Which of the following is NOT a characteristic of a good fuel?

The correct answer is B, spontaneous. In spontaneous combustion a substance catches fire by itself, without any external flame being brought near it. This happens when the slow oxidation of the substance in air releases more heat than the substance needs to reach its own ignition temperature, so the heat accumulates until the material bursts into flame. White phosphorus is the standard laboratory example, and heaps of coal dust, oily rags and damp haystacks catch fire in the same way. A, rapid combustion, is wrong because there the fuel burns quickly only after it is ignited by an outside source, as cooking gas does when a match is brought to it. C, complete combustion, describes the quality of burning, not how it starts: it means the carbon of the fuel is fully converted to carbon dioxide because oxygen is plentiful. D, explosive combustion, is wrong because it describes a reaction so fast that gas, heat, light and sound are produced in an instant, as in a firecracker.

The correct answer is A, Ghee. Calorific value is the heat released on completely burning one gram of a substance, and among the four foods listed ghee has the highest, because it is almost entirely fat. Fats yield roughly nine kilocalories per gram, more than twice what carbohydrates and proteins give, since the carbon and hydrogen in fat are far less oxidised to begin with and so have more energy to release on oxidation. B, maize, and C, rice, are wrong because both are mainly starch, a carbohydrate, which gives about four kilocalories per gram. D, meat, is wrong because it is largely protein and water; protein also gives about four kilocalories per gram, and the water in meat contributes nothing at all. The same reasoning explains why oils and butter are called energy-dense foods and why hydrogen, the lightest and least oxidised fuel, tops the calorific value table for fuels.

The correct answer is C, the outermost blue non-luminous zone. In this zone the supply of air is unrestricted, so combustion is complete and all the carbon of the fuel is converted to carbon dioxide, releasing the maximum heat. It gives little light and is faintly blue, which is why it is called the non-luminous zone, and a goldsmith directs exactly this part of the flame on gold or silver through a blowpipe to melt it. A is wrong because the innermost dark zone around the wick holds unburnt vaporised wax and receives the least oxygen, making it the coldest part of the flame. B is wrong because the middle yellow zone shows only partial combustion, where glowing unburnt carbon particles give the flame its light but not its greatest heat. D is wrong because the three zones differ sharply in both temperature and composition, and the whole point of studying flame structure is that difference.

The correct answer is A, Hydrogen. Its calorific value is about 150 kilojoules per gram, the highest of any known fuel, roughly three times that of petrol, and its only combustion product is water vapour, which makes it a clean fuel as well. For these reasons hydrogen is called the fuel of the future, although it is costly to produce and hard to store and transport safely because it is a very light gas that must be kept at high pressure or very low temperature. B is wrong because liquefied petroleum gas, a mixture of butane and propane, gives about 55 kilojoules per gram. C is wrong because petrol gives about 45 kilojoules per gram, the same order as kerosene and diesel. D is wrong because coal gives only about 25 to 33 kilojoules per gram, depending on its grade, and also leaves ash and smoke behind.

The correct answer is B, butane and propane. Liquefied petroleum gas is obtained during the refining of petroleum and from natural gas processing, and it is chiefly a mixture of butane and propane, which liquefy under moderate pressure at ordinary temperature and so can be stored in a cylinder. Because the gas has no smell of its own, a strong-smelling substance called ethyl mercaptan is added so that a leak can be detected. A is wrong because methane with some ethane is the composition of natural gas, which is supplied as compressed natural gas or through pipelines, not of cylinder gas. C is wrong because a mixture of carbon monoxide and hydrogen is water gas, an industrial fuel, and coal gas has a similar origin. D is wrong because ethylene and acetylene are industrial gases, acetylene being used with oxygen for welding because of its very hot flame.