General Science Questions and Answers

Answer: D

An acid reacting with a metal carbonate gives a salt, water and carbon dioxide:

\[ \mathrm{Na_2CO_3 + 2HCl \rightarrow 2NaCl + H_2O + CO_2} \]

The effervescence of carbon dioxide is the standard laboratory test for a carbonate; the gas turns limewater milky.

Answer: B

Hydrogen is not a noble gas. It sits in group 1 of the periodic table and is highly reactive, burning in air and combining with most elements.

Helium, neon, argon, krypton, xenon and radon make up group 18, the noble gases, whose full outer shells make them almost entirely unreactive.

Answer: C

Chloroquine is used to prevent and treat malaria where the parasite is still sensitive to it.

It kills the parasite in the red blood cells by interfering with its digestion of haemoglobin. Resistance is now widespread, so artemisinin combinations have largely replaced it for treatment.

Answer: C

Hydrochloric acid is secreted by the stomach to aid digestion. It brings the stomach contents to about pH 1.5 to 3.5.

That acidity activates pepsinogen into pepsin, which digests protein, and it also kills most swallowed bacteria.

Answer: B

Glass is made from a mixture of sand (silica) and silicates - sand fused with sodium carbonate and limestone, which give sodium and calcium silicates.

The soda lowers the melting point of silica from about 1,700°C to a workable temperature, and the lime keeps the product from dissolving in water.

Answer: D

85 decibels is the level beyond which prolonged exposure damages hearing, and is the usual occupational safety limit.

Because the scale is logarithmic, the safe exposure time halves for every 3 dB above it. The threshold of pain is far higher, around 120 to 130 dB.

Answer: C

Laughing gas is nitrous oxide, N₂O.

Humphry Davy noticed its euphoric effect in 1799 and gave it the name; it has been used as a dental and surgical anaesthetic since the 1840s. It is also a potent greenhouse gas.

Answer: B

Graphite is not a mixture - it is an element, pure carbon in one of its allotropic forms.

Glass and steel are mixtures, and the aluminium of everyday use is an alloy rather than the pure metal.

Answer: A

Rusting needs both water and oxygen. Neither alone will do it: iron kept in dry air or in boiled, air-free water does not rust.

The product is hydrated iron(III) oxide, Fe₂O₃·nH₂O. Dissolved salt or acid speeds the process by making the water a better electrolyte.

Answer: C

The modern atomic weight scale is based on ₆C¹² - carbon-12 - whose atomic mass is defined as exactly 12 units.

Oxygen-16 was the earlier standard, but chemists and physicists used slightly different versions of it; carbon-12 was adopted in 1961 to settle the discrepancy.