Class 10 Science Acids Bases Salts Complete Guide 2027 — pH Scale, Reactions, Indicators + Tricks

In short: Acids, Bases and Salts (Chapter 2) carries 7-10 marks in CBSE Class 10 Science 2027 and is the most practical-exam-linked Chemistry chapter. Master the five types of reactions, memorise the pH scale (0-14) and indicator colour chart, know the preparation and uses of four important salts (NaOH, NaHCO₃, Na₂CO₃, Ca(OCl)Cl), and avoid the six mistakes below. This chapter rewards memorisation as much as understanding.

Acids, Bases and Salts is Chapter 2 in CBSE Class 10 Science and it is one of the heaviest Chemistry chapters on the board paper. It carries 7-10 marks depending on the year, and it connects directly to your practical exam — experiments with litmus paper, pH indicators, and neutralisation reactions all come from this chapter.

The chapter covers a lot of ground: properties of acids and bases, five types of reactions, the pH scale, everyday chemicals (baking soda, washing soda, Plaster of Paris, bleaching powder), and their preparation processes. It can feel overwhelming, but the question patterns are very predictable once you know what the board asks. For the full marks breakdown, see the CBSE Class 10 Science chapter-wise weightage.

If you have already gone through Chemical Reactions and Equations, many reaction patterns here will look familiar.

What You Need to Know Before Starting

This chapter builds on:

• Chemical equations and balancing — you will be writing balanced equations for acid-base reactions
• Metals and their reactivity — acids react differently with different metals
• Basic understanding of ions — H⁺ ions make acids acidic, OH⁻ ions make bases basic
• Water as a solvent — acids and bases show their properties only in aqueous (water) solution

Acids and Bases — The Core Chemistry

What Makes Something an Acid or a Base?

Acid: A substance that produces H⁺ (hydrogen) ions when dissolved in water.

• HCl → H⁺ + Cl⁻
• H₂SO₄ → 2H⁺ + SO₄²⁻

Base: A substance that produces OH⁻ (hydroxide) ions when dissolved in water.

• NaOH → Na⁺ + OH⁻
• Ca(OH)₂ → Ca²⁺ + 2OH⁻

Key point the board loves to test: Dry HCl gas does NOT turn blue litmus red. Only when dissolved in water (forming H⁺ ions) does it behave as an acid. This shows that acids need water to show acidic behaviour.

Indicators — The Colour Chart You Must Memorise

Memory trick for litmus: "Acid turns Blue to Red — ABR." The opposite does the base: "Base turns Red to Blue — BRB (be right back)."

Memory trick for phenolphthalein: "Phenolphthalein is shy — it shows colour (pink) only in bases. In acids and neutral solutions, it stays colourless."

The Five Types of Reactions

1. Acid + Metal → Salt + Hydrogen gas

2HCl + Zn → ZnCl₂ + H₂↑

The hydrogen gas can be tested with a burning splinter — it makes a "pop" sound. This reaction is why metals dissolve in acids.

Board exam note: Copper (Cu) does NOT react with dilute HCl or H₂SO₄ because it is below hydrogen in the reactivity series. This is a common trick question.

2. Acid + Metal Carbonate → Salt + Water + CO₂

2HCl + Na₂CO₃ → 2NaCl + H₂O + CO₂↑

The CO₂ turns lime water milky — this is the standard test for CO₂.

3. Acid + Metal Hydrogen Carbonate → Salt + Water + CO₂

HCl + NaHCO₃ → NaCl + H₂O + CO₂↑

Same products as above. This is why baking soda (NaHCO₃) fizzes when you add lemon juice (citric acid).

4. Acid + Base → Salt + Water (Neutralisation)

HCl + NaOH → NaCl + H₂O

This is the most important reaction type. Neutralisation produces a salt and water. The heat produced is called the "heat of neutralisation."

5. Acid + Metal Oxide → Salt + Water

2HCl + CuO → CuCl₂ + H₂O

Metal oxides are basic in nature — they react with acids like bases do.

Memory trick for all five reactions: "M, MC, MHC, B, MO" — Metal, Metal Carbonate, Metal Hydrogen Carbonate, Base, Metal Oxide. All react with acids. The product is always a SALT plus something else (H₂, H₂O + CO₂, or just H₂O).

The pH Scale

The pH scale measures how acidic or basic a solution is. It runs from 0 to 14.

• pH 0-6: Acidic (lower = stronger acid). pH 0 is the strongest acid.
• pH 7: Neutral (pure water)
• pH 8-14: Basic/alkaline (higher = stronger base). pH 14 is the strongest base.

Memory trick: "pH = Power of Hydrogen." Lower pH = more H⁺ ions = more acidic.

pH Values the Board Loves to Test

Why pH Matters — Real-Life Applications

• Our stomach uses HCl (pH ~2) for digestion. Antacids (Milk of Magnesia) neutralise excess acid.
• Our blood pH is tightly maintained at 7.35-7.45. Even small changes are dangerous.
• Tooth decay happens when mouth pH drops below 5.5 (bacteria produce acids from food). Toothpaste is basic to neutralise this.
• Acid rain has pH below 5.6 and damages buildings, plants, and aquatic life.
• Soil pH affects crop growth. Farmers add quick lime (CaO) to acidic soil and organic matter to basic soil.

Important Salts — Preparation and Uses

Four salts appear in board exams repeatedly. Know their chemical formula, preparation method, and uses.

1. Common Salt — NaCl (Sodium Chloride)

• Source: sea water, rock salt deposits
• Uses: cooking, preserving food, raw material for NaOH, Na₂CO₃, NaHCO₃

2. Baking Soda — NaHCO₃ (Sodium Hydrogen Carbonate)

• Preparation: NaCl + H₂O + CO₂ + NH₃ → NaHCO₃ + NH₄Cl
• Uses: baking (releases CO₂ when heated), antacid, fire extinguisher (soda-acid type)
• Heating: 2NaHCO₃ →(heat)→ Na₂CO₃ + H₂O + CO₂

3. Washing Soda — Na₂CO₃·10H₂O (Sodium Carbonate Decahydrate)

• Preparation: heat baking soda to get Na₂CO₃, then recrystallise with water
• Uses: cleaning agent, softening hard water, manufacturing glass and paper
• Water of crystallisation: the 10H₂O is the water of crystallisation — this gives washing soda its crystal form

4. Bleaching Powder — Ca(OCl)Cl (Calcium Oxychloride)

• Preparation: Ca(OH)₂ + Cl₂ → Ca(OCl)Cl + H₂O (dry slaked lime + chlorine gas)
• Uses: bleaching textiles, disinfecting water, oxidising agent

5. Plaster of Paris — CaSO₄·½H₂O (Calcium Sulphate Hemihydrate)

• Preparation: heat gypsum (CaSO₄·2H₂O) at 373 K
• CaSO₄·2H₂O →(373K)→ CaSO₄·½H₂O + 1½H₂O
• Uses: plastering fractured bones, making moulds, decorative items
• Key property: when mixed with water, it sets into a hard mass (gypsum) — this is exothermic

Common Mistakes That Cost Marks

1. Writing wrong chemical formulas. NaHCO₃ is baking soda, Na₂CO₃ is washing soda. Students swap these constantly. Remember: "Baking = Bi (bi-carbonate, HCO₃⁻), Washing = just CO₃²⁻."

2. Forgetting the water of crystallisation. Washing soda is Na₂CO₃·10H₂O, not just Na₂CO₃. Plaster of Paris is CaSO₄·½H₂O, not CaSO₄. The "·nH₂O" part is essential for full marks.

3. Confusing "strong" with "concentrated." A strong acid (HCl, H₂SO₄) dissociates completely in water. A concentrated acid just means more acid dissolved per litre. You can have a dilute solution of a strong acid. Board exams test this distinction.

4. Getting the pH direction wrong. Lower pH = more acidic, NOT more basic. Students under pressure sometimes reverse this. Remember: "pH goes down, acidity goes up."

5. Omitting the state symbols in equations. Board examiners expect (aq), (s), (l), (g) in equations: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l). Missing state symbols can cost half a mark per equation.

6. Not knowing which metals DON'T react with dilute acids. Copper, silver, and gold do not react with dilute HCl or dilute H₂SO₄. They are below hydrogen in the reactivity series. This appears in MCQs almost every year.

4-Week Practice Plan

Week 1: Acids, Bases and Indicators

• Read NCERT Sections 2.1 and 2.2
• Memorise the indicator colour chart (litmus, phenolphthalein, methyl orange, turmeric)
• Write all five reaction types with one example each — daily for 3 days
• Solve all NCERT in-text questions for Sections 2.1-2.2

Week 2: pH Scale and Neutralisation

• Read NCERT Section 2.3 (pH and importance)
• Memorise the pH table (stomach acid to NaOH)
• Practice 10 questions on pH-related real-life applications
• Solve all NCERT in-text questions and Exercise questions up to Q.15
• Link this to your practical exam prep — see the Science practical exam guide

Week 3: Important Salts and Their Preparation

• Read NCERT Section 2.4 and 2.5
• Write the preparation equations for all 5 salts from memory
• Make a comparison table: salt name, formula, preparation, uses
• Solve remaining NCERT Exercise questions
• Attempt CBSE Class 10 Science important questions for this chapter

Week 4: Board Paper Practice

• Solve this chapter's questions from the last 5 years of CBSE board papers. See how to use previous year papers.
• Focus on 3-mark and 5-mark questions — these carry the most weight
• Time yourself: 3-mark answers should take 5-6 minutes
• Solve the CBSE 2027 sample paper Science questions
• Final revision: write all 5 salt preparations from memory in under 10 minutes

Quick Revision Card for Exam Day

• Acids produce H⁺, Bases produce OH⁻ — only in aqueous solution
• pH 0-6 = acidic, 7 = neutral, 8-14 = basic
• Litmus: ABR (Acid turns Blue to Red), BRB (Base turns Red to Blue)
• Phenolphthalein: pink only in base, colourless everywhere else
• 5 reaction types: Acid + Metal / Metal Carbonate / Metal HCO₃ / Base / Metal Oxide
• Neutralisation: Acid + Base → Salt + Water
• Baking soda = NaHCO₃ (bi = HCO₃), Washing soda = Na₂CO₃·10H₂O
• Bleaching powder = Ca(OCl)Cl — from slaked lime + chlorine
• Plaster of Paris = CaSO₄·½H₂O — from heating gypsum at 373 K
• Cu, Ag, Au do NOT react with dilute acids
• State symbols (aq, s, l, g) are mandatory in equations

For step-by-step practice with reaction equations, pH problems, and salt preparation questions, try Padhaao's AI tutor — it covers every CBSE Class 10 Science chapter with instant feedback on your answers.

Frequently Asked Questions

How many marks does Acids, Bases and Salts carry in CBSE Class 10 Science 2027?

Acids, Bases and Salts carries 7-10 marks in the CBSE Class 10 Science board exam 2027. It is part of the Chemical Substances unit, which is one of the highest-weighted Chemistry sections. You can expect one 1-mark MCQ, one 2-3 mark short answer, and often a 5-mark long answer from this chapter. It also connects to practical exam questions on indicators and pH.

What is the pH scale and why is it important for board exams?

The pH scale runs from 0 to 14 and measures how acidic or basic a solution is. pH below 7 is acidic, pH 7 is neutral, and pH above 7 is basic. Board exams test pH through real-life applications: stomach acid (pH 1-2), tooth decay when mouth pH drops below 5.5, acid rain having pH below 5.6, and blood pH being maintained at 7.35-7.45. Memorise the pH values of common substances for MCQ and assertion-reason questions.

What is the difference between a strong acid and a concentrated acid?

A strong acid is one that dissociates completely into ions in water (like HCl, H₂SO₄, HNO₃). A concentrated acid simply means a large amount of acid is dissolved per unit volume of solution. You can have a dilute solution of a strong acid (small amount of HCl in lots of water — still fully dissociates) or a concentrated solution of a weak acid (lots of acetic acid in little water — still partially dissociates). This distinction appears frequently in board exam questions.

How do I remember the preparation of all important salts?

Group them by raw material. Common salt (NaCl) from sea water is the starting point for the Solvay process chain: NaCl → NaHCO₃ (baking soda) → Na₂CO₃ (washing soda). Bleaching powder comes from slaked lime (Ca(OH)₂) plus chlorine gas. Plaster of Paris comes from heating gypsum (CaSO₄·2H₂O) at 373 K. Learn the chain: salt → baking soda → washing soda, and separately: slaked lime → bleaching powder, gypsum → Plaster of Paris.

Which reactions of acids are most important for board exams?

The five reaction types (acid with metal, metal carbonate, metal hydrogen carbonate, base, and metal oxide) are all tested. Neutralisation (acid + base → salt + water) is the most frequently asked. The reaction of acids with metal carbonates (producing CO₂ that turns lime water milky) connects to practical exam questions. Know at least two balanced equations for each reaction type, including state symbols.

Why does dry HCl gas not change the colour of litmus paper?

Dry HCl gas does not contain H⁺ ions — it exists as HCl molecules. Acids show their properties only when dissolved in water because water helps dissociate the acid into H⁺ and Cl⁻ ions. Without water, there are no free H⁺ ions to turn blue litmus red. This experiment demonstrates that hydrogen ions (not HCl molecules) are responsible for acidic behaviour. This is a very common 2-3 mark question in board exams.

Acids, Bases and Salts is a chapter where memorisation and equation-writing practice pay off directly in marks. Know your reactions, remember your pH values, and write complete balanced equations with state symbols — those 7-10 marks are well within reach.