Class 10 Science — Chapter 2 : Acids, Bases and Salts Notes
1. Acids and Bases — Basic Idea
Acids
Acids are substances that generally:
- have a sour taste,
- turn blue litmus red,
- produce H⁺/H₃O⁺ ions in water,
- react with certain metals to produce hydrogen gas.
Examples:
- Hydrochloric acid — HCl
- Sulphuric acid — H₂SO₄
- Nitric acid — HNO₃
- Acetic acid — CH₃COOH
Bases
Bases generally:
- are bitter,
- feel soapy/slippery,
- turn red litmus blue,
- produce OH⁻ ions in water.
Examples:
- Sodium hydroxide — NaOH
- Potassium hydroxide — KOH
- Calcium hydroxide — Ca(OH)₂
- Magnesium hydroxide — Mg(OH)₂
Alkalis
A base that dissolves in water is called an alkali.
Remember:
Every alkali is a base, but every base is not necessarily an alkali.
2. Indicators
An indicator is a substance that helps identify whether a solution is acidic or basic, usually through a colour or odour change.
Common indicators
| Indicator | In acidic medium | In basic medium |
|---|---|---|
| Blue litmus | Turns red | Remains blue |
| Red litmus | Remains red | Turns blue |
| Phenolphthalein | Colourless | Pink |
| Methyl orange | Red | Yellow |
| Litmus | Red | Blue |
| Turmeric | Yellow | Reddish-brown |
Natural indicators
Examples include:
- litmus,
- turmeric,
- red cabbage,
- coloured flower petals.
Litmus is obtained from lichens.
Olfactory indicators
Some substances change their smell depending on whether the medium is acidic or basic.
Examples studied in the chapter:
- onion,
- vanilla,
- clove.
3. Chemical Properties of Acids
A. Acids + Metals → Salt + Hydrogen
Acids react with many metals and release hydrogen gas.
General reaction
Acid + Metal → Salt + Hydrogen gas
Example:
Zn + H₂SO₄ → ZnSO₄ + H₂
Zinc reacts with dilute sulphuric acid to form zinc sulphate and hydrogen.
Test for hydrogen
Hydrogen is identified by bringing a burning flame near the gas. It burns with a characteristic ‘pop’ sound.
Important point
The metal displaces hydrogen from the acid, so hydrogen gas is liberated.
B. Bases + Certain Metals
Some metals can also react with strong bases.
For example, zinc reacts with sodium hydroxide:
2NaOH + Zn → Na₂ZnO₂ + H₂
The salt formed is sodium zincate, and hydrogen gas is released.
Important: Such reactions do not occur with every metal.
4. Acids + Metal Carbonates and Hydrogencarbonates
Acids react with metal carbonates and metal hydrogencarbonates to produce:
- a salt,
- carbon dioxide,
- water.
General equations
Metal carbonate + Acid → Salt + CO₂ + Water
Metal hydrogencarbonate + Acid → Salt + CO₂ + Water
Example 1: Sodium carbonate
Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂
Example 2: Sodium hydrogencarbonate
NaHCO₃ + HCl → NaCl + H₂O + CO₂
Test for carbon dioxide
When CO₂ is passed through lime water:
Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
Calcium carbonate forms a white precipitate, making lime water appear milky.
With excess CO₂:
CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂
The calcium hydrogencarbonate formed is soluble, so the milkiness disappears.
5. Neutralisation Reaction
A reaction between an acid and a base that produces salt and water is called a neutralisation reaction.
General equation
Acid + Base → Salt + Water
Example:
HCl + NaOH → NaCl + H₂O
At the ionic level:
H⁺ + OH⁻ → H₂O
Thus, the acidic effect of H⁺ and basic effect of OH⁻ cancel each other.
6. Acids + Metallic Oxides
Metallic oxides react with acids to form salt and water.
General reaction
Metal oxide + Acid → Salt + Water
Example:
CuO + 2HCl → CuCl₂ + H₂O
Copper oxide dissolves in hydrochloric acid, producing copper(II) chloride.
Since metallic oxides react with acids in a manner similar to bases, they are called basic oxides.
7. Non-metallic Oxides + Bases
Non-metallic oxides can react with bases to form salt and water.
Example:
CO₂ + Ca(OH)₂ → CaCO₃ + H₂O
Since CO₂ reacts with a base like an acid does, non-metallic oxides are considered acidic in nature.
8. What Do All Acids Have in Common?
The common chemical feature of acids is their ability to produce hydrogen ions in water.
For example:
HCl contains hydrogen, but simply containing hydrogen does not make a substance acidic.
Acids produce ions in aqueous solution, allowing the solution to conduct electricity.
HCl in water
HCl + H₂O → H₃O⁺ + Cl⁻
The H⁺ ion does not exist independently in water; it combines with water to form the hydronium ion (H₃O⁺).
So acidic character is associated with:
H⁺(aq) / H₃O⁺ ions
9. Why Alcohol and Glucose Are Not Acids
Alcohol and glucose contain hydrogen atoms, but their aqueous solutions do not produce H⁺ ions in the same way acids do.
Therefore, merely having hydrogen in a compound does not make it an acid.
Key idea
Acidic nature depends on the formation of H⁺/H₃O⁺ ions in aqueous solution.
10. Why Does Acidic Solution Conduct Electricity?
Acids ionise in water and produce ions.
For example:
HCl → H⁺ + Cl⁻
These charged particles allow electric current to pass through the solution.
Therefore:
Aqueous acidic solutions conduct electricity because of ions present in them.
Glucose and alcohol solutions do not show the same behaviour because they do not produce such ions in solution.
11. What Happens to Bases in Water?
Bases produce OH⁻ ions when dissolved in water.
Examples:
NaOH → Na⁺ + OH⁻
KOH → K⁺ + OH⁻
Mg(OH)₂ → Mg²⁺ + 2OH⁻
Thus:
Basic nature is due to OH⁻ ions in aqueous solution.
12. Dry HCl Gas Is Not Acidic
Hydrogen ions are produced from HCl only when water is present.
Therefore, dry HCl gas does not show acidic behaviour with dry litmus paper.
In water:
HCl + H₂O → H₃O⁺ + Cl⁻
Hence:
Water is essential for HCl to exhibit its acidic properties.
13. Dilution of Acids and Bases
Mixing an acid or base with water causes dilution.
During dilution:
- the concentration of H₃O⁺ ions in an acid decreases,
- the concentration of OH⁻ ions in a base decreases.
Safety rule
When diluting a concentrated acid:
Always add acid slowly to water with constant stirring.
Never add water directly to concentrated acid.
The process releases considerable heat. Adding water rapidly can cause sudden heating and splashing.
14. pH Scale
The pH scale is used to express the acidic or basic nature of a solution.
The usual pH range is:
0 to 14
| pH | Nature |
|---|---|
| Less than 7 | Acidic |
| 7 | Neutral |
| Greater than 7 | Basic/alkaline |
Important relationship
Higher H₃O⁺ concentration → Lower pH
Lower H₃O⁺ concentration → Higher pH
A universal indicator can be used to estimate pH by its colour.
15. Strong and Weak Acids
The strength of an acid depends on the amount of H⁺ ions it produces in solution.
Strong acid
Produces a relatively larger amount of H⁺ ions.
Weak acid
Produces a relatively smaller amount of H⁺ ions.
For example, when solutions of hydrochloric acid and acetic acid of the same concentration are compared, hydrochloric acid produces more hydrogen ions.
Important:
Strength is not simply the same thing as concentration.
16. Strong and Weak Bases
Similarly, bases can differ in strength according to the amount of OH⁻ ions they produce.
More OH⁻ ions → stronger basic character
Less OH⁻ ions → weaker basic character
17. Importance of pH in Everyday Life
A. pH of our body
The chapter states that the human body functions within a narrow pH range, approximately 7.0–7.8.
Living organisms are sensitive to changes in pH.
B. Acid rain
Rainwater having pH below 5.6 is called acid rain.
When acid rain enters rivers and other water bodies, it can lower their pH and make survival difficult for aquatic organisms.
C. Soil pH
Plants require a suitable pH range for healthy growth.
Farmers can test soil using a universal indicator and determine whether the soil is suitable for particular plants.
D. pH in the stomach
The stomach produces hydrochloric acid, which assists digestion.
Excess acid can cause indigestion, pain and irritation.
Antacids are mild bases used to neutralise excess stomach acid.
One example given in the chapter is magnesium hydroxide (milk of magnesia).
E. Tooth decay
Tooth decay begins when the pH of the mouth falls below about 5.5.
Bacteria act on food particles and sugars left in the mouth and produce acids.
These acids can damage tooth enamel.
Prevention
- Clean the mouth after eating.
- Use toothpaste, which is generally basic.
- Basic toothpaste helps neutralise excess acid.
F. Bee sting and nettle sting
A bee sting introduces an acidic substance that causes pain and irritation.
A mild base such as baking soda can provide relief by neutralising the acid.
Nettle plants can cause irritation because of methanoic acid.
18. Naturally Occurring Acids
| Natural source | Acid |
|---|---|
| Vinegar | Acetic acid |
| Curd/sour milk | Lactic acid |
| Orange | Citric acid |
| Lemon | Citric acid |
| Tamarind | Tartaric acid |
| Tomato | Oxalic acid |
| Ant sting | Methanoic acid |
| Nettle sting | Methanoic acid |
19. Salts
Salts are formed in many chemical reactions, especially neutralisation reactions.
Example:
HCl + NaOH → NaCl + H₂O
Here, sodium chloride is the salt.
20. Family of Salts
Salts having the same positive or negative ion/radical can be grouped into a family.
Sodium salts
- NaCl
- Na₂SO₄
Both contain Na⁺.
Chloride salts
- NaCl
- KCl
Both contain Cl⁻.
Thus, salts can belong to different families depending on their common ions.
21. pH of Salts
The nature of a salt solution depends on the acid and base from which the salt is formed.
Strong acid + Strong base
Produces a neutral salt.
Example:
- HCl + NaOH → NaCl
- pH approximately 7
Strong acid + Weak base
Produces an acidic salt solution.
pH < 7
Strong base + Weak acid
Produces a basic salt solution.
pH > 7
22. Common Salt — Sodium Chloride
The common salt used in food is sodium chloride (NaCl).
It can be obtained from seawater and is also found naturally as rock salt.
Rock salt consists of solid salt deposits formed from ancient seas and may contain impurities.
Common salt is important because it acts as a raw material for producing several useful chemicals.
23. Chlor-Alkali Process
When electricity is passed through aqueous sodium chloride solution (brine), it produces:
- sodium hydroxide,
- chlorine gas,
- hydrogen gas.
Equation
2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂
This is called the chlor-alkali process.
The name comes from:
- chlor → chlorine
- alkali → sodium hydroxide
Products and electrodes
- Chlorine is released at the anode.
- Hydrogen is released at the cathode.
- Sodium hydroxide solution forms near the cathode.
24. Bleaching Powder
Chlorine produced during the chlor-alkali process is used to manufacture bleaching powder.
It is represented in the chapter as:
Ca(ClO)₂
It is made by passing chlorine over dry slaked lime.
Equation given in the chapter
2Ca(OH)₂ + 2Cl₂ → Ca(ClO)₂ + CaCl₂ + 2H₂O
Uses of bleaching powder
- Bleaching cotton and linen.
- Bleaching wood pulp in paper industries.
- Bleaching washed clothes.
- Acting as an oxidising agent in chemical industries.
- Helping make drinking water free from germs.
25. Baking Soda
Chemical name
Sodium hydrogencarbonate
Formula
NaHCO₃
It is a mild, non-corrosive basic salt.
Preparation
One industrial preparation uses sodium chloride as a raw material:
NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃
Heating Baking Soda
When heated:
2NaHCO₃ → Na₂CO₃ + H₂O + CO₂
Products:
- sodium carbonate,
- water,
- carbon dioxide.
Uses of baking soda
1. Baking
Baking powder contains baking soda and an edible weak acid such as tartaric acid.
The reaction produces CO₂.
CO₂ makes bread and cakes rise, making them soft and spongy.
2. Antacids
Baking soda can neutralise excess acid in the stomach.
3. Fire extinguishers
It is used in soda-acid fire extinguishers, where the reaction generates CO₂.
26. Washing Soda
Chemical name
Sodium carbonate decahydrate
Formula
Na₂CO₃·10H₂O
Sodium carbonate can be obtained by heating baking soda, and recrystallisation produces washing soda.
Important property
It is a basic salt.
Uses
- Glass industry.
- Soap industry.
- Paper industry.
- Manufacture of sodium compounds such as borax.
- Domestic cleaning.
- Removing permanent hardness of water.
Easy memory
Washing soda → Washing/cleaning + industries + hard water
27. Water of Crystallisation
Some salt crystals contain a fixed number of water molecules within their crystal structure.
This fixed number of water molecules associated with one formula unit of a salt is called water of crystallisation.
Copper sulphate
Hydrated copper sulphate:
CuSO₄·5H₂O
It is blue.
When heated, it loses its water of crystallisation and becomes white.
Adding water restores the blue colour.
Important concept
The water present in a hydrated salt is not simply moisture or wetness. It is chemically associated with the crystal structure.
28. Washing Soda and Water of Crystallisation
The formula:
Na₂CO₃·10H₂O
contains 10 water molecules as water of crystallisation.
The “10H₂O” does not mean that the salt is simply wet.
29. Gypsum
Gypsum is another salt containing water of crystallisation.
Formula
CaSO₄·2H₂O
It contains two water molecules per formula unit.
When gypsum is heated, it loses part of its water and forms Plaster of Paris.
30. Plaster of Paris (POP)
Chemical name
Calcium sulphate hemihydrate
Formula
CaSO₄·½H₂O
It is produced by heating gypsum at about 373 K.
Reaction
CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O
When POP is mixed with water
It changes back into gypsum and forms a hard solid mass:
CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O
Uses of Plaster of Paris
- Supporting fractured bones as a plaster.
- Making toys.
- Decorative materials.
- Making surfaces smooth.
Why is “½H₂O” possible?
It does not mean that an individual water molecule is cut in half.
The formula represents the average composition: two CaSO₄ units share one water molecule.
31. Must-Know Chemical Equations
Acid + Metal
Zn + H₂SO₄ → ZnSO₄ + H₂
Acid + Base
HCl + NaOH → NaCl + H₂O
Acid + Metal Carbonate
Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂
Acid + Metal Hydrogencarbonate
NaHCO₃ + HCl → NaCl + H₂O + CO₂
CO₂ + Lime Water
Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
Acid + Metal Oxide
CuO + 2HCl → CuCl₂ + H₂O
HCl in Water
HCl + H₂O → H₃O⁺ + Cl⁻
Sodium hydroxide in water
NaOH → Na⁺ + OH⁻
Chlor-alkali process
2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂
Baking soda on heating
2NaHCO₃ → Na₂CO₃ + H₂O + CO₂
Gypsum → Plaster of Paris
CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O
POP + Water → Gypsum
CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O
32. High-Value Comparisons
Acid vs Base
| Acid | Base |
|---|---|
| Produces H⁺/H₃O⁺ in water | Produces OH⁻ in water |
| Blue litmus → red | Red litmus → blue |
| pH < 7 | pH > 7 |
| Often sour | Often bitter |
| Neutralises bases | Neutralises acids |
Strong vs Weak Acid
| Strong acid | Weak acid |
|---|---|
| Produces more H⁺ ions in solution | Produces fewer H⁺ ions |
| Greater ionisation | Lower ionisation |
| Example from chapter comparison: HCl | Acetic acid |
Base vs Alkali
Base: A substance with basic properties; not necessarily soluble in water.
Alkali: A base that dissolves in water.
33. Reaction Patterns to Memorise
These patterns can help you predict products in many Class 10 questions:
Acid + Metal → Salt + H₂
Acid + Metal Carbonate → Salt + CO₂ + H₂O
Acid + Metal Hydrogencarbonate → Salt + CO₂ + H₂O
Acid + Base → Salt + H₂O
Acid + Metal Oxide → Salt + H₂O
Non-metallic oxide + Base → Salt + H₂O
34. Important “Why?” Questions
Why should curd and sour substances not be stored in brass or copper vessels?
They contain acids that can react with the metals, potentially forming unwanted compounds.
Why does dry HCl not change dry blue litmus?
Water is required for HCl to produce H₃O⁺ ions.
Why does acid solution conduct electricity?
It contains ions that carry electric current.
Why does distilled water conduct electricity poorly while rainwater can conduct?
Distilled water has very few ions, whereas rainwater contains dissolved substances that provide ions.
Why is acid added to water during dilution?
Dilution releases heat. Adding acid slowly to water helps distribute the heat safely.
Why does tooth decay begin below pH 5.5?
Acidic conditions below this level can damage tooth enamel.
Why is baking soda used as an antacid?
It is mildly basic and can neutralise excess stomach acid.
Why does baking powder make cakes rise?
Its reaction produces CO₂ gas, which creates bubbles and makes the baked product rise.
Why is Plaster of Paris stored in moisture-proof containers?
Moisture can cause it to react with water and convert into gypsum, reducing its usefulness as POP.
35. Exam-Focused Facts
Memorise these numbers and facts:
- pH range: generally 0–14
- Neutral pH: 7
- Acidic: pH < 7
- Basic: pH > 7
- Acid rain: pH < 5.6
- Tooth decay becomes significant: below about pH 5.5
- Body working range stated in chapter: about pH 7.0–7.8
- Gypsum: CaSO₄·2H₂O
- Plaster of Paris: CaSO₄·½H₂O
- Baking soda: NaHCO₃
- Washing soda: Na₂CO₃·10H₂O
- Common salt: NaCl
- Bleaching powder: represented as Ca(ClO)₂
- Hydrogen ion in water: H₃O⁺
- Hydroxide ion: OH⁻
36. One-Page Chapter Revision
ACIDS
Acid → H⁺/H₃O⁺ in water → pH below 7
Acid + metal → salt + H₂
Acid + carbonate → salt + CO₂ + H₂O
Acid + base → salt + H₂O
Acid + metal oxide → salt + H₂O
BASES
Base → OH⁻ in water → pH above 7
Some bases dissolve in water → alkalis
Base + acid → salt + water
Some bases react with metals → salt + H₂
INDICATORS
- Blue litmus → red in acid
- Red litmus → blue in base
- Phenolphthalein → pink in base
- Methyl orange → red in acid, yellow in base
- Turmeric → reddish-brown in base
pH
0 ← acidic | 7 neutral | basic → 14
Lower pH → greater H₃O⁺ concentration
Higher pH → greater basic character
SALTS
Strong acid + strong base → neutral salt
Strong acid + weak base → acidic salt
Strong base + weak acid → basic salt
COMMON SALT → IMPORTANT PRODUCTS
NaCl
↓ electrolysis of brine
NaOH + Cl₂ + H₂
From these and related processes:
- Bleaching powder
- Baking soda
- Washing soda
- Other useful chemicals
IMPORTANT FORMULAE
NaHCO₃ → Baking soda
Na₂CO₃·10H₂O → Washing soda
CaSO₄·2H₂O → Gypsum
CaSO₄·½H₂O → Plaster of Paris
NaCl → Common salt
37. Final Concept Map
ACIDS, BASES AND SALTS
→ Indicators
- Litmus
- Turmeric
- Phenolphthalein
- Methyl orange
- Olfactory indicators
→ Acid reactions
- With metals → H₂
- With carbonates → CO₂
- With bases → neutralisation
- With metal oxides → salt + water
→ Base reactions
- With acids → neutralisation
- Some metals → H₂
- Non-metallic oxides → salt + water
→ Ions
- Acids → H₃O⁺
- Bases → OH⁻
→ pH
- <7 acidic
- 7 neutral
- 7 basic
→ Everyday importance
- Digestion
- Antacids
- Tooth decay
- Soil
- Acid rain
- Stings
→ Salts
- Salt families
- pH of salts
- Common salt
→ Common salt products
- NaOH
- Bleaching powder
- Baking soda
- Washing soda
→ Water of crystallisation
- CuSO₄·5H₂O
- Na₂CO₃·10H₂O
- CaSO₄·2H₂O
→ Plaster of Paris
- CaSO₄·½H₂O
- Used for casts, toys and decoration
What to Learn First for an Exam
If you are short on time, prioritise these:
1. All major reaction patterns
2. pH scale and relation with H₃O⁺/OH⁻
3. Strong vs weak acids/bases
4. Neutralisation
5. Reactions of acids with metals and carbonates
6. Chlor-alkali process
7. Baking soda and washing soda
8. Water of crystallisation
9. Gypsum and Plaster of Paris
10. Everyday applications of pH
11. Important chemical equations
12. Indicator colour changes
Introduction
- Acids: Substances that release H⁺ ions in water.
- Examples: HCl, H₂SO₄
- Bases: Substances that release OH⁻ ions in water.
- Examples: NaOH, Ca(OH)₂
- Salts: Compounds formed when an acid reacts with a base.
- Example: HCl + NaOH → NaCl + H₂O
Properties of Acids and Bases
| Property | Acid | Base |
|---|---|---|
| Taste | Sour | Bitter |
| Touch | Corrosive | Slippery |
| Litmus Test | Turns blue → red | Turns red → blue |
| Conductivity | Conduct electricity | Conduct electricity |
pH Scale
- Measures acidic or basic nature of a solution
- Scale ranges from 0 to 14:
- pH < 7 → Acidic
- pH = 7 → Neutral
- pH > 7 → Basic
- Examples:
- Lemon juice pH ≈ 2
- Soap solution pH ≈ 12
Neutralization Reaction
- Reaction between an acid and a base producing salt and water
- Example: HCl + NaOH → NaCl + H₂O
- Applications:
- Treating acidity with antacids
- Making fertilizers
Types of Salts
- Normal salts: Formed when all H⁺ of an acid is replaced by metal ions
- Example: NaCl
- Acidic salts: Some H⁺ remains unreacted
- Example: NaHSO₄
- Basic salts: Some OH⁻ remains unreacted
- Example: Cu₂(OH)Cl
Preparation of Salts
- Reaction of acids with metals:
- Zn + H₂SO₄ → ZnSO₄ + H₂
- Reaction of acids with bases:
- HCl + NaOH → NaCl + H₂O
- Reaction of acids with metal carbonates:
- CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O
Importance and Uses
- Salts: Table salt (NaCl), Baking soda (NaHCO₃)
- Acids: HCl in digestion, H₂SO₄ in industry
- Bases: Ca(OH)₂ in construction, NaOH in soap making