-pH ranges, which vary by species.
- Excessively acidic soil may limit the availability of some nutrients and increase the availability of certain harmful metal ions.
- Excessively alkaline soil may reduce the availability of nutrients such as iron.
Farmers may use soil testing to decide whether amendments are needed. For acidic soils, agricultural lime can help raise the pH.
4. pH and self-defence in organisms
Some organisms produce acidic or irritating substances as a defence mechanism. For example, certain ant stings can cause irritation, but the chemical composition varies by species. It is not scientifically accurate to assume that every ant sting is caused by formic acid.
5. pH of rainwater
Rainwater naturally dissolves carbon dioxide from the atmosphere and is therefore slightly acidic. Clean rainwater in equilibrium with ordinary atmospheric carbon dioxide has a pH around 5.6 under typical conditions.
Acid rain is commonly defined as precipitation with a pH below about 5.6, often because of sulphur dioxide and nitrogen oxides that form acids in the atmosphere.
Acid rain can affect lakes, soil, forests, buildings and monuments.
8. Salts and Their Classification
A salt is an ionic compound made up of positive and negative ions. It is often formed during an acid-base reaction, although salts can also be produced through other chemical reactions.
8.1 How are salts formed?
Consider the neutralization of hydrochloric acid with sodium hydroxide:
The positive sodium ions and negative chloride ions form sodium chloride.
Different acids and bases produce different salts. For example:
| Acid | Base | Salt formed |
|---|---|---|
| Hydrochloric acid (HCl) | Sodium hydroxide (NaOH) | Sodium chloride (NaCl) |
| Sulphuric acid (H₂SO₄) | Sodium hydroxide (NaOH) | Sodium sulphate (Na₂SO₄) |
| Nitric acid (HNO₃) | Potassium hydroxide (KOH) | Potassium nitrate (KNO₃) |
| Hydrochloric acid (HCl) | Calcium hydroxide | Calcium chloride (CaCl₂) |
Notice that the formula of a salt depends on the charges of its ions and must be electrically neutral overall.
8.2 Families of salts
Salts can be grouped according to the positive or negative ions they share.
Sodium salts: NaCl, Na₂SO₄ and Na₂CO₃ share the sodium ion.
Chloride salts: NaCl, KCl and CaCl₂ contain chloride ions.
This classification is useful when studying the preparation, properties and uses of salts.
8.3 Acidic, basic and neutral salt solutions
A salt solution is not always neutral. Its pH depends on how the ions interact with water.
| Type of salt solution | Common school-level example | Approximate nature |
|---|---|---|
| Neutral | Sodium chloride (NaCl) | pH near 7 |
| Basic | Sodium carbonate (Na₂CO₃) | pH above 7 |
| Acidic | Ammonium chloride (NH₄Cl) | pH below 7 |
The nature of a salt solution is often related to the strengths of the acid and base from which the salt is derived.
- A salt of a strong acid and a strong base is generally neutral.
- A salt of a strong acid and a weak base is generally acidic.
- A salt of a weak acid and a strong base is generally basic.
These are useful rules for common school-level examples, though the exact pH also depends on the salt’s ions, concentration and temperature.
9. Common Salt and Important Chemicals Derived from It
Common salt, or sodium chloride (NaCl), is an important raw material for manufacturing several useful chemicals.
It is obtained from seawater, salt lakes and underground deposits of rock salt. Rock salt deposits formed when ancient bodies of water evaporated and left salt behind.
9.1 Sodium hydroxide: The chlor-alkali process
Sodium hydroxide is manufactured by passing electricity through an aqueous solution of sodium chloride, called brine.
This industrial process is known as the chlor-alkali process because it produces chlorine and an alkali, sodium hydroxide.
Overall reaction:
The three main products are:
- Sodium hydroxide (NaOH)
- Chlorine gas (Cl₂)
- Hydrogen gas (H₂)
Uses of sodium hydroxide:
- Manufacture of soaps and detergents.
- Manufacture of paper.
- Production of certain synthetic fibres.
- Cleaning and industrial chemical processes.
Chlorine and hydrogen are also valuable industrial raw materials.
9.2 Bleaching powder
Bleaching powder is commonly represented at Class 10 level by the formula CaOCl₂. It is prepared by passing chlorine over dry slaked lime, calcium hydroxide.
The equation is the conventional school-level representation of its preparation.
Uses of bleaching powder:
- Bleaching cotton and linen in the textile industry.
- Bleaching wood pulp in paper manufacture.
- Disinfecting water under appropriate conditions.
- Acting as an oxidizing agent in certain chemical processes.
Bleaching powder should not be mixed with acids, toilet cleaners or ammonia-containing products because hazardous gases may be released.
9.3 Baking soda
Baking soda is sodium hydrogencarbonate, also called sodium bicarbonate.
Its chemical formula is:
It is a mild basic salt and is widely used in food preparation.
Preparation:
In the Solvay process, sodium hydrogencarbonate precipitates when carbon dioxide is passed through ammoniated brine under suitable conditions.
A simplified equation is:
Uses of baking soda:
- As a component of baking powder.
- In some antacid preparations.
- In certain fire extinguishers.
- As a chemical reagent in laboratories.
Why does baking soda make cakes rise?
On heating, sodium hydrogencarbonate decomposes to produce sodium carbonate, carbon dioxide and water.
The released carbon dioxide forms bubbles in the dough or batter, helping it expand.
Baking powder generally contains baking soda and one or more suitable acids, often with a starch-based ingredient. The acid helps generate carbon dioxide during preparation and baking.
9.4 Washing soda
Washing soda is hydrated sodium carbonate. Its common crystalline formula is:
It is also called sodium carbonate decahydrate.
Uses of washing soda:
- Cleaning clothes and household surfaces.
- Softening hard water by helping remove calcium and magnesium ions.
- Manufacturing glass, soap and certain chemicals.
- Acting as a useful industrial raw material.
Washing soda is different from baking soda: washing soda contains sodium carbonate, whereas baking soda contains sodium hydrogencarbonate.
9.5 Plaster of Paris
Plaster of Paris (POP) is calcium sulphate hemihydrate.
Its formula is:
It is prepared by heating gypsum, calcium sulphate dihydrate, at a suitable temperature.
The reaction removes part of the water of crystallization from gypsum.
Uses of Plaster of Paris:
- Making casts and moulds.
- Producing decorative articles.
- Making statues and ornamental designs.
- Creating plaster casts for certain medical applications.
Why does Plaster of Paris harden when mixed with water?
When water is added, Plaster of Paris changes back into gypsum and forms a hard mass.
The setting process explains why POP is useful for making moulds and casts.
Important distinction: Gypsum contains two water molecules per formula unit, while Plaster of Paris contains half a water molecule per formula unit.
10. Water of Crystallization
Some salts contain a fixed number of water molecules as part of their crystal structure. This water is called water of crystallization.
It is not simply moisture clinging to the outside of the crystals. It is associated with the crystalline structure of the substance.
Examples
| Salt | Formula | Water molecules per formula unit |
|---|---|---|
| Copper(II) sulphate pentahydrate | CuSO₄·5H₂O | 5 |
| Washing soda | Na₂CO₃·10H₂O | 10 |
| Gypsum | CaSO₄·2H₂O | 2 |
| Plaster of Paris | CaSO₄·½H₂O | ½ |
Heating hydrated copper sulphate
Blue hydrated copper sulphate crystals contain water of crystallization.
When heated, they lose water and become pale or white anhydrous copper sulphate.
When water is added to anhydrous copper sulphate, the blue hydrated form can be restored.
This experiment demonstrates the role of water of crystallization in the appearance and composition of certain salts.
Why is water of crystallization important?
- It can influence the colour and structure of crystals.
- It helps distinguish hydrated salts from anhydrous salts.
- It is important in the preparation and use of substances such as gypsum and Plaster of Paris.
- It is part of the chemical formula and must be included when calculating the formula mass of a hydrated salt.
11. Important Differences for Examinations
Acids versus bases
| Acids | Bases |
|---|---|
| Produce hydronium ions in water through proton transfer | Many bases produce hydroxide ions in water |
| Turn blue litmus red | Turn red litmus blue |
| Have pH below 7 in ordinary dilute aqueous solutions | Have pH above 7 in ordinary dilute aqueous solutions |
| React with many metals to release hydrogen | Certain strong bases react with metals such as zinc and aluminium |
| React with bases in neutralization reactions | React with acids in neutralization reactions |
Baking soda versus washing soda
| Baking soda | Washing soda |
|---|---|
| Sodium hydrogencarbonate | Sodium carbonate decahydrate |
| NaHCO₃ | Na₂CO₃·10H₂O |
| Used in baking and some antacids | Used in cleaning and water softening |
| Produces carbon dioxide on heating | Hydrated crystals can lose water of crystallization |
Gypsum versus Plaster of Paris
| Gypsum | Plaster of Paris |
|---|---|
| Calcium sulphate dihydrate | Calcium sulphate hemihydrate |
| CaSO₄·2H₂O | CaSO₄·½H₂O |
| Contains two water molecules per formula unit | Contains half a water molecule per formula unit |
| Used in construction and as a raw material for POP | Used in casts, moulds and decorative articles |
| Heating under suitable conditions produces POP | Addition of water causes it to set into gypsum |
Strong acids versus concentrated acids
A strong acid ionizes extensively in water, whereas a concentrated acid contains a large amount of acid per unit volume of solution.
These terms describe different properties and should never be treated as synonyms.
12. Common Misconceptions
1. Misconception: All acids are dangerous and all bases are safe. Correction: Hazards depend on the substance and its concentration. Strong acids and strong bases can both cause severe burns.
2. Misconception: Every acid has the same strength. Correction: Acids differ in how extensively they ionize in water.
3. Misconception: A strong acid must be concentrated. Correction: A strong acid can be present in a dilute solution.
4. Misconception: All salts are neutral. Correction: Some salt solutions are acidic or basic.
5. Misconception: All metal oxides react in the same way. Correction: Many metal oxides are basic, but some are amphoteric or show other behaviour.
6. Misconception: Neutralization always produces only water. Correction: The usual acid-base neutralization reaction produces salt and water.
7. Misconception: Baking soda and washing soda are the same substance. Correction: Their formulas, properties and uses differ.
8. Misconception: Water of crystallization is just surface moisture. Correction: It forms part of the crystal structure of a hydrated salt.
9. Misconception: A pH of 7 is always neutral at every temperature. Correction: Neutral pH depends on temperature. Pure water is approximately pH 7 at 25°C.
10. Misconception: Litmus paper gives the exact pH of a solution. Correction: Litmus indicates acidic or basic behaviour, while pH paper or a pH meter is used to estimate or measure pH.
Quick Revision Notes
- Acids produce hydronium ions (H₃O⁺) in water through proton transfer.
- Bases neutralize acids; many water-soluble bases produce hydroxide ions (OH⁻).
- Alkalis are bases that dissolve in water and produce hydroxide ions.
- Blue litmus turns red in acids, while red litmus turns blue in bases.
- Phenolphthalein is colourless in acidic solution and pink in basic solution.
- Acids react with many metals to produce salt and hydrogen gas.
- Acids react with metal carbonates and hydrogencarbonates to produce salt, water and carbon dioxide.
- Carbon dioxide turns limewater milky due to the formation of calcium carbonate.
- Acid-base neutralization generally produces salt and water.
- Strong and weak acids differ in their extent of ionization; strength is different from concentration.
- A pH below 7 indicates acidity, pH 7 indicates neutrality at approximately 25°C, and pH above 7 indicates basicity in ordinary dilute aqueous solutions.
- The pH scale is important in digestion, dental health, soil chemistry and environmental science.
- Sodium chloride is an important raw material for the chlor-alkali process.
- The chlor-alkali process produces sodium hydroxide, chlorine and hydrogen.
- Bleaching powder: CaOCl₂ (conventional school-level formula).
- Baking soda: NaHCO₃; used in baking and certain antacid preparations.
- Washing soda: Na₂CO₃·10H₂O; used in cleaning and water softening.
- Plaster of Paris: CaSO₄·½H₂O; hardens on adding water to form gypsum.
- Gypsum: CaSO₄·2H₂O.
- Water of crystallization is the fixed number of water molecules associated with the crystalline structure of a hydrated salt.
- Strong acids are not necessarily concentrated, and weak acids are not necessarily dilute.
- Salt solutions may be acidic, basic or neutral depending on their chemical properties.
Practice MCQs
Test your understanding of the chapter. Each question has one correct answer, followed by a short explanation.
1. Which ion is principally associated with the acidic nature of an aqueous acid solution?
A. Sodium ion (Na⁺)
B. Hydronium ion (H₃O⁺)
C. Hydroxide ion (OH⁻)
D. Chloride ion (Cl⁻)
Correct Answer: B. Hydronium ion (H₃O⁺)
Explanation: Acids transfer protons to water molecules, producing hydronium ions, which are responsible for their acidic behaviour.
2. What happens when blue litmus paper is dipped into an acidic solution?
A. It turns green.
B. It becomes colourless.
C. It turns red.
D. It remains blue in every acid.
Correct Answer: C. It turns red.
Explanation: Acids turn blue litmus red. Litmus identifies the acidic nature of a solution but does not measure its exact pH.
3. Which of the following is a weak acid?
A. Hydrochloric acid
B. Nitric acid
C. Sodium hydroxide
D. Acetic acid
Correct Answer: D. Acetic acid
Explanation: Acetic acid ionizes only partially in water. Hydrochloric acid and nitric acid are strong acids, while sodium hydroxide is a strong base.
4. Zinc reacts with dilute hydrochloric acid. Which gas is released?
A. Hydrogen
B. Oxygen
C. Nitrogen
D. Carbon dioxide
Correct Answer: A. Hydrogen
Explanation: Zinc displaces hydrogen from dilute hydrochloric acid.
Zn + 2HCl → ZnCl₂ + H₂↑
5. Which products are formed when sodium carbonate reacts with hydrochloric acid?
A. Sodium hydroxide and hydrogen
B. Sodium chloride, water and carbon dioxide
C. Sodium oxide and oxygen
D. Sodium chloride and oxygen
Correct Answer: B. Sodium chloride, water and carbon dioxide
Explanation: Acids react with metal carbonates to produce a salt, water and carbon dioxide.
6. Which statement about strong acids and concentrated acids is correct?
A. Every strong acid is concentrated.
B. Every concentrated acid is weak.
C. Strength and concentration describe different properties.
D. Strength and concentration mean exactly the same thing.
Correct Answer: C. Strength and concentration describe different properties.
Explanation: Strength concerns ionization, whereas concentration concerns the amount of solute in a given amount of solution.
7. What is the approximate pH of pure water at 25°C?
A. 1
B. 5
C. 10
D. 7
Correct Answer: D. 7
Explanation: Pure water is approximately neutral at 25°C, with equal hydrogen-ion and hydroxide-ion activities.
8. Which indicator turns pink in a basic solution?
A. Phenolphthalein
B. Blue litmus
C. Methyl orange
D. Turmeric in an acidic solution
Correct Answer: A. Phenolphthalein
Explanation: Phenolphthalein is colourless in acidic solution and pink in basic solution.
9. What is the correct balanced equation for the neutralization of hydrochloric acid with sodium hydroxide?
A. HCl + NaOH → Na + H₂O
B. HCl + NaOH → NaCl + H₂O
C. HCl + NaOH → NaCl + H₂
D. 2HCl + NaOH → NaCl + O₂
Correct Answer: B. HCl + NaOH → NaCl + H₂O
Explanation: Hydrochloric acid reacts with sodium hydroxide to form sodium chloride and water. The equation is balanced.
10. Which salt commonly produces a basic aqueous solution?
A. Sodium chloride
B. Ammonium chloride
C. Sodium carbonate
D. Potassium nitrate
Correct Answer: C. Sodium carbonate
Explanation: Carbonate ions react with water in a way that produces hydroxide ions, making the solution basic.
11. Which chemicals are the principal products of the chlor-alkali process?
A. Sodium hydroxide, chlorine and hydrogen
B. Sodium chloride, oxygen and nitrogen
C. Sodium carbonate, chlorine and oxygen
D. Calcium hydroxide, hydrogen and carbon dioxide
Correct Answer: A. Sodium hydroxide, chlorine and hydrogen
Explanation: Electrolysis of brine produces sodium hydroxide, chlorine gas and hydrogen gas.
12. What is the chemical formula of baking soda?
A. Na₂CO₃·10H₂O
B. CaSO₄·½H₂O
C. CaOCl₂
D. NaHCO₃
Correct Answer: D. NaHCO₃
Explanation: Baking soda is sodium hydrogencarbonate, also known as sodium bicarbonate.
13. Which substance is commonly used to soften hard water?
A. Hydrochloric acid
B. Washing soda
C. Plaster of Paris
D. Copper sulphate
Correct Answer: B. Washing soda
Explanation: Washing soda helps remove calcium and magnesium ions responsible for water hardness.
14. What is the formula of Plaster of Paris?
A. CaSO₄·2H₂O
B. Na₂CO₃·10H₂O
C. CaSO₄·½H₂O
D. NaHCO₃
Correct Answer: C. CaSO₄·½H₂O
Explanation: Plaster of Paris is calcium sulphate hemihydrate. Gypsum, by contrast, is calcium sulphate dihydrate.
15. Which gas is responsible for making limewater milky?
A. Carbon dioxide
B. Hydrogen
C. Oxygen
D. Nitrogen
Correct Answer: A. Carbon dioxide
Explanation: Carbon dioxide reacts with limewater to form insoluble calcium carbonate, which makes the solution milky.
16. Which statement about water of crystallization is correct?
A. It always means water present on the surface of crystals.
B. It is found only in acids.
C. It has no effect on the formula of a hydrated salt.
D. It refers to a fixed number of water molecules associated with a salt’s crystal structure.
Correct Answer: D. It refers to a fixed number of water molecules associated with a salt’s crystal structure.
Explanation: Water of crystallization is part of the composition of hydrated crystalline salts, such as CuSO₄·5H₂O.
17. What happens when hydrated copper sulphate is heated sufficiently?
A. It forms sodium sulphate.
B. It loses water of crystallization and becomes anhydrous copper sulphate.
C. It produces hydrogen gas.
D. It turns into calcium carbonate.
Correct Answer: B. It loses water of crystallization and becomes anhydrous copper sulphate.
Explanation: Blue hydrated copper sulphate loses its water and becomes pale or white anhydrous copper sulphate.
18. Why can acidic soil be unsuitable for some plants?
A. It always contains no water.
B. Acidity makes every nutrient unavailable.
C. Its pH can affect nutrient availability and the solubility of potentially harmful metal ions.
D. Acidic soil always has a temperature above 100°C.
Correct Answer: C. Its pH can affect nutrient availability and the solubility of potentially harmful metal ions.
Explanation: Soil pH influences nutrient availability and the chemical behaviour of metals. Different plants have different suitable pH ranges.
19. A solution has a pH of 3. Which statement is correct?
A. It is acidic.
B. It is neutral.
C. It is basic.
D. It must contain no hydrogen ions.
Correct Answer: A. It is acidic.
Explanation: A pH below 7 indicates an acidic solution under ordinary dilute aqueous conditions.
20. Why does Plaster of Paris harden after water is added?
A. It produces sodium chloride.
B. It releases only hydrogen gas.
C. It changes into anhydrous sodium carbonate.
D. It reacts with water to form gypsum.
Correct Answer: D. It reacts with water to form gypsum.
Explanation: Plaster of Paris rehydrates to form calcium sulphate dihydrate, creating a hard mass.
Frequently Asked Questions (FAQs)
1. What is the difference between an acid and a base?
An acid produces hydronium ions in water through proton transfer, while a base accepts protons or, in the Arrhenius definition, produces hydroxide ions in aqueous solution. Acids and bases can react with each other through neutralization, usually forming salt and water.
2. Why does dry HCl gas not change the colour of dry blue litmus paper?
Dry hydrogen chloride gas does not produce the hydronium ions needed for the usual acidic behaviour without water. When HCl dissolves in water, it produces hydronium ions, and the solution turns blue litmus red.
3. What is the difference between pH and acid strength?
pH describes the acidity or basicity of a solution, whereas acid strength refers to how extensively an acid ionizes in water. The pH of a solution depends on factors such as concentration, ionization and temperature. Therefore, acid strength and pH are related but are not the same thing.
4. Why is baking soda used in baking?
Baking soda releases carbon dioxide gas when heated or when it reacts with a suitable acid. The gas forms bubbles in the batter, helping it expand and making baked products lighter. Baking powder combines baking soda with suitable acidic ingredients.
5. What is the difference between baking soda and baking powder?
Baking soda is sodium hydrogencarbonate (NaHCO₃), a single chemical compound. Baking powder is a mixture that generally contains baking soda, one or more dry acids and usually a starch-based ingredient. The acid helps baking soda release carbon dioxide.
6. Why is Plaster of Paris stored in moisture-resistant containers?
Plaster of Paris reacts with moisture and gradually converts into gypsum. This reaction can cause it to harden prematurely, making it unsuitable for preparing moulds and casts. Keeping it dry helps preserve its usefulness.
7. Why are acids added to water during dilution rather than water added to acids?
Diluting concentrated acids can release considerable heat. Adding acid slowly to a larger quantity of water helps distribute the heat and reduces the risk of sudden boiling and splashing. Adding water directly to concentrated acid can cause dangerous splashes.
8. Can a salt solution be acidic or basic?
Yes. The nature of a salt solution depends on the chemical properties of its ions in water. Sodium chloride generally produces a nearly neutral solution, ammonium chloride produces an acidic solution, and sodium carbonate produces a basic solution.
9. What is the difference between gypsum and Plaster of Paris?
Gypsum is calcium sulphate dihydrate, CaSO₄·2H₂O. Plaster of Paris is calcium sulphate hemihydrate, CaSO₄·½H₂O. Gypsum loses part of its water of crystallization when heated under suitable conditions to produce Plaster of Paris, which hardens when water is added.
Conclusion
The chapter Acids, Bases and Salts connects fundamental chemical principles with familiar substances and important industrial processes. Understanding how acids and bases produce ions, how neutralization works, how the pH scale describes solutions, and how salts are formed provides the foundation for studying chemistry in greater depth.
For effective revision, focus on the balanced chemical equations, the difference between strength and concentration, the applications of the pH scale, and the formulas and uses of bleaching powder, baking soda, washing soda and Plaster of Paris. Practise explaining why each reaction occurs rather than memorizing equations alone.
A student who understands the relationship between chemical composition, observable changes and practical applications will be better prepared to solve both direct factual questions and conceptual problems in Class 10 Science.
References / Further Reading
- NCERT, Class 10 Science, Chapter 2: Acids, Bases and Salts — the primary textbook reference for the chapter. Read the official NCERT chapter PDF.
- CBSE Academic, Class X Science Curriculum — for the prescribed syllabus and learning outcomes. Read the CBSE Science curriculum.