General Science Questions and Answers

Answer: B

The two strands of DNA are held together by hydrogen bonds between the paired bases - two between adenine and thymine, three between guanine and cytosine.

Each bond is weak, but there are millions along the molecule, so the double helix is stable while still coming apart strand by strand for replication.

Answer: D

Radiocarbon dating estimates the age of fossils and other once-living material - wood, bone, charcoal, cloth.

Carbon-14 has a half-life of 5,730 years, so the method works back to about 50,000 years. It cannot date rock or soil, which never contained living carbon.

Answer: C

Carbon monoxide is not a greenhouse gas. A greenhouse gas must absorb infrared radiation, which needs a molecule whose dipole moment changes as it vibrates.

Carbon monoxide absorbs very weakly and lasts only a few months in the atmosphere. Carbon dioxide, methane and water vapour are all effective greenhouse gases.

Answer: D

Bauxite is the chief ore of aluminium, a mixture of hydrated aluminium oxides.

The oxide is extracted from it by the Bayer process and then reduced to the metal by electrolysis in molten cryolite. Calamine is a zinc ore, calcite calcium carbonate, and galena lead sulphide.

Answer: D

Ammonia is lighter than air. Its molar mass is 17 g/mol against the average 29 g/mol of air.

Oxygen (32), carbon dioxide (44) and chlorine (71) are all heavier, which is why carbon dioxide can be poured to smother a flame and chlorine settles in low ground.

Answer: B

The electron microscope revealed the internal structure of the cell. Using electrons instead of light, it resolves detail down to a fraction of a nanometre - a thousand times finer than a light microscope.

Mitochondria, the endoplasmic reticulum, ribosomes and the structure of membranes were all discovered with it.

Answer: A

The dark Fraunhofer lines in the solar spectrum are caused by the absorption of particular wavelengths by the sun's cooler outer layers.

The hot interior emits a continuous spectrum; atoms in the photosphere and chromosphere above absorb exactly the wavelengths they would themselves emit, leaving dark lines. Their positions identify the elements present - which is how helium was discovered.

Answer: A

The prefix iso- indicates sameness, equality or constancy, from the Greek isos, equal.

Hence isotherm (equal temperature), isobar (equal pressure), isotope (same place in the periodic table) and isomer (same formula). Mega- means large, meta- beyond or changed, and quasi- resembling.

Answer: A

Silicon is the element used to convert solar energy to electricity. A photovoltaic cell is a silicon wafer doped to make a p-n junction; light striking it frees electrons that flow as current.

Silicon is used because its band gap of 1.1 eV suits the solar spectrum well and because it is abundant and easily purified.

Answer: D

Gallium (Ga) is the metal. It is a soft, silvery element of group 13 that melts at 29.8°C - low enough to liquefy in the hand.

Sulphur and iodine are non-metals; selenium is a non-metal usually classed with the metalloids.