Ask any Class 10 student in Jaipur what confuses them most in Chemistry, and “Acids, Bases and Salts” almost always comes up — not because the concepts are hard, but because the chapter mixes theory (ions, pH), reactions (metal + acid, neutralisation), and daily-life applications (baking soda, bleaching powder, Plaster of Paris) all in one place. Students often learn these as three separate things instead of one connected story — and that’s exactly where marks get lost.
At Convex Classes, Jaipur, we teach this chapter by connecting the dots first — what an ion actually is, why it matters, and only then the reactions and everyday uses — so nothing feels like a random fact to memorise. Here’s the complete breakdown, the way we teach it in our own Class 10 Science batches.
Why This Chapter Is a Guaranteed Scorer (If You Understand the Logic)
- It regularly contributes 8–10 marks across MCQs, short answers, and one long numerical/reasoning question in the CBSE board paper.
- The pH concept and ionic reactions here connect directly to Chapter 3 (Metals and Non-metals) and Class 11 Equilibrium — so this is a foundational chapter, not a standalone one.
- A large chunk of questions are daily-life application based (bleaching powder, baking soda, curd turning sour) — these need reasoning, not formulas, so once you understand the “why,” you can answer any twist the exam throws at you.
Concept Breakdown — Explained the Convex Classes Way
1. What Actually Makes Something an Acid or a Base?
Forget “sour = acid, bitter = base” as your only definition — that’s true, but it’s not what the exam actually tests. The real definition is about ions released in water:
- Acids release H⁺ ions (which immediately combine with water to form H₃O⁺, called hydronium ions) when dissolved in water.
- Bases release OH⁻ ions (hydroxide ions) when dissolved in water.
Why this matters more than you’d think: this single idea explains almost every “why” question in this chapter — why dry HCl gas doesn’t turn litmus paper red (no water = no H⁺ ions formed), why alcohol and glucose aren’t acids despite having hydrogen (their hydrogen doesn’t ionise in water), and why distilled water doesn’t conduct electricity but rainwater does (no free ions vs dissolved ionic impurities).
2. The pH Scale — Read It Like a Number Line, Not a Formula
pH tells you how many H⁺ ions are present in a solution, on a scale of 0–14:
| pH Range | Nature | Example |
|---|---|---|
| 0 – below 7 | Acidic (lower number = stronger acid) | Lemon juice (~2), gastric juice (~1.5) |
| Exactly 7 | Neutral | Pure/distilled water |
| Above 7 – 14 | Basic (higher number = stronger base) | Soap (~9-10), NaOH (~13-14) |
The trap examiners love: pH and H⁺ concentration move in opposite directions — lower pH means more H⁺ ions, not fewer. Students who rush often write this backwards. Always pause and think “lower pH = more acidic = more H⁺” before answering.
3. What Happens When Metals Meet Acids
General reaction: Metal + Acid → Salt + Hydrogen gas
Example: Zn + H₂SO₄ → ZnSO₄ + H₂
Test for hydrogen gas (asked almost every year): bring a burning matchstick near the gas — it burns with a characteristic “pop” sound. This single line is worth marks in at least one question every year, so never skip writing it.
4. Neutralisation — Not Just “Acid + Base = Salt + Water”
A neutralisation reaction is when an acid and a base react to cancel each other’s properties, forming a salt and water, and releasing heat (it’s exothermic — students often forget to mention this).
NaOH + HCl → NaCl + H₂O
Real-life application that examiners love testing: antacids (like milk of magnesia, Mg(OH)₂) neutralise excess stomach acid during indigestion — this is neutralisation happening inside your own body.
5. The Everyday Chemicals — Learn Them as a Connected Family, Not Isolated Facts
| Common Name | Chemical Name | Formula | Made From | Main Use |
|---|---|---|---|---|
| Baking soda | Sodium hydrogen carbonate | NaHCO₃ | — | Antacid, baking powder ingredient |
| Washing soda | Sodium carbonate (recrystallised) | Na₂CO₃·10H₂O | Heating baking soda further, then recrystallising | Softening hard water, cleaning agent |
| Bleaching powder | Calcium oxychloride | CaOCl₂ | Slaked lime + Chlorine gas | Bleaching, disinfecting water |
| Plaster of Paris | Calcium sulphate hemihydrate | CaSO₄·½H₂O | Heating gypsum | Setting fractures, moulds |
| Quicklime / Slaked lime | Calcium oxide / hydroxide | CaO / Ca(OH)₂ | — | Treating acidic soil, whitewashing |
Convex Classes’ trick to remember Plaster of Paris: it “wants” its water back — it was made by removing water from gypsum (CaSO₄·2H₂O) using heat, so naturally it reacts eagerly with water again, turning hard within minutes. That’s exactly why it must be stored in air-tight, moisture-proof containers.
Fully Worked NCERT Solutions
Q1. Identifying acid, base, and water using only red litmus paper
Dip red litmus paper in each: it stays red (slightly darker) in the acid, turns blue in the base, and barely changes (may look slightly lighter/washed out) in plain water — so the base is the only one that flips its colour, while acid keeps it red and water leaves it essentially unchanged.
Q2. Why shouldn’t curd/sour food be kept in brass or copper vessels?
Curd contains lactic acid, and sour foods contain other organic acids. These react with the copper and zinc in brass/copper vessels to form soluble metal salts, which are toxic if consumed — this is a food-safety chemistry question, not just a “chemical reaction” question.
Q3. Which gas is released when acid reacts with metal, and how do you test it?
Hydrogen gas. Example: Zn + 2HCl → ZnCl₂ + H₂. Test: bring a burning matchstick near the gas — it burns with a pop sound.
Q4. Compound ‘A’ reacts with dilute HCl, gas extinguishes a candle, one product is calcium chloride — identify A and write the equation
The gas that extinguishes a flame is CO₂, and CO₂ from an acid-carbonate reaction means ‘A’ is a carbonate of calcium — i.e., calcium carbonate (CaCO₃). CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
Q5. Why do HCl and HNO₃ show acidic behaviour in water but alcohol/glucose don’t?
HCl and HNO₃ dissociate in water to release H⁺ ions, which combine with water to form H₃O⁺ (hydronium ions) — this is what causes acidic behaviour. Alcohol and glucose also contain hydrogen, but their hydrogen atoms don’t ionise in water, so no H⁺ is ever released.
Q6. Why does an aqueous acid solution conduct electricity?
Because acids dissociate into ions in water, and moving ions are what carry electric current through a solution.
Q7. Why doesn’t dry HCl gas change dry litmus paper’s colour?
Because ionisation (release of H⁺) only happens in the presence of water. Without water, there are no free H⁺ ions to react with the litmus paper.
Q8. Why should acid be added to water, and not water to acid, while diluting?
Dissolving acid in water is a highly exothermic process. Adding acid slowly into a large volume of water lets the heat spread out safely. Doing it the reverse way can cause the mixture to splash violently due to the sudden localised heat, which can cause burns.
Q9. How does diluting an acid affect H₃O⁺ concentration?
Dilution decreases the concentration of H₃O⁺ ions per unit volume, which means the acid becomes weaker (less acidic) than before.
Q10. Effect of adding excess base to a NaOH solution on OH⁻ concentration?
The concentration of OH⁻ ions increases further, making the solution more strongly basic.
Q11. Solution A has pH 6, Solution B has pH 8 — which is acidic, which has more H⁺?
Solution A (pH 6) is acidic and has a higher H⁺ ion concentration. Solution B (pH 8) is basic. Remember: lower pH always means more H⁺ ions.
Q12. Effect of H⁺ concentration on the nature of a solution
More H⁺ ions → more acidic solution. Fewer H⁺ ions → more basic (alkaline) solution.
Q13. Do basic solutions also contain H⁺ ions?
Yes — every aqueous solution contains both H⁺ and OH⁻ ions. A solution is basic when the OH⁻ concentration is much higher than the H⁺ concentration, not because H⁺ is completely absent.
Q14. When would a farmer treat soil with quicklime, slaked lime, or chalk?
When the soil is too acidic for healthy crop growth. These substances are basic in nature and neutralise the excess acidity, bringing the soil back to a suitable pH range for cultivation.
Q15. Common name of CaOCl₂?
Bleaching powder.
Q16. Which substance gives bleaching powder on reacting with chlorine?
Slaked lime, Ca(OH)₂. Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O
Q17. Sodium compound used to soften hard water?
Sodium carbonate (washing soda), Na₂CO₃·10H₂O.
Q18. What happens when sodium hydrogen carbonate solution is heated?
It decomposes to form sodium carbonate, water, and carbon dioxide gas: 2NaHCO₃ →(heat) Na₂CO₃ + H₂O + CO₂
Q19. Equation for Plaster of Paris + water
CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O (gypsum, sets into a hard mass)
Q20. A solution turns red litmus blue — likely pH?
(d) 10 — since only a basic solution turns red litmus blue, and among the given options only 10 is above 7.
Q21. A solution reacts with crushed egg-shells to release a gas that turns lime water milky — the solution contains?
(b) HCl — egg-shells contain calcium carbonate, and CaCO₃ + acid releases CO₂, which turns lime water milky. Among the given options, HCl is the acid.
Q23. 10 mL NaOH needs 8 mL HCl to neutralise; how much HCl needed for 20 mL NaOH (same solutions)? (d) 16 mL —
since the ratio must stay constant (double the base needs double the acid): 8 mL × 2 = 16 mL.
Q24. Medicine used for treating indigestion?
(c) Antacid — it neutralises excess stomach acid.
Q25. Word and balanced equations for acid-metal reactions
a) Zinc + Sulphuric acid → Zinc sulphate + Hydrogen Zn + H₂SO₄ → ZnSO₄ + H₂
b) Magnesium + Hydrochloric acid → Magnesium chloride + Hydrogen Mg + 2HCl → MgCl₂ + H₂
c) Aluminium + Sulphuric acid → Aluminium sulphate + Hydrogen 2Al + 3H₂SO₄ → Al₂(SO₄)₃ + 3H₂
d) Iron + Hydrochloric acid → Iron(II) chloride + Hydrogen Fe + 2HCl → FeCl₂ + H₂
Q26. Activity to prove alcohol/glucose are not acids
Set up a circuit with a battery, bulb, two electrodes (nails), and a switch, dipped in the test liquid inside a beaker.
- With ethyl alcohol: the bulb does not glow.
- With glucose solution: the bulb does not glow either.
Since no current flows, no ions are present — proving neither substance ionises in water, and hence neither behaves as an acid despite containing hydrogen.
Q27. Why doesn’t distilled water conduct electricity while rainwater does?
Distilled water is pure H₂O with no dissolved ionic substances, so there’s nothing to carry charge. Rainwater picks up dissolved gases and mineral traces (including mild acids) as it falls through the atmosphere, giving it free ions that conduct electricity.
Q28. Why do acids not show acidic behaviour without water?
Because the release of H⁺ ions (ionisation) only occurs when the acid is dissolved in water. Without water, there’s no ionisation, and hence no acidic behaviour.
Q29. Five solutions with pH 4, 1, 11, 7, 9 — identify each
Given: A=4, B=1, C=11, D=7, E=9
(a) Neutral → D (pH 7) (b) Strongly alkaline → C (pH 11) (c) Strongly acidic → B (pH 1) (d) Weakly acidic → A (pH 4) (e) Weakly alkaline → E (pH 9)
Increasing order of H⁺ concentration (H⁺ increases as pH decreases): C < E < D < A < B
Q30. Mg ribbon + HCl (tube A) vs Mg ribbon + CH₃COOH (tube B) — which fizzes more?
HCl is a strong acid (fully ionises), while acetic acid is a weak acid (partially ionises). So tube A (HCl) has a higher H⁺ concentration and reacts faster, producing more vigorous fizzing than tube B.
Q31. Fresh milk has pH 6 — how does pH change as it turns to curd?
The pH decreases further (becomes more acidic) because bacteria convert milk sugars into lactic acid during curdling, adding more H⁺ ions to the solution.
Q32. Milkman adds baking soda to fresh milk
a) Baking soda shifts milk’s pH from 6 (slightly acidic) to slightly alkaline, because bacteria that curdle milk thrive better in acidic conditions — making it slightly alkaline slows down curdling and keeps the milk fresh longer. b) Since the milk starts out slightly basic, the acid produced by bacteria first has to neutralise this extra alkalinity before the milk can turn sufficiently acidic to set as curd — hence it takes longer to set.
Q33. Why should Plaster of Paris be stored in a moisture-proof container?
Because it reacts readily with atmospheric moisture, converting back into gypsum and hardening into a solid, unusable mass right inside the container if exposed to humid air.
Q34. What is a neutralisation reaction? Give two examples.
A reaction between an acid and a base that produces a salt and water, releasing heat (exothermic). i) NaOH + HCl → NaCl + H₂O ii) Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O (this is exactly how antacids relieve indigestion)
Q35. Two uses each of washing soda and baking soda
Washing soda: (i) used as a cleansing/washing agent, (ii) used to remove permanent hardness of water. Baking soda: (i) used as an ingredient in baking powder, (ii) used as a mild antacid for relieving indigestion.
Common Mistakes Convex Classes Students Make (and How to Fix Them)
- Mixing up pH direction — “lower pH = more acidic = more H⁺” needs to become automatic; write it as a small note on your answer sheet if it helps during practice tests.
- Forgetting the water condition for acidic behaviour — nearly every “why doesn’t X show acidic properties” question comes back to “no water, no ionisation.” Always mention this explicitly, not just “because it’s not an acid.”
- Not naming the gas-testing method — writing “hydrogen gas is formed” without describing the “burning matchstick + pop sound” test loses easy marks.
- Confusing washing soda and baking soda’s formulas — remember baking soda is the simpler one (NaHCO₃), and washing soda is the “upgraded,” recrystallised, water-containing version (Na₂CO₃·10H₂O) made from baking soda.
- Forgetting exothermic nature of neutralisation — many students write only “salt + water is formed” and skip that heat is released, which examiners specifically look for.
Extra Practice Questions (Beyond NCERT) — For Board-Level Practice
- Why is it dangerous to breathe in chlorine gas near an area where bleaching powder is being manufactured, and how is bleaching powder itself useful despite this?
- A student tests four unknown solutions and records pH values of 2, 7, 9, and 13. Arrange them in decreasing order of OH⁻ ion concentration and justify.
- Explain why tooth decay starts when the pH inside the mouth falls below 5.5, and suggest one chemical way (using a basic substance) to counter it.
- Two test tubes contain dilute HCl and dilute acetic acid of the same molar concentration. Equal-sized zinc granules are added to both. Predict and explain which will finish reacting first.
- Write the reaction of Plaster of Paris setting into gypsum, and explain in your own words why a doctor plastering a broken arm needs to work quickly once water is mixed in.
(Convex Classes students: these reasoning-based HOTS questions are solved step by step on the board in our regular batches — ask your subject teacher or message us on WhatsApp for the full solved set.)
Convex Classes’ Quick Revision Checklist Before Your Test
- Can you explain acids and bases in terms of ions released in water, not just taste/litmus colour?
- Do you know the pH-to-H⁺ relationship well enough to never get the direction backwards?
- Can you write the metal + acid reaction and describe the hydrogen gas test without missing the “pop sound” line?
- Do you know the formula, source, and use of all five common chemicals (baking soda, washing soda, bleaching powder, Plaster of Paris, quicklime/slaked lime)?
- Can you explain at least 3 daily-life applications (milk turning to curd, antacids, egg-shell + acid) using the concepts, not just recall them as facts?
Need Personal Guidance on This Chapter?
Understanding the “why” behind acids, bases and salts is what separates a good score from an average one — and that’s exactly the kind of teaching we focus on at Convex Classes, Jaipur in our Class 10 Science batches, backed by regular chapter tests and doubt-clearing sessions.
For a free demo class, chapter-wise test series, or personal doubt support in Class 10 Science, message us directly on WhatsApp: 8290601516.
FAQs
Q1. Is Acids, Bases and Salts a difficult chapter for Class 10 board exams?
Not if you learn it as one connected concept (ions in water) rather than separate facts — most questions are reasoning-based and repeat the same core logic every year with different examples.
Q2. What is the easiest way to remember the pH scale direction?
Lower number = more acidic = more H⁺ ions. Higher number = more basic = more OH⁻ ions. 7 is exactly neutral.
Q3. What is the difference between baking soda and washing soda?
Baking soda is sodium hydrogen carbonate (NaHCO₃), while washing soda is sodium carbonate with water molecules attached (Na₂CO₃·10H₂O), made by further processing baking soda.
Q4. Where can Jaipur students get in-person doubt support for this chapter?
Convex Classes, Jaipur offers dedicated Class 10 Science batches with chapter tests and doubt sessions — WhatsApp 8290601516 for details.
