Chemistry Questions and Answers

Answer: C

A is true but R is false.

Anodizing is an electrolytic process that thickens the natural oxide layer on a metal, usually aluminium. The tough oxide coat protects the metal from corrosion, so the Assertion is correct.

The Reason describes a different process. Coating iron with a layer of zinc is galvanization, not anodizing. Since R is false, it cannot explain A, and the options calling it true or calling A false are wrong.

Answer: B

Silver turns black due to reaction with hydrogen sulphide present in air.

Silver reacts with traces of hydrogen sulphide gas in the air to form a thin black layer of silver sulphide. This tarnishing is a form of corrosion.

Silver is a noble metal and does not react with oxygen in air under ordinary conditions, so the oxygen option is wrong. Nitrogen oxides and carbon monoxide are not what blackens silver articles. Compare copper, which gains a green coating from moist carbon dioxide and air.

Answer: C

The correct set is A - \(\mathrm{NH_{3}}\); B - NaOH; C - HCl; D - NaOH.

A salt is formed when an acid reacts with a base. Ammonium chloride comes from HCl and ammonia, \(\mathrm{NH_{3} + HCl \rightarrow NH_{4}Cl}\). Sodium nitrate comes from nitric acid and sodium hydroxide, and sodium chloride from hydrochloric acid and sodium hydroxide.

Each other option has an error. The first gives \(\mathrm{Na(OH)_{2}}\), which does not exist because sodium has a valency of 1, and NaH, a hydride rather than the base that gives sodium chloride. The second gives \(\mathrm{NH_{4}}\), which on its own is an ion, not a base. The fourth gives NaO, which is not a real compound. Only the third gets all four right, with ammonia acting as the base.

Answer: A

Step 1 is a physical change and Step 2 is a chemical change.

Dissolving sugar in water is a physical change: no new substance forms, and the sugar can be recovered by evaporating the water.

Caramelization is a chemical change: heating breaks down the sugar into new substances with a different colour, smell and taste, and the change cannot be reversed. That rules out the options calling both steps the same kind of change, and the option that reverses them.

Answer: C

The ion has 11 electrons.

The number of protons is the mass number minus the number of neutrons. A neutral atom has as many electrons as protons, and a charge of 2+ means two electrons have been lost.

\[ \begin{aligned} \text{protons} &= 27 - 14 = 13 \\ \text{electrons in ion} &= 13 - 2 = 11 \end{aligned} \]

13 is the number of electrons in the neutral atom, 15 comes from adding the two instead of subtracting, and 10 is the result of subtracting three electrons, the charge of the familiar \(\mathrm{Al^{3+}}\) ion, instead of the two stated here.

Answer: C

The solutions are A - Neutral; B - Basic; C - Acidic.

China rose (hibiscus) indicator turns acidic solutions dark pink (magenta) and basic solutions green, and it does not change colour in a neutral solution.

So solution A, which caused no change, is neutral; B, which turned green, is basic; and C, which turned magenta, is acidic. The other options mix up which colour goes with acids and which with bases.

Answer: C

The key reads galvanization as a physical change and electroplating as a chemical change.

In electroplating, an electric current passed through a solution of a copper salt drives a chemical reaction: copper ions gain electrons at the object and are deposited as copper metal while copper dissolves at the other electrode. New substances form at the electrodes, so it is a chemical change.

Galvanization is treated as a protective coating: iron is covered with a layer of zinc, for example by dipping it in molten zinc, so that air and moisture cannot reach the iron. On this school-level reading no new substance is made in the iron; it is simply coated, which is why the key calls it physical. The options calling both changes the same kind, or reversing them, disagree with this reading.

Answer: C

The metals are Potassium, Iron, Copper.

Potassium reacts violently with cold water, so violently that the hydrogen released catches fire (A). Iron does not react with cold or hot water but reacts with steam to form iron oxide and hydrogen (B). Copper does not react with water at all (C).

The other options misplace a metal. In option 1, iron is put among metals that do not react with water, but it reacts with steam, and calcium reacts with cold water much less violently than potassium. In option 2, magnesium does not react violently with cold water. In option 4, copper is listed as reacting with steam, which it does not, and aluminium as not reacting with water, though it reacts with steam.

Answer: B

The mismatched set is Slaked lime - CaO - Used for lowering the acidity of soil.

CaO, calcium oxide, is quicklime. Slaked lime is calcium hydroxide, Ca(OH)₂, made by adding water to quicklime. The use given is right, since lime is spread on acidic soil to neutralise it, but the formula belongs to a different substance, so the set is mismatched.

The other sets match. Bleaching powder, CaOCl₂, is used for bleaching cotton and linen in laundries. Plaster of Paris, CaSO₄.½H₂O, is used for smoothing and coating walls. Gypsum, CaSO₄.2H₂O, is the raw material of the dental plasters and stones used to make casts of teeth.

Answer: B

The mixture that cannot be separated by crystallization is a solution of methyl alcohol and potassium permanganate.

Crystallization recovers a pure solid from its solution by evaporating the solvent until crystals form. Potassium permanganate is a strong oxidising agent and reacts with methyl alcohol, so the two do not stay a simple solution from which the permanganate can be crystallized back unchanged. Methyl alcohol, a volatile liquid, is in any case recovered by distillation, not crystallization.

The others are standard uses of crystallization: salt is obtained from impure salt or sea water, alum is purified from an impure sample, and zinc chloride and lead(II) chloride can be separated because lead(II) chloride is far less soluble in cold water and crystallizes out first.