Two reactants in a flask at room temperature are producing bubbles of a gas that turn limewater milky. The reactants could be

The correct answer is C, Cu + PbCl2 → CuCl2 + Pb. Copper is less reactive than lead, so it cannot push lead out of its compound.

In a displacement reaction, a more reactive metal takes the place of a less reactive metal in its salt solution. The reactivity series, from more to less reactive, runs: potassium, sodium, calcium, magnesium, aluminium, zinc, iron, lead, hydrogen, copper, mercury, silver, gold. Magnesium, zinc and iron are all above copper, so each of them displaces copper from copper sulphate; the blue solution fades and reddish-brown copper is deposited. Copper, however, lies below lead, so it has no power to displace lead from lead chloride.

A is wrong: iron is more reactive than copper, so this reaction takes place, the classic iron nail in copper sulphate experiment.

B is wrong: zinc is above copper in the series and readily displaces it.

D is wrong: magnesium is far more reactive than copper and displaces it easily.

Exam tip: a metal displaces only those below it in the reactivity series: K, Na, Ca, Mg, Al, Zn, Fe, Pb, H, Cu, Hg, Ag, Au.

The correct answer is C, 2 only. Graphite has free electrons that can move through it, so it conducts electricity; diamond and fullerene do not conduct well.

In graphite, each carbon atom is bonded to only three others, forming flat sheets of hexagons. The fourth electron of every atom is free to move along the sheets, which lets graphite carry current; that is why it is used for electrodes in dry cells and electric arcs. In diamond, every carbon atom is bonded to four others in a rigid three-dimensional network, leaving no free electrons, so diamond is an insulator even though it conducts heat very well. Fullerene, such as the football-shaped C60, has its electrons held within the molecule and is a poor conductor.

A is wrong: diamond is an electrical insulator.

B is wrong: it includes diamond, which does not conduct electricity.

D is wrong: neither diamond nor fullerene is a good conductor, and it leaves out graphite.

Exam tip: graphite is the only common non-metal that conducts electricity; diamond is the hardest natural substance.

The correct answer is C, increases heavily. In a short circuit the live and neutral wires touch, the resistance of the path falls almost to zero, and the current shoots up.

By Ohm's law, current I = V / R. Normally the appliances in a circuit provide resistance and keep the current within safe limits. When damaged insulation lets the live wire touch the neutral wire directly, the current bypasses the appliances and the resistance becomes very small, so for the same supply voltage the current becomes very large. This heats the wires sharply (heat ∝ I²Rt) and can start a fire. A fuse or a miniature circuit breaker (MCB) breaks the circuit when this happens.

A is wrong: lower resistance means more current, not less.

B is wrong: the resistance changes sharply, so the current cannot stay the same.

D is wrong: the current does not fluctuate; it rises to a very high value until a fuse or breaker cuts it.

Exam tip: short circuit means resistance near zero and huge current; overloading means too many appliances on one socket.

The correct answer is C, Blue. A leaf from a sunny day is full of starch made by photosynthesis, and iodine turns starch blue-black.

This is the standard school test for starch in a leaf. Boiling the leaf in water kills its cells and softens it so that the solutions can enter. Boiling it in alcohol, usually over a water bath because alcohol catches fire easily, dissolves the chlorophyll, leaving the leaf pale or nearly colourless. When iodine solution is then added, the starch stored in the leaf reacts with it and the leaf turns blue-black. A leaf kept in the dark for two days before the test shows no blue colour, proving that light is needed for photosynthesis.

A is wrong: the leaf is colourless only before iodine is added; the starch then turns it blue.

B is wrong: the green chlorophyll has already been removed by the boiling alcohol.

D is wrong: a starch-free leaf would show the brown colour of iodine, not white, and this leaf has starch.

Exam tip: iodine test: starch gives blue-black; alcohol removes chlorophyll before the test.

The correct answer is A, greater than the speed of sound. A body moving faster than sound outruns its own sound waves, which pile up into a shock wave heard on the ground as a sonic boom.

In air at room temperature, sound travels at about 343 m/s. A supersonic jet or a bullet moving faster than this leaves its sound waves behind, and they bunch together into a cone-shaped shock wave trailing the object. When the edge of this cone sweeps over a listener, the sudden change in air pressure is heard as a loud bang, strong enough to rattle or even crack window glass. Speeds are often given as a Mach number, the ratio of the object's speed to the speed of sound, so supersonic means Mach 1 or more.

B is wrong: no material object can travel faster than light.

C is wrong: below the speed of sound the waves spread ahead of the source, so no shock wave forms.

D is wrong: at exactly the speed of sound the waves bunch up at the nose, but the boom heard on the ground comes from supersonic flight.

Exam tip: Mach 1 = speed of sound; above Mach 1 is supersonic and above Mach 5 hypersonic.