Is it correct to say: Confusing 'strong' with 'concentrated' acid?

STRONG = fully ionises in water (HCl, H₂SO₄). CONCENTRATED = high amount of acid per litre. A dilute strong acid still ionises 100 %.

Is it correct to say: Writing acid + metal → salt + water?

Wrong. Acid + metal → salt + H₂. Water is a product only with a base or a carbonate.

Is it correct to say: Assuming lower pH means less acidic?

Lower pH ⇒ MORE H⁺ ⇒ MORE acidic. pH 1 is very acidic, pH 6 is mildly acidic.

Is it correct to say: Adding water to concentrated acid?

ALWAYS add acid to water (slowly, with stirring). Reverse can cause splashing and burns due to the exothermic mixing.

Is it correct to say: Calling all salts neutral?

Salt of strong acid + strong base = neutral. Strong acid + weak base (NH₄Cl) = acidic. Weak acid + strong base (Na₂CO₃) = basic.

Is it correct to say: Confusing bleaching powder Ca(OCl)Cl with lime CaO or slaked lime Ca(OH)₂?

Bleaching powder = calcium oxychloride, made from Cl₂ + Ca(OH)₂. Different chemistry, different uses.

Acids, Bases and Salts

Properties of acids and bases, pH scale, salts, water of crystallisation and chlor-alkali process — NCERT Class 10 Chemistry Ch 2

📐 9 formulas✏️ 3 examples🎯 6 practice⚖️ 5 marks🏫 CBSE📚 Class 10✓ 2025–26 syllabus

💡

Board Exam Tips

→pH scale numerical (pH from [H⁺]) — practise both directions.
→Chlor-alkali process (electrolysis of brine) — a favourite 3-mark diagram question.
→Preparation and uses of bleaching powder, baking soda, washing soda, plaster of Paris — one of these appears every year.
→Colour change of indicators — memorise litmus, phenolphthalein and methyl orange.
→Difference between strong/weak and dilute/concentrated acids — often confused; 2 marks safe.

📐 Formulas(9)

Acid Ionisation in Water★ Board fav

H C l (a q) + H_{2} O (l) ⟶ H_{3} O^{+} (a q) + C l^{-} (a q)

Base Ionisation in Water

N a O H (a q) ⟶ N a^{+} (a q) + O H^{-} (a q)

Neutralisation★ Board fav

A c i d + B a se ⟶ S a l t + W a t er

Acid + Metal

A c i d + M e t a l ⟶ S a l t + H_{2} (g)

Acid + Metal Carbonate

2 H C l + N a_{2} C O_{3} ⟶ 2 N a C l + H_{2} O + C O_{2} (g)

pH Scale★ Board fav

p H = - lo g_{10} [H^{+}]

Symbol	Meaning
$[H^{+}]$	Molar concentration of H⁺ ions (mol/L)

Chlor-Alkali Process

2 N a C l (a q) + 2 H_{2} O (l) electrolysis 2 N a O H (a q) + C l_{2} (g) + H_{2} (g)

Water of Crystallisation

C u S O_{4} \cdot 5 H_{2} O Δ C u S O_{4} + 5 H_{2} O

Setting of Plaster of Paris

C a S O_{4} \cdot \frac{1}{2} H_{2} O + \frac{3}{2} H_{2} O ⟶ C a S O_{4} \cdot 2 H_{2} O

✏️ Solved Examples

1Solved Exampleeasy3 steps

A solution has [H⁺] = 10⁻⁴ M. Find its pH and state whether acidic or basic.

Apply the pH formula

p H = - lo g_{10} [H^{+}] = - lo g_{10} (1 0^{- 4})

2Solved Exampleboard4 steps

Explain what happens when dilute sulphuric acid is added to sodium hydroxide solution. Write a balanced equation and identify the salt.

It is an acid-base neutralisation. H⁺ from H₂SO₄ combines with OH⁻ from NaOH to give water.

3Solved ExampleHOTS5 steps

Compound A (used in baking) on heating gives compound B, water and CO₂. B on dissolving in water and recrystallising gives compound C (washing soda). Identify A, B and C with balanced equations.

A is baking soda ⇒ NaHCO₃

A = N a H C O_{3} (sodium hydrogencarbonate)

⚠️ Traps & Common Mistakes

⚠️Common Mistakes6

1
Confusing 'strong' with 'concentrated' acid
✓STRONG = fully ionises in water (HCl, H₂SO₄). CONCENTRATED = high amount of acid per litre. A dilute strong acid still ionises 100 %.
2
Writing acid + metal → salt + water
✓Wrong. Acid + metal → salt + H₂. Water is a product only with a base or a carbonate.
3
Assuming lower pH means less acidic
✓Lower pH ⇒ MORE H⁺ ⇒ MORE acidic. pH 1 is very acidic, pH 6 is mildly acidic.
4
Adding water to concentrated acid
✓ALWAYS add acid to water (slowly, with stirring). Reverse can cause splashing and burns due to the exothermic mixing.
5
Calling all salts neutral
✓Salt of strong acid + strong base = neutral. Strong acid + weak base (NH₄Cl) = acidic. Weak acid + strong base (Na₂CO₃) = basic.
6
Confusing bleaching powder Ca(OCl)Cl with lime CaO or slaked lime Ca(OH)₂
✓Bleaching powder = calcium oxychloride, made from Cl₂ + Ca(OH)₂. Different chemistry, different uses.

🎯 Practice Yourself

🎯Practice Yourself6 questions

Q1
A solution has pH = 2. Find [H⁺]. Is it acidic or basic?
Q2
Give the chemical name and formula of (a) baking soda, (b) washing soda, (c) plaster of Paris.
Q3
State two uses of bleaching powder.
Q4
Explain the term 'water of crystallisation' with an example.
Q5
Why does an aqueous solution of NaCl conduct electricity but glucose solution does not?
Q6
Fresh milk has pH 6. What happens to its pH as it turns to curd? Why?

📝 Notes

Acids, Bases and Salts

Every kitchen, garden and laboratory reaction ultimately traces back to acids and bases — the two 'personalities' of aqueous chemistry.

Arrhenius definitions (Class 10 level)

Acid: substance that gives H⁺ (or H₃O⁺) ions in water.
Base: substance that gives OH⁻ ions in water.
Alkali: a water-soluble base.
Salt: ionic compound formed from the cation of a base and anion of an acid.

Reactions of acids — must-know four

With metals: acid + metal → salt + H₂↑
With carbonates / bicarbonates: acid + carbonate → salt + water + CO₂↑
With metal oxides / hydroxides (bases): acid + base → salt + water
In water: dissociation gives H⁺ (H₃O⁺) ions.

pH scale — practical numbers

| pH | Example | |---|---| | 1 | Gastric juice, HCl 0.1 M | | 3 | Vinegar, lemon | | 5.6 | 'Acid rain' threshold | | 7 | Pure water, blood pH ~7.4 | | 9 | Baking soda solution | | 11 | Household ammonia | | 14 | NaOH 1 M |

Living systems are sensitive to pH — human blood must stay between 7.35 and 7.45.

Industrial and household salts

| Salt | Made by | Use | |---|---|---| | Common salt NaCl | Sea water evaporation | Food, chlor-alkali feedstock | | NaOH (caustic soda) | Chlor-alkali process | Soap, paper, textiles | | Bleaching powder Ca(OCl)Cl | Cl₂ + slaked lime | Disinfectant, bleach | | Baking soda NaHCO₃ | Solvay process | Baking, antacid | | Washing soda Na₂CO₃·10H₂O | Recrystallising Na₂CO₃ | Softening water, glass | | Plaster of Paris CaSO₄·½H₂O | Heating gypsum ~373 K | Casts, moulds, boards |

Quick sanity checks

Universal indicator gives a rainbow: red (acid) → violet (base).
Neutralisation between strong acid + strong base is always exothermic (~57 kJ per mole H⁺).
Sudden change of pH by one unit = 10× change in acidity.