According to Boyle s law for a perfect gas

The correct answer is A, by absorbing radiating heat energy. A change of state is governed by the energy of the particles. When a solid absorbs heat its particles vibrate harder until the force holding them in the lattice is overcome and it melts; further heat lets the liquid particles escape as vapour. Withdrawing heat reverses both steps. Option B is wrong because electrolytes conduct electricity in solution and take part in chemical change; they do not convert a state. Option D is wrong for the same reason, as acids and bases bring about a chemical reaction that gives new substances, not a physical change of state. Option C is the tempting distractor: pressure does matter, and squeezing a gas hard enough liquefies it, but the option is incomplete because raising pressure on a solid or a liquid alone will not change its state and in fact raises the boiling point. Heat is the general agent the question is after.

The correct answer is B, 1924. In 1924 Satyendra Nath Bose sent Albert Einstein a paper deriving Planck s radiation law by counting indistinguishable light quanta. Einstein saw at once that the same statistics applied to atoms, translated the paper into German, had it published and extended it, predicting that a dilute gas of such particles cooled near absolute zero would collapse into a single lowest quantum state. That state is the Bose-Einstein condensate, and the statistics are Bose-Einstein statistics. Option A, 1935, is the year of the Einstein-Podolsky-Rosen paper on quantum entanglement and not of this prediction. Option D, 1919, is the year of the solar eclipse observations that confirmed general relativity. Option C, 1930, has no connection with the condensate. Remember the chain: Bose s statistics and Einstein s prediction in 1924, the first condensate made in the laboratory in 1995, and the Nobel Prize in Physics for it in 2001.

The correct answer is B, 4 degrees Celsius. Water contracts as it is cooled from higher temperatures like any other liquid, but only down to 4 degrees Celsius. Below that point the hydrogen bonds begin to lock the molecules into an open, cage-like arrangement and the water expands again. Its density is therefore greatest at 4 degrees Celsius, where one cubic centimetre has a mass of about one gram. This behaviour is called the anomalous expansion of water. Option A, 0 degrees Celsius, is the freezing point, where water has already expanded and ice forms that is about nine per cent less dense, so it floats. Option C, 10 degrees Celsius, is above the point of maximum density, so the water there is slightly less dense. Option D, 100 degrees Celsius, is the boiling point, where water is at its least dense as a liquid. The practical consequence is that a pond freezes from the top down and the bottom stays at 4 degrees Celsius, letting fish survive.

The correct answer is D, plasma. Plasma is a hot, electrically conducting mixture of free electrons and positive ions produced when a gas is heated or strongly ionised. Stars, including the Sun, are made of plasma, and since stars and the ionised gas between them hold the overwhelming bulk of the visible matter in the universe, plasma is the most abundant state. On the Earth it is uncommon and is seen in lightning, the aurora, the flame of an electric arc, neon signs and fluorescent tubes. Option A, solid, is the state of planets, moons and dust, which are a tiny fraction of cosmic matter. Option B, liquid, is rarer still in the universe, since a liquid needs a narrow band of temperature and pressure to exist. Option C, gas, is the usual wrong answer, because interstellar clouds do hold neutral gas, but far more of the universe is ionised than neutral. Plasma is often called the fourth state of matter.

The correct answer is A, bromine. Bromine is a dark reddish-brown, dense, fuming liquid at ordinary room temperature, with a melting point of about minus 7 degrees Celsius and a boiling point of about 59 degrees Celsius. It is the only non-metal that is liquid under these conditions, just as mercury is the only metal that is liquid at room temperature, and the pair is a standard one-mark question. Option B, iodine, is a shining greyish-black solid at room temperature; it is remembered instead as a solid that sublimes readily to a violet vapour. Option C, sulphur, is a brittle yellow solid that melts only near 115 degrees Celsius. Option D, phosphorus, is also a solid at room temperature, white phosphorus being waxy and red phosphorus powdery. Gallium and caesium are sometimes offered as near answers, but both are metals that merely melt a little above room temperature.