Chapter 2 of 13

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Rajasthan (RBSE)Class 10 Science★ 7–9 marks · RBSE Class 10 Science board (Unit — Chemical Substances: Nature & Behaviour)

Acids, Bases and Salts

Why does lemon sting and soap feel slippery? RBSE Class 10 Science Chapter 2 — indicators (including olfactory ones), the reactions of acids and bases, neutralisation, the pH scale and why it rules your stomach, teeth and soil, plus the chemistry of common salts — washing soda, baking soda, bleaching powder and plaster of Paris — with RBSE board-pattern problems and solutions.

⏱ 38 min readMedium difficulty✓ Free · NCERT-aligned

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Study Guide

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By the end of this chapter you'll be able to…

1Identify acids and bases using litmus, phenolphthalein, methyl orange and olfactory indicators
2Write the reactions of acids and bases with metals, carbonates, oxides and each other (neutralisation)
3Explain acidity/basicity in terms of H⁺(aq) and OH⁻(aq) ions and distinguish strong from weak
4Use the pH scale to compare acidity and explain its role in digestion, tooth decay, soil and stings
5State the preparation, formula and uses of common salt, baking soda, washing soda, bleaching powder and plaster of Paris
6Explain water of crystallisation with copper sulphate and gypsum/plaster of Paris as examples

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Why this chapter matters

The pH scale introduced here is one of the most reused ideas in all of school science — it returns in biology (blood, digestion, soil) and in agriculture. The chapter is also a reliable scorer in the RBSE board paper: the named salts (baking soda, washing soda, bleaching powder, plaster of Paris) and their formulae/uses appear almost every year.

Before you start — revise these

A 5-minute refresher here will save you 30 minutes of confusion below.

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Chemical Reactions and Equations (Class 10)

Balancing equations and the double-displacement/neutralisation pattern.

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Atoms and Molecules (Class 9)

Ions, valency and writing formulae of salts.

Acids, Bases and Salts — RBSE Class 10 (Science)

Squeeze a lemon and your tongue knows instantly — sour. Rub a slip of soap between wet fingers — slippery, slightly bitter if it touches your lips. Your senses are doing crude chemistry: detecting acids and bases. This chapter turns those gut reactions into a measurable, predictable science built around one number — the pH.

1. Indicators — how we see acids and bases

We can't taste chemicals in a lab (never do). Instead we use indicators — substances that change colour (or smell) in acids and bases.

Indicator	In acid	In base
Litmus (from lichen)	red	blue
Methyl orange	red/pink	yellow
Phenolphthalein	colourless	pink
Turmeric	unchanged (yellow)	red/brown

There are also olfactory indicators — substances whose smell changes with acidity. Onion and vanilla lose their smell in a base; clove oil's smell changes too. These are useful for visually impaired students and as a neat demonstration.

2. Chemical properties of acids and bases

Acids + metals → salt + hydrogen

$Zn + H_{2} SO_{4} \to ZnSO_{4} + H_{2} ↑$

The hydrogen gas burns with a "pop" sound when a lit matchstick is brought near — the standard test.

Bases + metals → salt + hydrogen

Reactive bases also release hydrogen with certain metals:

$2 NaOH + Zn \to Na_{2} ZnO_{2} + H_{2} ↑$

Acids + metal carbonates / hydrogencarbonates → salt + water + CO₂

$Na_{2} CO_{3} + 2 HCl \to 2 NaCl + H_{2} O + CO_{2} ↑$

The CO₂ turns limewater milky — the test for carbon dioxide.

Acids + bases → salt + water (neutralisation)

$NaOH + HCl \to NaCl + H_{2} O$

Metal oxides + acids; non-metal oxides + bases

Metal oxides are basic (CuO + 2HCl → CuCl₂ + H₂O); non-metal oxides are acidic (CO₂ + 2NaOH → Na₂CO₃ + H₂O). These reactions confirm the basic/acidic nature of the oxides.

3. What is an acid or a base, really?

Acids produce hydrogen ions, H⁺(aq) (which exist as hydronium ions, H₃O⁺) in water. Bases produce hydroxide ions, OH⁻(aq) in water.

Two key consequences:

Acids need water to show acidic behaviour — dry HCl gas does not turn blue litmus red. The H⁺ ion must be free in solution.
Diluting an acid or base is highly exothermic. Always add acid to water, slowly, while stirring — never water to acid, or the heat can splash the concentrated acid out.

Strong vs weak: a strong acid (HCl, H₂SO₄, HNO₃) ionises completely; a weak acid (acetic acid in vinegar, carbonic acid) ionises only partly. Same for strong bases (NaOH, KOH) and weak bases (NH₄OH).

4. The pH scale — strength on a ruler

The pH scale runs from 0 to 14 and measures the concentration of H⁺ ions:

pH < 7 → acidic (the lower, the more acidic)
pH = 7 → neutral (pure water)
pH > 7 → basic (the higher, the more basic)

A universal indicator gives a different colour at each pH. The scale is logarithmic: pH 3 is ten times more acidic than pH 4.

Why pH rules everyday life

In your stomach: dilute HCl (pH ~1.5) digests food. Excess acid → acidity; an antacid (a mild base like milk of magnesia, Mg(OH)₂) neutralises it.
In your mouth: below pH 5.5, tooth enamel (calcium phosphate) starts to dissolve → cavities. Bacteria make acid from leftover sugar; brushing with mildly basic toothpaste neutralises it.
In the soil: plants grow best in a narrow pH band; farmers in Rajasthan treat acidic soil with lime and alkaline soil with gypsum.
Self-defence in nature: a bee sting injects acid (soothe with baking soda); a nettle sting is acidic too. A wasp sting is alkaline.
Acid rain (pH < 5.6) lowers river pH and harms aquatic life.

5. Salts — a big, useful family

A salt is the product (besides water) of an acid–base neutralisation. The acid and base that "make" a salt give it a family name: all chlorides (NaCl, KCl) come from HCl; all sulphates from H₂SO₄.

pH of a salt depends on its parents:

strong acid + strong base → neutral salt (NaCl, pH 7)
strong acid + weak base → acidic salt (NH₄Cl)
weak acid + strong base → basic salt (Na₂CO₃)

Common salt (NaCl) — the raw material

From common salt we industrially make a whole shelf of chemicals (the chlor-alkali process electrolyses brine):

$2 NaCl (a q) + 2 H_{2} O (l) electricity 2 NaOH (a q) + Cl_{2} (g) + H_{2} (g)$

NaOH, chlorine and hydrogen — each feeds dozens of products.

The salts you must know cold

Salt	Formula	Made from	Uses
Baking soda	NaHCO₃	NaCl, ammonia, CO₂, water	Antacid, baking (releases CO₂), fire extinguishers
Washing soda	Na₂CO₃·10H₂O	heating then recrystallising baking soda	Cleaning, removing hard-water hardness, glass/soap manufacture
Bleaching powder	CaOCl₂	Cl₂ + dry slaked lime	Bleaching cloth/paper, disinfecting drinking water
Plaster of Paris	CaSO₄·½H₂O	heating gypsum to 373 K	Plaster casts for fractures, statues, smoothing walls

Water of crystallisation — some salts hold fixed water molecules in their crystals. Copper sulphate is CuSO₄·5H₂O (blue); heating drives off the water leaving white anhydrous CuSO₄, and adding water turns it blue again. Gypsum is CaSO₄·2H₂O; heating it to 373 K gives plaster of Paris, CaSO₄·½H₂O, which sets back to gypsum when mixed with water.

6. Closing thought

You started with a sour tongue and a slippery bar of soap. You end with:

a definition (acids give H⁺, bases give OH⁻ in water),
a measurement (the pH scale), and
a family tree of salts that explains your antacid, your baking, your blackboard chalk and the plaster on a broken arm.

The single most useful takeaway is the pH scale — it shows up again in biology (blood pH, soil), in agriculture, and in every chemistry exam you will ever sit. Internalise "below 7 acidic, above 7 basic," and the rest of this chapter hangs neatly off it.

Key formulas & results

Everything you need to memorise, in one card. Screenshot this for revision.

Acid + metal

Acid + Metal → Salt + H₂↑

H₂ gives a 'pop' with a lit splint. e.g. Zn + H₂SO₄ → ZnSO₄ + H₂.

Acid + carbonate

Acid + Carbonate → Salt + H₂O + CO₂↑

CO₂ turns limewater milky.

Neutralisation

Acid + Base → Salt + Water

NaOH + HCl → NaCl + H₂O.

Acidic / basic nature

Acid → H⁺(aq) (as H₃O⁺); Base → OH⁻(aq)

Needs water; defines acid/base behaviour.

pH scale

pH < 7 acidic · pH = 7 neutral · pH > 7 basic

Logarithmic: pH 3 is 10× more acidic than pH 4.

Chlor-alkali process

2NaCl(aq) + 2H₂O →(electricity) 2NaOH + Cl₂ + H₂

Brine electrolysis — source of NaOH, Cl₂, H₂.

Plaster of Paris

CaSO₄·½H₂O ; gypsum CaSO₄·2H₂O →(373 K) CaSO₄·½H₂O

Sets back to gypsum on adding water.

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Common mistakes & fixes

These are the exact errors that cost students marks in board exams. Read them once, save yourself the trouble.

WATCH OUT

✗ Writing the formula of plaster of Paris as CaSO₄·2H₂O

✓ Plaster of Paris is CaSO₄·½H₂O (half a water molecule per formula unit). CaSO₄·2H₂O is gypsum, the raw material.

WATCH OUT

✗ Saying dry HCl gas turns blue litmus red

✓ Acids need water to release H⁺ ions. Dry HCl gas does NOT change dry litmus; it only acts as an acid in aqueous solution.

WATCH OUT

✗ Diluting acid by adding water to concentrated acid

✓ Always add ACID to WATER, slowly with stirring. The reverse releases heat so fast it can splash hot, concentrated acid out.

WATCH OUT

✗ Confusing washing soda and baking soda

✓ Baking soda = NaHCO₃ (used in baking, antacids). Washing soda = Na₂CO₃·10H₂O (used in cleaning, made by heating + recrystallising baking soda).

WATCH OUT

✗ Treating pH as linear

✓ pH is logarithmic. A drop of one pH unit means 10× more acidic, not '1 more'.

WATCH OUT

✗ Thinking the salt of any acid and base is always neutral

✓ Only strong acid + strong base gives a neutral salt (NaCl). Strong acid + weak base → acidic salt; weak acid + strong base → basic salt.

NCERT exercises (with solutions)

Every NCERT exercise from this chapter — what it covers and how many questions to expect.

In-text Q (Section 2.1)

Indicators; reactions of acids and bases with metals and carbonates

Questions

In-text Q (Section 2.2)

Why acids/bases conduct; strong vs weak; dilution; pH

Questions

In-text Q (Section 2.3)

Common salt, chlor-alkali products, and the named salts

Questions

End-of-chapter Ex

End-of-chapter exercise

Mixed: indicators, pH applications, salt preparation and uses, water of crystallisation

Questions

Practice problems

Try each one yourself before tapping "Show solution". Active recall > rereading.

Q1EASY· Indicators

What colour does phenolphthalein turn in (a) an acid and (b) a base?

▶ Show solution

Phenolphthalein is colourless in acid and pink in base.

✦ Answer: (a) colourless, (b) pink.

Q2EASY· Gas test

Which gas is released when zinc reacts with dilute sulphuric acid, and how is it tested?

▶ Show solution

Hydrogen gas is released. Test: a burning splint brought to the mouth of the tube burns the gas with a 'pop' sound.

✦ Answer: H₂; gives a pop sound with a lit splint.

Q3EASY· Formula recall

Give the chemical formula and one use of washing soda.

▶ Show solution

Washing soda is Na₂CO₃·10H₂O.

Use (any one): cleaning, removing permanent hardness of water, or manufacturing glass/soap/paper.

✦ Answer: Na₂CO₃·10H₂O; used for cleaning / softening hard water.

Q4MEDIUM· Neutralisation

Write the equation for the reaction between sodium hydroxide and hydrochloric acid, and name the type of reaction.

▶ Show solution

Step 1 — Acid + base → salt + water.

Step 2 — NaOH + HCl → NaCl + H₂O.

✦ Answer: NaOH + HCl → NaCl + H₂O; neutralisation reaction.

Q5MEDIUM· CO₂ test

What happens when dilute hydrochloric acid is added to sodium carbonate? Write the equation and the confirmatory test for the gas.

▶ Show solution

Step 1 — Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑.

Step 2 — Brisk effervescence of CO₂; on passing it through limewater, the limewater turns milky (CaCO₃ forms).

✦ Answer: CO₂ is released; it turns limewater milky.

Q6MEDIUM· pH application

A farmer finds his field soil is too acidic. What should he add and why?

▶ Show solution

Step 1 — Acidic soil has pH below the ideal range for crops.

Step 2 — Adding a base such as quicklime (CaO), slaked lime [Ca(OH)₂] or chalk neutralises the excess acid and raises the pH.

✦ Answer: Add lime (a base) to neutralise the acidity and raise soil pH.

Q7MEDIUM· Water of crystallisation

Blue copper sulphate crystals turn white on heating and blue again on adding water. Explain.

▶ Show solution

Step 1 — CuSO₄·5H₂O contains 5 water molecules of crystallisation, giving the blue colour.

Step 2 — Heating drives off this water: CuSO₄·5H₂O → CuSO₄ (white) + 5H₂O.

Step 3 — Adding water restores the hydrate, so the blue colour returns.

✦ Answer: The blue colour is due to water of crystallisation; heating removes it (white), adding water restores it (blue).

Q8HARD· Tooth decay

Explain why tooth decay starts when the pH of the mouth falls below 5.5, and how toothpaste helps.

▶ Show solution

Step 1 — Bacteria in the mouth break down leftover sugar into acids, lowering the pH.

Step 2 — Below pH 5.5, this acid dissolves tooth enamel (made of calcium phosphate), starting decay.

Step 3 — Toothpastes are mildly basic; brushing neutralises the excess acid and stops the enamel from dissolving.

✦ Answer: Acid below pH 5.5 dissolves enamel; basic toothpaste neutralises the acid.

Q9HARD· Bleaching powder

How is bleaching powder prepared? Write its formula and two uses.

▶ Show solution

Step 1 — Chlorine is passed over dry slaked lime.

Step 2 — Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O. Formula: CaOCl₂.

Step 3 — Uses (any two): bleaching cotton/linen and wood pulp, disinfecting drinking water, oxidising agent in industry.

✦ Answer: Cl₂ + dry slaked lime → CaOCl₂; used for bleaching and disinfecting water.

Q10HARD· Salts family

Classify the following salts as acidic, basic or neutral and justify: (a) NaCl, (b) Na₂CO₃, (c) NH₄Cl.

▶ Show solution

Step 1 — Identify the parent acid and base of each.

(a) NaCl: from strong acid HCl + strong base NaOH → neutral (pH 7).
(b) Na₂CO₃: from weak acid H₂CO₃ + strong base NaOH → basic (pH > 7).
(c) NH₄Cl: from strong acid HCl + weak base NH₄OH → acidic (pH < 7).

✦ Answer: (a) neutral, (b) basic, (c) acidic.

Q11HARD· Plaster of Paris

(a) Name the raw material and the reaction used to make plaster of Paris. (b) Write its formula. (c) Why must it be stored in a moisture-proof container? (d) Give two uses.

▶ Show solution

Step 1 (a) — Gypsum (CaSO₄·2H₂O) is heated to about 373 K: CaSO₄·2H₂O →(373 K) CaSO₄·½H₂O + 1½H₂O.

Step 2 (b) — Formula: CaSO₄·½H₂O (calcium sulphate hemihydrate).

Step 3 (c) — On contact with moisture it sets into hard gypsum, becoming useless; so it must be kept dry/airtight.

Step 4 (d) — Uses (any two): setting fractured bones (plaster casts), making statues and decorative items, smoothing surfaces before painting.

✦ Answer: heat gypsum → CaSO₄·½H₂O; store dry because it sets with water; used for casts and statues.

5-minute revision

The whole chapter, distilled. Read this the night before the exam.

•Acids turn blue litmus red; bases turn red litmus blue. Phenolphthalein: colourless/pink. Methyl orange: red/yellow.
•Acid + metal → salt + H₂ (pop test). Acid + carbonate → salt + water + CO₂ (limewater milky).
•Acid → H⁺(aq); base → OH⁻(aq). Acids need water; always add acid to water.
•pH < 7 acidic, 7 neutral, > 7 basic; logarithmic scale.
•pH in life: stomach HCl, enamel dissolves below 5.5, soil treatment, bee sting acidic / wasp sting basic.
•Common salt → NaOH, Cl₂, H₂ by chlor-alkali electrolysis of brine.
•Baking soda NaHCO₃; washing soda Na₂CO₃·10H₂O; bleaching powder CaOCl₂; plaster of Paris CaSO₄·½H₂O.
•Water of crystallisation: CuSO₄·5H₂O (blue ⇌ white); gypsum CaSO₄·2H₂O → PoP CaSO₄·½H₂O at 373 K.

Rajasthan (RBSE) marks blueprint

Where the marks come from in this chapter — so you can plan your prep.

Typical chapter weightage: 7–9 marks

Question type	Marks each	Typical count	What it tests
MCQ / Assertion–Reason	1	1–2	Indicator colours, formulae, pH values
Short answer	2	2	Reactions of acids/bases; pH applications
Short answer	3	1	Preparation and uses of a named salt; water of crystallisation
Long / case-based	4–5	0–1	Plaster of Paris / chlor-alkali / pH in daily life

Prep strategy

Tabulate indicator colours in acid vs base and memorise them cold — they are direct MCQ marks
Learn the four named salts as formula + preparation + two uses each
Anchor every pH question to a real example: stomach, teeth, soil, stings, acid rain
Practise writing balanced equations for acid + metal, acid + carbonate, and neutralisation

Where this shows up in the real world

This chapter isn't just an exam topic — it lives in the world around you.

Antacids

Indigestion tablets are mild bases (milk of magnesia, Mg(OH)₂) that neutralise excess stomach HCl — neutralisation in your medicine cabinet.

Baking

Baking soda/powder releases CO₂ on heating, making cakes and bhaturas rise — a controlled acid–base reaction in the kitchen.

Water treatment

Bleaching powder (CaOCl₂) disinfects municipal drinking water and swimming pools by releasing chlorine.

Orthopaedic casts

Plaster of Paris sets hard within minutes of mixing with water — used to immobilise broken bones.

Soil correction

Lime is added to acidic farmland and gypsum to alkaline soil to bring the pH into the crop-friendly range.

First aid for stings

Baking soda (a base) soothes an acidic bee sting; mild acid like vinegar helps with an alkaline wasp sting.

Exam strategy

Battle-tested tips from teachers and toppers for this chapter.

For 'what is observed' questions, state the colour/gas change AND the equation — both carry marks.
When a named salt is asked, always give formula + preparation + at least one use.
In pH questions, name a concrete application (stomach/teeth/soil) for the application mark.
Write the 'pop test' for hydrogen and 'limewater milky' for CO₂ verbatim — they are standard mark phrases.
Double-check formulae of hydrated salts: the number of water molecules is frequently tested.
Mention the safety rule (acid to water) whenever dilution is involved.

Going beyond the textbook

For olympiad aspirants and curious learners — topics that build on this chapter.

The pH formula pH = −log[H⁺] and simple calculations of pH from H⁺ concentration (Class 11 preview).
Brønsted–Lowry definition: acids as proton donors and bases as proton acceptors, beyond the H⁺/OH⁻ picture.
Buffer solutions: how blood maintains a near-constant pH around 7.4 despite acid/base challenges.
Titration and the equivalence point: measuring exactly how much base neutralises a given acid.

Where else this chapter is tested

CBSE board isn't the only one — other exams test this chapter too.

RBSE Class 10 Board (BSER Ajmer)High — named salts and pH applications appear almost every year

NTSE / state scholarshipMedium — indicator and pH MCQs

NEET FoundationHigh — pH, buffers and digestion link to biology

Science Olympiad (NSO)Medium — salt chemistry and reaction prediction

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Questions students ask

The real ones — pulled from the Q&A community and tutor sessions.

Yes. RBSE prescribes the NCERT Science textbook, so the chapter content, activities and exercises are identical. Only the board exam pattern and marking are set by RBSE (BSER, Ajmer).

On average, two formula units of CaSO₄ share one molecule of water — so each unit 'owns' half. Written per formula unit, this is CaSO₄·½H₂O. It is a real, well-defined hemihydrate, not a rounding error.

Dilution is strongly exothermic. Adding water to concentrated acid releases heat so fast that the mixture can boil and splash hot acid out. Adding acid slowly to a large volume of water spreads the heat safely.

'Strong' refers to how completely it ionises (HCl fully ionises → strong). 'Concentrated' refers to how little water is present (high amount of acid per litre). A dilute solution of a strong acid is still a strong acid.

Acidic behaviour comes from free H⁺ ions, which form only in the presence of water. Without water, HCl cannot release H⁺, so dry litmus shows no change.

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