Where Mindful Learning Begins
Where Mindful Learning Begins
Class 10 Science NCERT Solutions Chapter 2 (CBSE 2026-27) — every in-text question, every embedded Activity question, and all 15 back-of-chapter Exercises on this one page, solved with the reasoning behind each answer.
Acids, Bases and Salts moves from acid-base indicators (litmus, olfactory, synthetic) through how acids and bases react with metals, metal carbonates, and each other, to what acids and bases actually have in common in water, the pH scale and its everyday importance, and finally the whole family of chemicals made from common salt — sodium hydroxide, bleaching powder, baking soda, washing soda, and Plaster of Paris. These NCERT solutions work through every Activity-based question and the full 15-question Exercise set in detail, making this one of the richest chapters in the chemistry portion of the Class 10 Science syllabus.
Acids, Bases and Salts builds directly on the equation-writing and reaction-classifying skills from Chapter 1, and applies them to one specific, richly practical family of substances. It moves from how we detect acids and bases using indicators, to how they react with metals, metal carbonates and each other, to what makes something acidic or basic at the level of ions in water, to the pH scale that lets us measure this precisely, and finally to the whole chain of everyday chemicals — washing soda, baking soda, bleaching powder, Plaster of Paris — that all trace back to one raw material: common salt. Every question woven through the chapter is solved here in the order the textbook presents it: the chapter-opening question, the four "QUESTIONS" boxes after Sections 2.1 through 2.4, the questions embedded directly inside Activities and "Do You Know" boxes, and the full 15-question Exercise set at the end.
Litmus, phenolphthalein, methyl orange and olfactory indicators; how acids and bases react with metals, carbonates, and each other (neutralisation).
Why acids and bases only show their character in water, hydronium and hydroxide ions, and the 0–14 pH scale with its everyday importance.
Families of salts, pH of salts, water of crystallisation, and the chlor-alkali process that turns common salt into NaOH, bleaching powder, baking soda and washing soda.
Dip a strip of red litmus paper into each of the three test tubes:
This works because red litmus doesn't change in either an acid or in neutral water — it only turns blue in a base. But once turned blue by the base, that same strip becomes sensitive to acid (turns red) while staying unchanged in neutral water, letting you tell all three apart with a single piece of litmus paper.
Curd and other sour substances contain acids (such as lactic acid). When stored in brass or copper vessels, this acid reacts with the metal to form toxic metal salts (copper/brass salts), which can contaminate the food and cause food poisoning if consumed. This reaction also corrodes and damages the vessel. Hence sour substances should not be kept in brass or copper vessels.
Hydrogen gas is usually liberated. Example:
Test: bring a burning candle or matchstick near the mouth of the test tube (or near a soap bubble filled with the gas). Hydrogen gas burns with a characteristic "pop" sound, confirming its presence.
Since one product is calcium chloride and the gas evolved extinguishes a flame (typical of CO2), compound A must be calcium carbonate (CaCO3).
HCl, HNO3, and similar acids ionise in water to release H+ (H3O+) ions, which are what produce acidic character. Alcohol and glucose also contain hydrogen atoms, but these are covalently bonded within the molecule and do not ionise or dissociate in water to release H+ ions. Since no free H+ ions are produced, alcohol and glucose solutions do not show acidic behaviour, despite containing hydrogen.
When an acid dissolves in water, it dissociates into free-moving ions (H+ and a corresponding negative ion, e.g., Cl– from HCl). These free ions can carry electric charge through the solution, allowing the acidic solution to conduct electricity.
HCl shows acidic behaviour only by releasing H+ ions, and this ionisation happens only in the presence of water. Dry HCl gas has no water present to ionise in, so it does not produce H+ ions and therefore does not show acidic character or change the colour of dry litmus paper.
Dissolving an acid in water is a highly exothermic process. If water is added to a concentrated acid, the large amount of heat generated can cause the mixture to splash out suddenly, potentially causing burns, and the glass container may crack due to excessive local heating. Adding the acid slowly to water (with constant stirring) instead lets the heat spread safely through the larger volume of water, avoiding dangerous splashing or overheating.
When an acid solution is diluted, the concentration of H3O+ ions decreases — the same number of ions gets spread through a larger volume of water, so the concentration per unit volume goes down.
When more base is dissolved into a solution of sodium hydroxide, the concentration of OH– ions increases, since additional base releases more hydroxide ions into the same solution.
A lower pH means a higher H+ ion concentration. Since solution A has the lower pH (6 < 8), solution A has more hydrogen ion concentration.
Solution A (pH 6, less than 7) is acidic; solution B (pH 8, greater than 7) is basic.
The higher the concentration of H+(aq) ions, the more acidic the solution (and the lower its pH). The lower the concentration of H+(aq) ions, the less acidic (or more basic) the solution.
Yes, basic solutions also contain a small amount of H+(aq) ions. However, they contain a much higher concentration of OH–(aq) ions compared to H+ ions. Since OH– ions far outnumber the H+ ions, the overall solution behaves as basic.
Quick lime (CaO), slaked lime (Ca(OH)2) and chalk (CaCO3) are all basic in nature. A farmer would use them when the soil is too acidic for healthy crop growth — adding these substances neutralises the excess acidity in the soil and raises its pH into a range suitable for plant growth.
Bleaching powder.
Dry slaked lime, Ca(OH)2 (calcium hydroxide).
Washing soda — sodium carbonate, Na2CO3·10H2O.
On heating, sodium hydrogencarbonate decomposes to form sodium carbonate, releasing water and carbon dioxide.
(Plaster of Paris + Water → Gypsum, a hard solid mass)
All three — onion, vanilla, and clove — can be used as olfactory indicators. Each shows a change (masking or disappearance) of its characteristic odour when treated with either an acid or a base, but not the other, so a change (or persistence) of smell reveals whether the medium is acidic or basic without needing to see a colour change.
Reaction of zinc with the different acids used in Activity 2.3:
(Dilute HNO3 is a strong oxidising agent, so with most metals it usually does not release hydrogen gas in the simple way shown above — this is a special case beyond what's needed to answer the Activity.)
The reaction with sodium hydroxide (Activity 2.4) is already given in the text: 2NaOH(aq) + Zn(s) → Na2ZnO2(s) + H2(g).
The bulb glows again, showing that alkaline (basic) solutions also conduct electricity. This is because bases also dissociate in water to produce free ions (e.g., Na+ and OH– for NaOH), and it is these ions that carry the electric current — just as H+ and the corresponding anion do in an acidic solution.
Since an alkali is a base that dissolves in water, NaOH, KOH, Ca(OH)2, and NH4OH — all of which are soluble in water — are alkalis. Mg(OH)2 is a base but is only very slightly soluble in water, so it is not usually classified as an alkali.
(i) Dry HCl gas does not show acidic character (dry litmus paper shows no colour change), because in the absence of water it cannot ionise to release H+ ions.
(ii) HCl solution (HCl dissolved in water) does show acidic character (moist/wet litmus paper turns red), because in the presence of water, HCl ionises to release H+ (as H3O+) ions, which are responsible for acidic behaviour.
A strong base ionises almost completely in water to produce a large number of OH– ions (e.g., NaOH, KOH). A weak base ionises only partially in water, producing fewer OH– ions for the same concentration (e.g., NH4OH, Mg(OH)2).
No, life as we know it cannot exist on Venus. Its atmosphere consists of thick, corrosive clouds of sulphuric acid, combined with extremely high surface temperature and atmospheric pressure — conditions far too harsh and toxic to support life.
Different plants grow best within a specific, narrow range of soil pH. By testing soil samples from places where particular plants are already growing well, you can conclude which pH range suits which plant — in general, most common crop plants prefer soil that is close to neutral (roughly pH 6 to 7.5), while some plants in a given region may specifically prefer slightly acidic or slightly alkaline soil, depending on the species.
Since the nettle sting is acidic (methanoic/formic acid) and rubbing the dock leaf relieves the pain, the dock plant's sap must be basic in nature — it neutralises the acid injected by the nettle.
Another common remedy for such acidic stings/burns is applying a paste of baking soda (sodium hydrogencarbonate), which is also mildly basic and neutralises the acid in the same way.
Three families can be identified, based on the common radical shared by the salts (a salt can belong to more than one family):
| Family | Common radical | Salts in this family |
|---|---|---|
| Sulphate family | SO42– | K2SO4, Na2SO4, CaSO4, MgSO4, CuSO4 |
| Sodium family | Na+ | Na2SO4, NaCl, NaNO3, Na2CO3 |
| Chloride family | Cl– | NaCl, NH4Cl |
"10H2O" is the water of crystallisation — a fixed number (here, 10) of water molecules chemically bound within one formula unit of the salt. Despite containing this water, the crystal does not feel wet to the touch, because this water is not loose/free water but is locked into the fixed structure of the crystal lattice.
Calcium sulphate hemihydrate is called 'Plaster of Paris' because it was originally manufactured by heating gypsum found in large deposits near Paris, France, and it became widely used there for making moulds, decorative material, and plaster casts — the name stuck as a reference to where the process was popularised.
Answer: (d) 10
Turning red litmus blue indicates a basic solution, and basic solutions have pH greater than 7. Among the options, only 10 is greater than 7.
Answer: (b) HCl
Egg-shells contain calcium carbonate. An acid (HCl) reacting with it releases CO2 gas, which turns lime water milky. NaCl, LiCl and KCl are neutral salts and would not react with the carbonate to release gas.
Answer: (d) 16 mL
Since 10 mL NaOH needs 8 mL HCl to neutralise it, doubling the NaOH volume to 20 mL (same concentration) requires double the acid: 8 mL × 2 = 16 mL.
Answer: (c) Antacid
Antacids are mild bases that neutralise the excess acid in the stomach responsible for indigestion.
(a) dilute sulphuric acid + zinc granules
Zinc + Sulphuric acid → Zinc sulphate + Hydrogen
(b) dilute hydrochloric acid + magnesium ribbon
Magnesium + Hydrochloric acid → Magnesium chloride + Hydrogen
(c) dilute sulphuric acid + aluminium powder
Aluminium + Sulphuric acid → Aluminium sulphate + Hydrogen
(d) dilute hydrochloric acid + iron filings
Iron + Hydrochloric acid → Iron(II) chloride + Hydrogen
Set up the conductivity apparatus from Activity 2.8: fix two nails on a cork placed in a beaker, and connect them through a bulb and switch to a 6-volt battery. Pour dilute HCl (or dilute H2SO4) into the beaker and switch on the current — the bulb glows, showing the acid conducts electricity (contains free ions).
Now empty the beaker, rinse it, and repeat with a glucose solution, then separately with an alcohol solution, in place of the acid. In both cases the bulb does not glow, showing that these solutions do not conduct electricity — i.e., they do not produce ions in water, even though both contain hydrogen. This proves that the mere presence of hydrogen in a compound doesn't make it acidic; acidic character requires the compound to ionise in water and release H+ ions, which glucose and alcohol do not do.
Distilled water is pure H2O with no dissolved ions, so it cannot conduct electricity. Rain water, as it falls through the atmosphere, dissolves small amounts of carbon dioxide and other gases, forming weak acids (like carbonic acid), which ionise to produce ions in the water. These dissolved ions allow rain water to conduct electricity.
Acidic behaviour comes from the release of H+ (H3O+) ions, and this ionisation can only take place in the presence of water. In the absence of water, the acid molecules do not ionise, so no H+ ions are released, and the acid does not show its acidic character.
A = pH 4, B = pH 1, C = pH 11, D = pH 7, E = pH 9
Increasing order of H+ ion concentration (lower pH = higher H+ concentration, so this is the reverse of increasing pH order):
Test tube A (with HCl) will show more vigorous fizzing.
HCl is a strong acid and ionises almost completely in water, producing a higher concentration of H+ ions. CH3COOH (acetic acid) is a weak acid and ionises only partially, producing fewer H+ ions for the same concentration. A higher H+ ion concentration means a faster reaction with magnesium, producing hydrogen gas more vigorously.
The pH will decrease (the milk becomes more acidic, dropping below 6) as it turns into curd. This is because bacteria present in milk convert milk sugar (lactose) into lactic acid during fermentation, which increases the acidity — and hence lowers the pH — of the milk as it sets into curd.
(a) Baking soda (sodium hydrogencarbonate) is a mild base. Adding it shifts the milk's pH from slightly acidic (6) to slightly alkaline, which helps delay the natural souring of the milk and keep it fresh for longer.
(b) Milk curdles because bacteria produce lactic acid, lowering its pH. Since the milk's starting pH is now slightly alkaline, more lactic acid has to be produced before the pH drops low enough to curdle it, so the milk takes longer to set as curd.
Plaster of Paris (calcium sulphate hemihydrate) readily absorbs moisture from the air. If exposed to moist/humid air, it reacts with the water vapour and converts back into gypsum (CaSO4·2H2O), turning into a hard, solid, unusable lump before it can be put to use. Storing it in a moisture-proof/airtight container prevents this reaction and keeps it usable as a fine powder.
A neutralisation reaction is a reaction between an acid and a base that produces a salt and water, cancelling out their acidic and basic properties:
Examples:
Washing soda (Na2CO3·10H2O):
Baking soda (NaHCO3):
Next up is Chapter 3 — Metals and Non-Metals. Or explore the full chapter list, browse the Class 10 Science hub, or book a free demo class for personalised coaching.
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