Chemistry Questions and Answers

Answer: C

X is hydrochloric acid and Y is milk of magnesia.

The stomach secretes hydrochloric acid, which creates the acidic medium needed for the enzyme pepsin to digest proteins and also kills germs. Too much acid causes acidity and indigestion. It is treated with an antacid, a mild base such as milk of magnesia (magnesium hydroxide), which neutralises the excess acid.

Milk of magnesia is not secreted in the stomach, so options naming it as X are wrong. Sodium carbonate (washing soda) is too strongly alkaline to take as an antacid; the antacid in that family is sodium hydrogencarbonate (baking soda). Ascorbic acid, vitamin C, is an acid and would make acidity worse.

Answer: D

The statement that is NOT correct is crystallisation is a chemical change.

In crystallisation, pure solid crystals separate from a solution, as when sugar or copper sulphate crystals form from a saturated solution. The substance remains the same, and the crystals can be dissolved again, so it is a physical change.

Digestion breaks food into new, simpler substances; photosynthesis makes glucose and oxygen from carbon dioxide and water; respiration converts glucose and oxygen into carbon dioxide and water with release of energy. All three produce new substances, so they are chemical changes.

Answer: B

Only statement B is correct.

A more reactive metal displaces a less reactive metal from a solution of its salt. The reactivity order of these metals is zinc > iron > copper.

Zinc is more reactive than copper, so it displaces copper from copper sulphate solution, and the blue colour fades (B). Iron is less reactive than zinc, so it cannot displace zinc from zinc sulphate (A is wrong), and copper is less reactive than iron, so it cannot displace iron from iron sulphate (C is wrong).

Answer: C

The correct matching is A-IV, B-I, C-II, D-III.

• A. Acetic acid is found in vinegar (IV).
• B. Lactic acid is found in curd (I).
• C. Oxalic acid is found in spinach (II).
• D. Ascorbic acid (vitamin C) is found in citrus fruits such as oranges and amla (III).

The other codes give vinegar, curd or spinach the wrong acid; for example, citric acid rather than oxalic acid is the acid of lemons.

Answer: D

The product not obtained from petroleum is Jatropa (jatropha) oil.

Jatropha oil is pressed from the seeds of the Jatropha curcas plant. It is a plant oil, and in India it has been promoted as a raw material for biodiesel, an alternative to petroleum-based diesel.

The other three are petroleum products. Kerosene is a fraction obtained in the refining of crude oil. Paraffin wax is separated from the heavy fractions of petroleum, and grease is made by thickening a petroleum lubricating oil.

Answer: B

The answer is (i) Manure (ii) Fevicol.

(i) Urea is a chemically made, inorganic fertiliser. Cow dung is an organic material that is rotted down and used as manure, so manure completes the analogy.

(ii) Gum from plants is a natural adhesive; Fevicol, a synthetic glue, is an artificial adhesive.

Rubber is a natural product, not an artificial adhesive, which rules out options 3 and 4. 'Fertilizer' alone repeats the category without the natural-versus-chemical contrast the analogy sets up.

Answer: C

X is magnesium (Mg), and the reaction is a combination reaction.

Magnesium ribbon burns in air with a dazzling white flame and leaves a white powder, magnesium oxide:

\[ 2\text{Mg} + \text{O}_{2} \rightarrow 2\text{MgO} \]

Two substances join to form one product, which is the definition of a combination reaction (it is also exothermic, not endothermic). Burning is never a decomposition, which splits one substance into several, so the calcium and aluminium options fail on the reaction type, and burning zinc is exothermic, not endothermic, and does not give the dazzling white flame described.

Answer: D

Both statements are true and Statement II is the correct reason for Statement I.

Isotopes are atoms of the same element with the same number of protons and electrons but different numbers of neutrons, such as carbon-12 and carbon-14. Chemical properties depend on the electrons, in particular the arrangement of the outer (valence) electrons, which isotopes share.

Neutrons sit in the nucleus and take no part in chemical bonding; they change only the mass and physical properties such as density. That is exactly why isotopes behave the same chemically, so II explains I.

Answer: D

The correct description is R1 is exothermic and R2 is a decomposition.

R1 is the lime-water reaction. Slaked lime reacts with carbon dioxide to form calcium carbonate and water, and heat is released:

\[ \begin{aligned} &\text{Ca(OH)}_{2} + \text{CO}_{2} \\ &\rightarrow \text{CaCO}_{3} + \text{H}_{2}\text{O} \end{aligned} \]

R2 breaks one compound into three on heating, which is a decomposition:

\[ \begin{aligned} &\text{Ca(HCO}_{3})_{2} \\ &\rightarrow \text{CaCO}_{3} + \text{H}_{2}\text{O} + \text{CO}_{2} \end{aligned} \]

R2 is not a combination, and R1 is neither endothermic nor a decomposition, which rules out the other three options.

Answer: D

The true statement is X has pH more than 7 and Y has pH less than 7.

Phenolphthalein is colourless in acidic and neutral solutions and turns pink in basic ones. So X, which turns it pink, is basic, with pH above 7. When Y is added the pink colour disappears, which means the base has been neutralised; Y must be acidic, with pH below 7.

Adding a weak base to a basic solution would keep it basic and pink. And X cannot be acidic, because an acid would not turn phenolphthalein pink in the first place.