General Science Questions and Answers

Answer: B

Sunlight splits into seven colours - violet, indigo, blue, green, yellow, orange and red.

The sequence is remembered as VIBGYOR. It is a continuous spectrum rather than seven discrete bands; Newton chose seven divisions partly by analogy with the notes of a musical scale.

Answer: C

Cement is made chiefly from limestone and clay.

They are ground and burned together at about 1,450°C to form clinker, which is then ground with a little gypsum. Limestone supplies the calcium and clay the silica, alumina and iron.

Answer: D

In a laser all the atoms emit light of the same frequency, amplitude and phase.

This is what stimulated emission means: a passing photon triggers an excited atom to emit a photon identical to it in every respect. The result is a beam that is monochromatic, coherent and almost perfectly parallel.

Answer: A

Potassium-40 is the radioactive isotope most commonly present in the human body.

It makes up about 0.012 per cent of all natural potassium, and since the body contains around 140 g of potassium, a person carries roughly 4,000 becquerels of it - the largest single source of internal radiation.

Answer: B

Reactor fuel is used mainly as uranium dioxide, UO₂, pressed and sintered into ceramic pellets.

The oxide is chosen because it has a very high melting point, about 2,865°C, and is chemically stable under irradiation. UF₆ is the gas used in enrichment, not in the reactor.

Answer: B

Water has the greatest heat capacity of these - about 4,186 J per kg per K, several times that of iron, gold or benzene.

The energy goes into breaking hydrogen bonds rather than into raising the temperature. This is why water is used as a coolant, and why coastal climates are so much milder than continental ones.

Answer: A

Washing soda is hydrated sodium carbonate, Na₂CO₃·10H₂O.

It softens water by precipitating the calcium and magnesium as carbonates, so soap can lather. Baking soda is a different compound, sodium bicarbonate.

Answer: A

A cleared tropical rain forest regrows slowly because its soil is poor in nutrients.

Almost all the nutrients in a rain forest are held in the living vegetation, not in the ground; heavy rainfall leaches the soil continuously. Once the trees are removed the little that remains washes away, leaving nothing for regrowth.

Answer: A

The sky is blue because of the scattering of sunlight by the molecules of the air.

Rayleigh scattering varies as \( 1/\lambda^4 \), so short wavelengths are scattered far more strongly than long ones - blue about sixteen times more than red. The scattered blue light reaches the eye from every direction, and the sun looks red at sunset because that blue has been scattered out of the direct beam.

Answer: C

Heavy water is deuterium oxide, D₂O, in which both hydrogen atoms are the heavier isotope with a proton and a neutron.

It occurs naturally as about one part in 6,400 of ordinary water and is about 11 per cent denser. Because it absorbs very few neutrons it is used as a moderator in nuclear reactors, including India's PHWRs.