Whose oxidation number is always -1?

The correct answer is D, Charles’s law. Charles’s law says that at constant pressure the volume of a fixed mass of gas is directly proportional to its absolute temperature, so V by T stays constant. Jacques Charles studied this in 1787, and Joseph Gay-Lussac published the quantitative form in 1802. Written as a ratio it gives V1 by T1 equals V2 by T2, with the temperature always taken in kelvin. Extending the volume-temperature graph backwards to zero volume gives absolute zero, minus 273.15 degrees Celsius, which is how the kelvin scale was fixed. A is wrong because Boyle’s law relates pressure and volume at constant temperature, with PV constant. B is wrong because Dalton’s law of partial pressures adds the pressures of the gases in a mixture. C is wrong because Avogadro’s law links equal volumes of gases to equal numbers of molecules. Exam tip: keep the three apart - Boyle for P and V, Charles for V and T, Gay-Lussac for P and T.

The correct answer is B, Coulomb’s Law. Coulomb’s law gives the force between two point charges at rest. It states that the force is directly proportional to the product of the two charges and inversely proportional to the square of the distance between them, and acts along the line joining them. Charles-Augustin de Coulomb established it in 1785 with a torsion balance. In free space the constant of proportionality is about 9 into 10 raised to 9 newton metre squared per coulomb squared, and the force is repulsive for like charges and attractive for unlike charges. A is wrong because Lenz’s law gives the direction of an induced current, which opposes the change in flux. C is wrong because Gauss’ law relates the electric flux through a closed surface to the charge inside it. D is wrong because Ohm’s law connects voltage, current and resistance in a conductor. Exam tip: remember the shape - Coulomb’s law is an inverse square law, like Newton’s law of gravitation.

The correct answer is A, Femto. In the SI system the prefix femto, written f, stands for 10 raised to minus 15. The name comes from the Danish and Norwegian word femten, meaning fifteen. It is the prefix used for the size of an atomic nucleus, which is a few femtometres across, and a femtometre is also called a fermi after the physicist Enrico Fermi. Laser pulses lasting a few femtoseconds are used to watch chemical bonds break. B is wrong because zepto, z, stands for 10 to the minus 21. C is wrong because atto, a, stands for 10 to the minus 18, the step between femto and zepto. D is wrong because yocto, y, is the smaller 10 to the minus 24. Exam tip: learn the descending ladder - milli, micro, nano, pico, femto, atto, zepto, yocto, each three powers of ten apart.

The correct answer is B, Galileo Galilei. Galileo was the first to argue that, with air resistance removed, every body falls with the same acceleration regardless of its mass. He reached this by rolling balls down inclined planes and timing them, and the idea is linked in popular accounts with his experiment from the Leaning Tower of Pisa. This overturned the older belief of Aristotle that heavier bodies fall faster. The acceleration he described is g, about 9.8 metres per second squared near the earth. A is wrong because Einstein built on this in the general theory of relativity through the equivalence principle, long afterwards. C is wrong because Edison was an inventor of the electric lamp and phonograph and did no work on falling bodies. D is wrong because Newton, born the year Galileo died, later explained why this happens through his law of gravitation. Exam tip: remember the order - Galileo found that all bodies fall alike, Newton explained the cause.

The correct answer is A, Petiole. The petiole is the stalk that joins a leaf blade to the stem; it belongs to the leaf, not to the flower, and a leaf without one is called sessile. A typical flower is built of four whorls borne on the thalamus. The calyx is the outermost ring of sepals, which protect the bud. Inside it the corolla of petals attracts pollinators. Then comes the androecium, the male whorl, whose units are the stamens, each with an anther on a filament. At the centre is the gynoecium, the female whorl, whose unit is the pistil or carpel, made of stigma, style and ovary. Option B is wrong because the pistil is the female part of the flower. Option C is wrong because the stamen is its male part. Option D is wrong because sepals form the calyx, the flower's outermost whorl. Exam tip: the four floral whorls are calyx, corolla, androecium and gynoecium; the petiole is a part of the leaf.