Acids, Bases and Salts notes

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CLASS 10 • SCIENCE • CHAPTER 2

Acids, Bases
& Salts

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📖 Chapter Roadmap

1. Acids and Bases2. Indicators3. Properties of Acids & Bases4. Acids + Metals5. Carbonates & Hydrogencarbonates6. Neutralisation7. Metal & Non-metal Oxides8. pH Scale9. Importance of pH10. What are Salts?11. Common Salt12. Sodium Hydroxide13. Bleaching Powder14. Baking Soda15. Washing Soda16. Plaster of Paris17. Quick Comparison18. Final Revision

Study rule: Learn the definitions first, then practise the reaction equations, pH values and uses of important salts.

1. What are Acids and Bases?

What are acids, bases and salts? Acids are substances that produce hydrogen ions in aqueous solution, bases produce hydroxide ions in aqueous solution, and salts are generally formed when an acid reacts with a base.

Acids

Acids are substances that generally have a sour taste and turn blue litmus red. In water, acids produce hydrogen ions. For example, hydrochloric acid gives hydrogen ions in aqueous solution.

\[HCl(aq)\rightarrow H^+(aq)+Cl^-(aq)\]

Common acids: hydrochloric acid (HCl), sulphuric acid (H₂SO₄), nitric acid (HNO₃) and ethanoic acid (CH₃COOH).

Bases

Bases are substances that are generally bitter, feel soapy and turn red litmus blue. Soluble bases are called alkalis.

\[NaOH(aq)\rightarrow Na^+(aq)+OH^-(aq)\]

Common bases: sodium hydroxide, potassium hydroxide, calcium hydroxide and magnesium hydroxide.

3-Step Understanding

1. Step 1: Acidic solutions contain hydrogen ions in water.
2. Step 2: Basic solutions contain hydroxide ions in water.
3. Step 3: Their properties can be tested using indicators and pH.

2. Indicators: How to Identify Acids and Bases?

An indicator is a substance that changes colour in acidic or basic medium.

Litmus

Blue litmus turns red in acid. Red litmus turns blue in base.

Common Indicators

Indicator	Acidic medium	Basic medium
Blue litmus	Turns red	Remains blue
Red litmus	Remains red	Turns blue
Phenolphthalein	Colourless	Pink
Methyl orange	Red	Yellow
Universal indicator	Shows a colour related to pH	Shows a colour related to pH

Olfactory Indicators

Some substances change or lose their smell in acidic or basic conditions. Onion, vanilla essence and clove oil are commonly discussed examples.

Exam tip: Remember: Acid → blue litmus becomes red; Base → red litmus becomes blue.

3. Chemical Properties of Acids and Bases

Acid + Base → Salt + Water

This reaction is called neutralisation.

\[HCl+NaOH\rightarrow NaCl+H_2O\]

Acids in Water

Acids show acidic behaviour when dissolved in water because they produce hydrogen ions. Dry HCl gas does not change dry litmus paper because ions are not produced without water.

Why do acids conduct electricity?

Aqueous acids contain mobile ions, so their solutions can conduct electric current.

Safety: Never taste laboratory chemicals. Always follow laboratory safety instructions when handling acids and bases.

4. Reaction of Acids with Metals

General reaction: Acid + Metal → Salt + Hydrogen gas.

\[\text{Acid}+\text{Metal}\rightarrow\text{Salt}+H_2\uparrow\]

Example: Zinc + Hydrochloric Acid

\[Zn+2HCl\rightarrow ZnCl_2+H_2\uparrow\]

1. Step 1: Zinc reacts with hydrochloric acid.
2. Step 2: Zinc chloride is formed.
3. Step 3: Hydrogen gas is released.

Acids + Metals: Important Point

Not every metal reacts with every acid in the same way. Metals below hydrogen in the reactivity series generally do not displace hydrogen from dilute non-oxidising acids.

Test for hydrogen: Hydrogen burns with a characteristic pop sound when a burning splint is brought near the gas.

5. Acids with Metal Carbonates and Hydrogencarbonates

General reaction: Acid + Metal carbonate → Salt + Water + Carbon dioxide.

\[\text{Acid}+\text{Metal carbonate}\rightarrow\text{Salt}+H_2O+CO_2\]

Example: Hydrochloric Acid + Sodium Carbonate

\[Na_2CO_3+2HCl\rightarrow2NaCl+H_2O+CO_2\uparrow\]

Example: Hydrochloric Acid + Sodium Hydrogencarbonate

\[NaHCO_3+HCl\rightarrow NaCl+H_2O+CO_2\uparrow\]

The carbon dioxide gas turns lime water milky.

\[Ca(OH)_2+CO_2\rightarrow CaCO_3\downarrow+H_2O\]

3-Step Identification

1. Step 1: Add acid to the carbonate or hydrogencarbonate.
2. Step 2: Effervescence occurs because CO₂ is released.
3. Step 3: Pass the gas through lime water; it turns milky.

6. Neutralisation Reaction

A reaction between an acid and a base in which salt and water are formed is called a neutralisation reaction.

\[\text{Acid}+\text{Base}\rightarrow\text{Salt}+\text{Water}\]

Example

\[HCl+NaOH\rightarrow NaCl+H_2O\]

Uses of Neutralisation

• Indigestion: Antacids neutralise excess acid in the stomach.
• Ant bite: A suitable mild base can help neutralise formic acid.
• Soil treatment: Acidic soil can be treated with bases such as quicklime or slaked lime.
• Factory waste: Acidic or basic industrial waste can be neutralised before disposal, as required by safety procedures.

7. Reactions with Metal and Non-metal Oxides

Acids + Metal Oxides

Metal oxides are generally basic. They react with acids to form salt and water.

\[CuO+2HCl\rightarrow CuCl_2+H_2O\]

Bases + Non-metal Oxides

Many non-metal oxides are acidic. They react with bases to form salt and water.

\[Ca(OH)_2+CO_2\rightarrow CaCO_3+H_2O\]

Memory trick: Acid + Basic oxide → Salt + Water. Base + Acidic oxide → Salt + Water.

8. The pH Scale

What is pH? pH is a scale used to express how acidic or basic an aqueous solution is. At 25°C, pH 7 is neutral; values below 7 are acidic and values above 7 are basic.

\[pH=-\log[H^+]\]

pH Range

pH	Nature	Meaning
0–6	Acidic	More acidic as pH decreases
7	Neutral	Neither acidic nor basic
8–14	Basic	More basic as pH increases

Universal Indicator

A universal indicator gives different colours at different pH values, allowing a rough comparison of acidity or basicity.

Remember: Lower pH = stronger acidity. Higher pH = stronger basicity, within the usual aqueous pH scale.

9. Importance of pH in Everyday Life

pH in the Stomach

The stomach contains hydrochloric acid. Excess acid can cause discomfort and is commonly treated using suitable antacids.

pH and Tooth Decay

When the pH in the mouth falls below about 5.5, tooth enamel can start to be damaged. Basic toothpaste helps neutralise excess acids.

pH of Soil

Plants grow well only within suitable pH ranges. Farmers may add materials to acidic or basic soil to adjust its pH.

pH and Self-defence by Animals/Plants

Some stings and bites involve acidic or basic substances. Knowledge of neutralisation helps explain suitable traditional first-aid approaches, but severe stings require medical attention.

Natural Water Systems

Changes in pH can affect aquatic organisms. Therefore, pH is an important environmental parameter.

10. What are Salts?

A salt is an ionic compound often formed by the reaction of an acid with a base. Salts can also be formed in other chemical reactions.

Example

\[HCl+NaOH\rightarrow NaCl+H_2O\]

Here, NaCl is the salt.

Family of Salts

Salts having the same positive or negative ion can be considered a family. For example, sodium salts include sodium chloride and sodium sulphate, while chloride salts include sodium chloride and calcium chloride.

pH of Salt Solutions

Different salts can produce acidic, basic or nearly neutral solutions depending on the acid and base from which they are formed.

11. Common Salt and Its Important Products

Common salt is sodium chloride, NaCl. It is an important raw material for making several useful chemicals.

Chlor-alkali Process

Electrolysis of concentrated aqueous sodium chloride (brine) produces sodium hydroxide, chlorine and hydrogen.

\[2NaCl+2H_2O\xrightarrow{electrolysis}2NaOH+Cl_2+H_2\]

Products and Uses

Product	Formula	One important use
Sodium hydroxide	NaOH	Soaps, detergents, paper and chemical industries
Chlorine	Cl₂	Water treatment and manufacture of chemicals
Hydrogen	H₂	Fuel and industrial uses

12. Sodium Hydroxide (NaOH)

Sodium hydroxide is a strong base obtained industrially by electrolysis of brine.

\[2NaCl+2H_2O\rightarrow2NaOH+Cl_2+H_2\]

Uses

• Manufacture of soaps and detergents
• Paper making
• Artificial textile fibres
• Several chemical manufacturing processes

Safety: Sodium hydroxide is highly corrosive. Never handle concentrated NaOH without proper laboratory safety equipment and supervision.

13. Bleaching Powder

Bleaching powder is commonly represented as CaOCl₂ at school level. It is prepared by passing chlorine over dry slaked lime.

\[Ca(OH)_2+Cl_2\rightarrow CaOCl_2+H_2O\]

Uses

• Disinfecting drinking water
• Bleaching cotton and linen in the textile industry
• Bleaching wood pulp in paper industries
• As an oxidising/disinfecting chemical in suitable applications

14. Baking Soda

Baking soda is sodium hydrogencarbonate, NaHCO₃.

On Heating

\[2NaHCO_3\xrightarrow{\Delta}Na_2CO_3+H_2O+CO_2\uparrow\]

Uses

• In baking powder to make cakes and bread rise
• As a mild antacid in suitable formulations
• In some fire extinguishing systems

Why does cake become soft and spongy?

On heating, baking soda produces carbon dioxide. The gas forms bubbles in the dough, helping it rise and become porous.

15. Washing Soda

Washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O.

Preparation from Sodium Hydrogencarbonate

\[2NaHCO_3\xrightarrow{\Delta}Na_2CO_3+H_2O+CO_2\]

The sodium carbonate can then crystallise with water to form washing soda:

\[Na_2CO_3+10H_2O\rightarrow Na_2CO_3\cdot10H_2O\]

Uses

• Cleaning agent
• Removing permanent hardness of water
• Manufacture of glass, soap and paper

16. Plaster of Paris (POP)

Plaster of Paris is calcium sulphate hemihydrate, CaSO₄·½H₂O.

Preparation

Gypsum is heated carefully to about 373 K to form Plaster of Paris.

\[CaSO_4\cdot2H_2O\xrightarrow{heat}CaSO_4\cdot\tfrac12H_2O+\tfrac32H_2O\]

Setting of POP

When water is added, POP changes back into hard gypsum.

\[CaSO_4\cdot\tfrac12H_2O+\tfrac32H_2O\rightarrow CaSO_4\cdot2H_2O\]

Uses

• Making casts for fractured bones
• Making moulds and decorative items
• Making false ceilings and models

Remember: POP + water → gypsum → hard setting mass.

17. Acids, Bases and Salts: Quick Comparison Table

Use this table for fast Class 10 Science revision and to identify important substances, reactions and uses.

Topic	Key point	Example / Formula	Exam clue
Acid	Produces H⁺ in aqueous solution	HCl	Blue litmus → red
Base	Produces OH⁻ in aqueous solution	NaOH	Red litmus → blue
Neutralisation	Acid + base → salt + water	HCl + NaOH → NaCl + H₂O	Salt + water formed
Acid + metal	Salt + hydrogen gas	Zn + 2HCl → ZnCl₂ + H₂	Pop sound test
Acid + carbonate	Salt + water + CO₂	NaHCO₃ + HCl → NaCl + H₂O + CO₂	Effervescence
pH	Measures acidity/basicity	7 = neutral	Lower pH = more acidic
Baking soda	Sodium hydrogencarbonate	NaHCO₃	Releases CO₂ on heating/with acid
Washing soda	Sodium carbonate decahydrate	Na₂CO₃·10H₂O	Cleaning + hard water
Bleaching powder	Chlorine product of slaked lime	CaOCl₂	Water disinfection
POP	Calcium sulphate hemihydrate	CaSO₄·½H₂O	Fracture casts

↔ Swipe left/right on the table on a small screen

18. ⭐ Final Board Exam Revision

Must Learn Definitions

• Acid
• Base and alkali
• Indicator
• Neutralisation
• pH
• Salt
• Water of crystallisation
• Baking soda
• Washing soda
• Bleaching powder
• Plaster of Paris

Must Learn Equations

\[Zn+2HCl\rightarrow ZnCl_2+H_2\]

\[Na_2CO_3+2HCl\rightarrow2NaCl+H_2O+CO_2\]

\[HCl+NaOH\rightarrow NaCl+H_2O\]

\[CuO+2HCl\rightarrow CuCl_2+H_2O\]

\[2NaCl+2H_2O\xrightarrow{electrolysis}2NaOH+Cl_2+H_2\]

\[2NaHCO_3\xrightarrow{\Delta}Na_2CO_3+H_2O+CO_2\]

\[CaSO_4\cdot2H_2O\xrightarrow{heat}CaSO_4\cdot\tfrac12H_2O+\tfrac32H_2O\]

⚠ Common Mistakes

• Do not confuse pH 7 with a strongly basic solution; pH 7 is neutral at 25°C.
• Do not write CO instead of CO₂ in carbonate reactions.
• Do not confuse baking soda (NaHCO₃) with washing soda (Na₂CO₃·10H₂O).
• Remember POP is CaSO₄·½H₂O, while gypsum is CaSO₄·2H₂O.
• Use correct coefficients when balancing chemical equations.

📝 Last-Minute Checklist

• ☐ Indicators
• ☐ Acid + metal
• ☐ Acid + carbonate
• ☐ Neutralisation
• ☐ Metal oxides
• ☐ pH scale
• ☐ Importance of pH
• ☐ Common salt
• ☐ Chlor-alkali process
• ☐ Bleaching powder
• ☐ Baking soda
• ☐ Washing soda
• ☐ POP
• ☐ Water of crystallisation

🎯 Exam method: Write the definition first, then the balanced equation, then one observation/use. For 3-mark questions, keep the answer in clear numbered steps.

✍ Practice Corner

Try these yourself before checking your textbook:

1. What is an acid? Give two examples.
2. What is a base? How is an alkali different?
3. What happens when zinc reacts with dilute hydrochloric acid?
4. Write the reaction of sodium hydrogencarbonate with HCl.
5. What is neutralisation? Give one everyday use.
6. What is pH? What does pH 7 indicate?
7. Why does tooth decay begin below about pH 5.5?
8. Explain the chlor-alkali process with its products.
9. What is baking soda? Give its formula and two uses.
10. Differentiate between washing soda and Plaster of Paris.

Answer-writing tip: For a 3-mark answer, use definition + equation/example + explanation. Keep each point short and scientifically correct.

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19. Acids, Bases and Salts Important Questions

What are acids and bases?
Acids produce hydrogen ions in aqueous solution, while bases produce hydroxide ions in aqueous solution.

What is the pH of a neutral solution?
At 25°C, a neutral aqueous solution has pH 7.

What happens when an acid reacts with a metal carbonate?
A salt, water and carbon dioxide are formed.

What is baking soda?
Baking soda is sodium hydrogencarbonate, NaHCO₃.

What is washing soda?
Washing soda is sodium carbonate decahydrate, Na₂CO₃·10H₂O.

What is Plaster of Paris?
POP is calcium sulphate hemihydrate, CaSO₄·½H₂O, used for casts, moulds and decorative work.

✍ Class 10 Science • Chapter 2 • Acids, Bases and Salts
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