Class 10 Science Chapter 2 Acids, Bases and Salts NCERT Solutions 2026 PDF Download

Ans:

1. Dip the red litmus paper into each test tube. The solution that turns it blue is the Base.
2. Now, use this blue litmus paper (changed from red). Dip it into the remaining two tubes. The one that turns it back to red is the Acid.
3. The remaining tube which causes no change in either red or blue litmus is Distilled Water (Neutral).

Ans: Curd and sour substances contain acids (like lactic acid). Acids react with metals like copper and brass to form toxic compounds. These compounds can spoil the food and make it unfit for consumption, potentially causing food poisoning.

Ans: Hydrogen gas (H₂) is evolved.
Example: Zinc granules reacting with Sulphuric acid.
Zn + H₂SO₄ → ZnSO₄ + H₂↑
The presence of Hydrogen gas is confirmed by bringing a burning candle near the gas bubbles; it burns with a pop sound.

Ans: Acids dissociate in water to produce charged particles called ions (H⁺ ions). These free-moving ions are responsible for conducting electric current through the solution.

Ans: Acids show acidic properties only in the presence of water because they need water to dissociate into H⁺ ions. Dry HCl gas does not contain H⁺ ions, so it cannot change the colour of dry litmus paper.

Ans: Diluting a concentrated acid is a highly exothermic process. If water is added to a concentrated acid, the sudden release of huge heat may cause the mixture to splash out and cause burns, or the glass container may break. Adding acid drop-wise to water with constant stirring dissipates the heat safely.

Ans: Plaster of Paris (CaSO₄·½H₂O) absorbs moisture from the air and sets into a hard solid mass called Gypsum. This makes it useless for moulding purposes.
Equation: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O (Hard Gypsum)

Ans: A reaction where an acid and a base react to form salt and water, thereby cancelling each other's effect, is called neutralization.
1. NaOH + HCl → NaCl + H₂O
2. KOH + HNO₃ → KNO₃ + H₂O

Ans:

• Washing Soda (Na₂CO₃·10H₂O): Used in glass/soap/paper industries and for removing permanent hardness of water.
• Baking Soda (NaHCO₃): Used as an ingredient in antacids (neutralizes stomach acid) and in making baking powder for cakes.

Ans: (i) Ant sting: Methanoic acid (Formic acid)
(ii) Tomato: Oxalic acid
(iii) Tamarind: Tartaric acid
(iv) Sour Milk (Curd): Lactic acid

Ans: Tooth enamel (Calcium hydroxyapatite) is the hardest substance in the body. However, it gets corroded when the pH in the mouth falls below 5.5 because the bacteria present in the mouth produce acids by degrading sugar and food particles. This acid demineralizes the enamel.

Ans: When electricity is passed through an aqueous solution of Sodium Chloride (Brine), it decomposes to form Sodium Hydroxide (Alkali), Chlorine gas, and Hydrogen gas.
2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)

Ans: Fresh milk has a pH of 6. By adding baking soda (a base), he shifts the pH to slightly alkaline. This prevents the milk from turning sour (forming lactic acid) quickly, thus increasing its shelf life.

Ans: Formula: CaOCl₂ (Calcium Oxychloride).
It is prepared by the action of Chlorine gas on dry Slaked Lime.
Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O

Ans:

• Soil: Plants require a specific pH range for healthy growth. If soil is too acidic, farmers add quicklime (base) to neutralize it.
• Digestion: Our stomach produces HCl (pH ~1.2) to digest food. Excess acid causes pain (acidity), which is treated with antacids (bases like Mg(OH)₂).
• Aquatic Life: Acid rain lowers river water pH. If it becomes too acidic, survival of aquatic life becomes difficult.

Ans: Lower pH = Higher H⁺ concentration.
pH values: A(4), B(1), C(11), D(7), E(9).
Order: C (11) < E (9) < D (7) < A (4) < B (1).

Ans: Water of crystallization is the fixed number of water molecules chemically attached to a salt.
Proof: Heat blue copper sulphate crystals in a test tube. Water droplets appear on the tube walls, and the salt turns white (anhydrous). If you add a few drops of water back to the white powder, it turns blue again, proving the presence of water of crystallization.

Ans: The gas that extinguishes fire and produces effervescence is Carbon Dioxide (CO₂). Since CO₂ is produced by reacting with acid, compound 'A' must be a carbonate, specifically Calcium Carbonate (CaCO₃).
Equation: CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O

Ans: Distilled water is a pure covalent compound with no free ions, so it does not conduct electricity. Rain water, while falling, dissolves acidic gases like CO₂ and SO₂ from the atmosphere, forming carbonic acid etc., which dissociate into ions. These ions allow rain water to conduct electricity.

Ans: Substances whose smell (odour) changes in acidic or basic solutions are called Olfactory Indicators.
Example: Vanilla essence retains its smell in acid but loses its smell in a base. Onion also works similarly.

Ans:

• Baking Soda: Pure Sodium Hydrogen Carbonate (NaHCO₃).
• Baking Powder: A mixture of Baking Soda and a mild edible acid like Tartaric Acid. The acid neutralizes the bitter taste produced when soda is heated.

Ans: It is called Plaster of Paris because it was historically prepared from Gypsum which was found in abundance near Paris, France.

Ans: Initially, lime water turns milky due to insoluble Calcium Carbonate. On passing excess CO₂, the milkiness disappears because soluble Calcium Hydrogen Carbonate is formed.
CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂ (Soluble)

Ans: (a) Both contain Acids (Acetic acid and Lactic acid). They turn blue litmus red.
(b) Both contain Bases. They turn red litmus blue.