Chemistry Questions and Answers

Answer: A

Metals are the most numerous class of known elements.

Of the 118 elements, about 90 are metals, some 17 non-metals and 7 or 8 metalloids. Everything below and to the left of the metalloid staircase in the periodic table is metallic, including all the d- and f-block elements.

Answer: D

All the above are chemical weathering.

Oxidation turns iron minerals to rust and crumbles the rock; carbonation dissolves limestone as \[\mathrm{CaCO_3 + H_2O + CO_2 \rightarrow Ca(HCO_3)_2}\] which is how caves form; and acid rain accelerates both. Each alters the rock's chemical composition, unlike physical weathering by frost or heat.

Answer: A

The pH rises on adding ammonia.

Ammonia is a weak base: \[\mathrm{NH_3 + H_2O \rightleftharpoons NH_4^+ + OH^-}\] The extra hydroxide lowers \([\mathrm{H^+}]\) and so raises the pH above 7. Cane sugar and common salt are neutral, and hydrochloric acid would lower the pH.

Answer: B

The number of waves equals \(n\), the number of the orbit.

De Broglie's standing-wave condition for a stable orbit is \[2\pi r = n\lambda\] so an orbit of principal quantum number \(n\) accommodates exactly \(n\) complete wavelengths, whatever the multiplier in front of it.

Answer: A

This is the molality of the solution.

\[m=\frac{\text{moles of solute}}{\text{mass of solvent in kg}}\] Because it is defined by mass rather than volume, molality does not change with temperature - which is why the colligative-property formulae use it. Molarity is moles per litre of solution and does vary with temperature.

Answer: C

Chromic acid also renders iron passive.

Strong oxidising acids build a thin, adherent, impervious film of oxide on the iron surface, and that film prevents further attack - which is why concentrated nitric acid can be carried in steel tankers. Acetic, oxalic and citric acids are weak and non-oxidising.

Answer: C

Iron is not supplied by ordinary fertilisers.

Fertilisers supply the three macronutrients - nitrogen, phosphorus and potassium, the N-P-K of the label - because plants need them in quantity. Iron is a micronutrient, needed in traces and usually present in the soil already; manures supply it when it is lacking.

Answer: D

Phosphorus exists as \(\mathrm{P_4}\).

White phosphorus is a tetrahedral molecule of four atoms with P–P–P angles of 60°. That severe angular strain is why white phosphorus is so reactive that it ignites in air and must be stored under water.

Answer: B

Secondary structure is the alpha helix and the beta pleated sheet.

These are local, regular foldings of the polypeptide backbone, held by hydrogen bonds between the carbonyl oxygen of one residue and the amide hydrogen of another. The primary structure is the amino-acid sequence, the tertiary the overall fold of one chain, and the quaternary the assembly of several chains.

Answer: A

The chemical changes are (a) and (b).

Rusting forms hydrated iron(III) oxide and burning magnesium forms magnesium oxide - new substances, and neither change is reversible by simple physical means. Melting ice and dissolving sugar produce no new substance and are reversed by freezing and by evaporation.