In a vacuum flask, the vacuum between the two walls prevents heat transfer mainly by

The correct answer is B, the joule. Heat is a form of energy in transit, so it is measured in the same unit as work and energy, the joule, which is one newton metre.

Option A, the calorie, is an older and still familiar unit of heat, defined as the heat needed to raise the temperature of one gram of water by one degree Celsius; it equals about 4.2 joules, but it is not the SI unit. Option C, the kelvin, is the SI unit of temperature, not of heat, and the difference between heat and temperature is exactly what this question tests. Option D, the watt, is the unit of power, that is the rate of doing work or of transferring energy, one joule per second, so a heater rated in watts tells you how fast it supplies heat, not how much.

The correct answer is B, 4 degrees Celsius. Water behaves abnormally between zero and four degrees: it contracts as it is heated in that range and expands again above four degrees, so the same mass occupies its least volume, and therefore has its greatest density, at four degrees Celsius.

Option A is the freezing point, where water turns to ice; ice is in fact less dense than water, which is why it floats. Option C is the boiling point at normal atmospheric pressure, where water is at its most expanded as a liquid. Option D lies below the freezing point, where pure water at ordinary pressure is solid. The property matters in nature because in a pond in winter the water at four degrees sinks, the colder water floats and freezes at the top, and fish survive in the liquid water below.

The correct answer is B, about 540 calories per gram. This is the heat that one gram of water at a hundred degrees Celsius must absorb to become steam at the same temperature, with no rise in temperature at all; the same quantity is released when steam condenses back to water.

Option A, 80 calories per gram, is the latent heat of fusion of ice, the heat needed to melt one gram of ice at zero degrees, and interchanging the two values is the commonest error in this chapter. Option C, 336, is the same latent heat of fusion expressed in joules per gram rather than calories, offered here with the wrong unit. Option D, 2260, is the latent heat of vaporisation in joules per gram; it is the right quantity in the wrong unit, since 2260 joules is about 540 calories.

The correct answer is A, the conservation of energy. The first law says that the heat supplied to a system is equal to the increase in its internal energy plus the external work it does. Nothing is lost and nothing is created; energy only changes its form, which is the principle of conservation applied to heat.

Option B belongs to the second law, which introduces entropy and says that it never decreases in an isolated system. Option C belongs to the zeroth law, which says that two bodies each in thermal equilibrium with a third are in equilibrium with one another and which makes the measurement of temperature possible. Option D belongs to the third law, which says that the entropy of a perfect crystal tends to zero as the temperature tends to absolute zero and that absolute zero cannot be reached.

The correct answer is A, the zeroth law. It states that if body A is in thermal equilibrium with body C, and body B is also in thermal equilibrium with body C, then A and B are in thermal equilibrium with each other. That is precisely what allows a thermometer to be used: the thermometer plays the part of the third body, and two objects that give it the same reading have the same temperature.

Option B, the first law, deals with energy accounting and not with the meaning of temperature. Option C, the second law, fixes the direction in which heat flows and sets a limit on the efficiency of engines. Option D, the third law, concerns behaviour near absolute zero. The zeroth law was recognised and named after the other three had already been numbered, which is why it carries the number zero.