Chemistry Questions and Answers

Answer: B

The noble gases are monatomic.

Their outermost shells are already complete, so the atoms have no tendency to bond even to one another and exist as single atoms. Every other gaseous element - hydrogen, nitrogen, oxygen, the halogens - goes about in molecules.

Answer: C

Xenon is called the stranger gas.

The name comes from the Greek xenos, meaning stranger, and was given by Ramsay and Travers in 1898 because it was found only in vanishingly small traces in liquid air. It is also the noble gas that most readily forms compounds, such as \(\mathrm{XeF_4}\).

Answer: C

The pH of blood is about 7.4.

It is held within the narrow range 7.35–7.45 by the carbonic acid–bicarbonate buffer, supported by the lungs, which control \(\mathrm{CO_2}\), and the kidneys, which control \(\mathrm{HCO_3^-}\). Straying outside about 6.8–7.8 is life-threatening.

Answer: C

The pH of pure water is 7 at 25 °C.

Water self-ionises to the extent that \[[\mathrm{H^+}] = [\mathrm{OH^-}] = 10^{-7}\ \mathrm{mol\,L^{-1}}\] so \(\mathrm{pH} = -\log(10^{-7}) = 7\). The value drifts below 7 at higher temperatures, but the water stays neutral because the two concentrations remain equal.

Answer: A

Lemon contains citric acid.

Lemon juice is about 5–6 per cent citric acid, which gives it a pH near 2.2. Lactic acid is in sour milk, malic acid in apples and tartaric acid in tamarind and grapes.

Answer: C

Osmium is the heaviest - that is, the densest - metal, at \(22.59\ \mathrm{g\,cm^{-3}}\).

Iridium is a very close second at 22.56. Both are more than twice as dense as lead. Mercury, though it feels heavy, is only \(13.5\ \mathrm{g\,cm^{-3}}\).

Answer: B

Air contains most nitrogen, about 78 per cent by volume.

Oxygen makes up 21 per cent, argon 0.93 per cent and carbon dioxide about 0.04 per cent. Free hydrogen is present only in traces, being light enough to escape into space.

Answer: D

Chlorine bleaches by oxidation.

In water it gives hypochlorous acid, which releases nascent oxygen: \[\mathrm{Cl_2 + H_2O \rightarrow HCl + HOCl},\qquad \mathrm{HOCl \rightarrow HCl + [O]}\] The nascent oxygen oxidises the coloured compound to a colourless one. Because the change is oxidative it is permanent, unlike bleaching by sulphur dioxide, which works by reduction and reverses in air.

Answer: B

Gases is not a type of element.

Elements are classified by chemical character as metals, non-metals and metalloids. Gas, liquid and solid describe physical state, which changes with temperature and pressure and cuts across those classes - mercury is a liquid metal and bromine a liquid non-metal.

Answer: D

Water drops are spherical because of surface tension.

Molecules at the surface are pulled inwards by their neighbours, since there are none above them, and the liquid therefore contracts to the shape with the least surface area for a given volume. For a fixed volume that shape is a sphere.