Chemistry Questions and Answers

Answer: D

\(X^{3+}\) has 13 protons, 10 electrons and 14 neutrons.

The atomic number, 13, is the number of protons and does not change when an ion forms (the element is aluminium). A neutral atom has 13 electrons; losing three to form a 3+ ion leaves 10. Neutrons = mass number - atomic number = 27 - 13 = 14.

Option 3 (13, 13, 10) forgets that the ion has lost three electrons and puts the electron count in place of the neutrons. Options 1 and 2 change the number of protons, which would make the particle a different element.

Answer: A

The rubbing surface carries glass powder and red phosphorus.

In a safety match, the side of the box is coated with powdered glass, which provides friction, and a little red phosphorus. Striking the match converts some red phosphorus to white phosphorus, which catches fire and ignites the match head, made of antimony trisulphide and potassium chlorate.

White phosphorus is not used on the box, because it is poisonous and catches fire on its own in air; its use in matches was abandoned for safety. Magnesium and potassium are not part of the striking surface.

Answer: B

The group of elements only is Tin, Lithium, Sodium.

An element is a substance made of only one kind of atom. Tin (Sn), lithium (Li) and sodium (Na) are all metallic elements found in the periodic table.

Each other group contains an alloy, a mixture of metals (or of a metal and another element). Steel is an alloy of iron with carbon, brass is copper with zinc, and bronze is copper with tin. Iron, zinc, potassium, copper and aluminium are elements, but one alloy is enough to spoil a group.

Answer: C

The true statements are (B) and (D).

Photosynthesis makes new substances, glucose and oxygen, from carbon dioxide and water, and curd formation changes milk proteins through bacterial action. Both are chemical changes that cannot be reversed simply.

Crystallisation is a physical change, since the substance stays the same and only its form changes, so (A) is false. Ripening of fruit involves chemical changes in starches, acids and pigments, so (C) is false.

Answer: C

Only (A) is true.

The reactivity series runs \(\mathrm{K \gt Na \gt Ca \gt Mg \gt Al \gt Zn \gt Fe}\) and so on. Zinc lies between aluminium and iron, so (A) is correct.

Sodium is less reactive than potassium (though more reactive than calcium), so (B) is false, and aluminium is less reactive than magnesium, so (C) is false.

A simple test is displacement: a more reactive metal displaces a less reactive one from its salt solution, so zinc displaces iron from iron sulphate but cannot displace aluminium.

Answer: C

A, B, C, D, E, F are 19, 9, 9, 12, 12, 12.

For a neutral atom, protons = electrons = atomic number, and mass number = protons + neutrons.

\[ \begin{aligned} \text{Row 1: } C &= B = 9,\ A = 9 + 10 = 19 \\ \text{Row 2: } D &= E = 12,\ F = 24 - 12 = 12 \end{aligned} \]

Option 1 gives the atomic number as the mass number, and options 2 and 4 make the electron and proton counts differ, which is impossible for a neutral atom.

Answer: D

The statements that are not true are (A), (B) and (D).

(A) is false: a compound has properties different from its constituents, as water differs from hydrogen and oxygen. (B) is false: compounds also react with elements or other compounds to form new compounds. (D) is false: compounds can be separated into their elements only by chemical methods, not physical ones.

(C) is true, since every compound has a fixed composition by mass (the law of constant proportions), so any option including (C) is wrong.

Answer: D

The substances that give a flame are B and C.

A flame is produced only by substances that vaporise during burning, because it is the burning vapour that forms the flame. Wax and kerosene, for example, burn with a flame, while charcoal only glows.

B and C vaporise on burning, whichever their state, so they give a flame. A and D do not vaporise, so they burn without a flame.

Answer: C

The reactions are \(\mathrm{Na_2CO_3 + 2HCl \rightarrow 2NaCl + H_2O + CO_2}\) and \(\mathrm{Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O}\).

Sunita uses sodium carbonate, \(\mathrm{Na_2CO_3}\), not sodium hydrogen carbonate, \(\mathrm{NaHCO_3}\), so options 1 and 4 have the wrong first reaction. The carbon dioxide produced turns lime water, calcium hydroxide, milky by forming insoluble calcium carbonate.

The reaction \(\mathrm{CaCO_3 + H_2O + CO_2 \rightarrow Ca(HCO_3)_2}\) in options 1 and 2 happens only when excess carbon dioxide is passed and the milkiness disappears; it is not the first reaction with lime water.

Answer: A

The answer is A-thermosetting plastic; B-electrical switches; C-thermoplastic; D-containers.

Bakelite is a thermosetting plastic: once moulded it sets permanently and does not soften on heating. Being a poor conductor of heat and electricity, it is used for electrical switches, plugs and the handles of utensils. PVC (polyvinyl chloride) is a thermoplastic: it softens on heating and can be reshaped, and it is used for pipes, containers, wire insulation and toys.

Options C and D swap the two types of plastic. Option B gives bakelite the use of containers and PVC the use of utensils, which reverses the properties that matter.