Chemistry Questions and Answers

Answer: C

The order of reactivity is C \(\lt\) A \(\lt\) B \(\lt\) D.

A metal displaces a less reactive metal from its salt solution, so a deposit or coating means the added metal is more reactive than the metal in the salt.

• D displaces zinc from ZnSO₄ (white coating) and iron from FeSO₄, so it is more reactive than zinc: the most reactive.
• B displaces iron and copper but not zinc, so it lies between zinc and iron.
• A does not displace iron but displaces copper, so it lies between iron and copper.
• C does not even displace copper, so it is the least reactive.

That gives C \(\lt\) A \(\lt\) B \(\lt\) D. The other orders put D below B, or A below C, which contradicts the observed displacements.

Answer: A

'Y' could be rayon.

Rayon is made from cellulose obtained from wood pulp, a natural source, which is chemically treated to make a fibre. That is why it is often called artificial silk and described as a man-made fibre from a natural source.

Nylon and acrylic are fully synthetic fibres made from petrochemicals, so their source is not natural. Jute is a natural plant fibre used as it is, not a man-made fibre.

Answer: C

The fuel best suited for domestic use is C.

A good domestic fuel should be easy to handle and burn cleanly, so a gas is preferred to a solid or liquid. Its ignition temperature should be moderate: low enough to light easily, but not so low that it catches fire accidentally. And it should give reasonable heat.

Fuel C is a gas with moderate calorific value and moderate ignition temperature, so it meets all three. Fuel A is a solid with a high ignition temperature, hard to light. Fuel B is a gas but also hard to ignite. Fuel D has a low calorific value and a low ignition temperature, which makes it both weak and dangerous to store.

Answer: C

The most reactive metal is Z.

A more reactive metal displaces a less reactive metal from a solution of its salt. X displaces Y, so X is more reactive than Y. X cannot displace Z, so Z is more reactive than X.

The order of reactivity is therefore Z, then X, then Y. X is in the middle, Y is the least reactive, and the information given is enough to decide, so 'cannot be determined' is wrong.

Answer: D

The set of chemical changes is rusting of iron, burning of paper, digestion of food.

In a chemical change new substances are formed. Rusting forms iron oxide, burning paper forms ash, carbon dioxide and water vapour, and digestion breaks food into simpler substances. None can be reversed by simple means.

Each other set includes a physical change: boiling water and breaking glass change only state or shape; dissolving salt or sugar in water can be reversed by evaporation; shredding paper changes only its size.

Answer: B

He notices that the lemon floats in water.

A lemon is only slightly denser than plain water, so at first it sinks. Dissolving more and more salt makes the water denser. Once the salt water is denser than the lemon, the upward push of the water (buoyancy) is greater than the lemon's weight and it rises to float.

The lemon does not split, and it would hang mid-way only if the densities happened to be exactly equal; the result of adding a lot of salt is that it floats.

Answer: D

Statements a, b and c are true, so the answer is (a), (b) and (c).

Rusting is a chemical reaction in which iron combines with oxygen (in the presence of water) to form hydrated iron(III) oxide (a). Like other oxidation reactions it releases heat, though so slowly that it is not felt, so it is exothermic (b), and it happens at ordinary room temperature (c).

Statement d is false: rust does not turn back into iron on its own; getting the metal back requires reduction in a furnace. Options including d are therefore wrong.

Answer: A

The correct matching is (a)-(iv), (b)-(vi), (c)-(iii), (d)-(v), (e)-(i), (f)-(ii).

• Slaked lime: calcium hydroxide, Ca(OH)₂.
• Table salt: sodium chloride, NaCl.
• Limestone: calcium carbonate, CaCO₃.
• Caustic soda: sodium hydroxide, NaOH.
• Quick lime: calcium oxide, CaO.
• Baking soda: sodium bicarbonate, NaHCO₃.

The common confusion is between quick lime (CaO) and slaked lime (Ca(OH)₂), formed when quick lime is 'slaked' with water; options C and D mix these or the sodium compounds.

Answer: C

Both statements are correct, so the answer is Both (a) and (b).

Non-metals generally do not react with water; some, like phosphorus, are even stored under water to keep them from reacting with air. Non-metals also generally do not react with dilute acids, because they cannot give up electrons to the hydrogen ions to release hydrogen gas, as metals do.

These two properties are standard ways of distinguishing non-metals from metals at the upper primary level, so the options rejecting either statement are wrong.

Answer: A

X and Y are Bitumen, Coal Tar.

Bitumen is a thick black residue obtained in the refining of petroleum and is now the main binder for metalling (surfacing) roads. Coal tar is a thick black liquid obtained when coal is processed (heated in the absence of air); it was used for road surfacing before bitumen replaced it.

Charcoal is a solid fuel from wood, not a liquid, and option D reverses the sources of the two liquids.