Question

Which of the following does not have a fixed melting point/boiling point?
(a) Gold
(b) Ethanol
(c) Air
(d) Oxygen

Answer 1

Hint: The melting point of a solid is the temperature at which the liquid phase and the solid phase exist in equilibrium with each other. Similarly the boiling point of a liquid is the temperature at which the vapour phase and the liquid phase exist in equilibrium with each other. Pure substances have a fixed melting/boiling point.

Complete step by step answer:
Before solving this question, we need to look into the definition of pure substances. Pure substances are built of only one kind of particle. They have a uniform structure. They are divided into elements and compounds. An element is only made up of only one type of atom. An element could be a metal, non-metal or a metalloid. For example silicon, iron etc. Compounds are formed from elements when they combine in a fixed ratio. For example potassium chloride is a compound.

Now, Let us look into all the options one by one:
(a) Gold: It is an element and hence a pure substance. Its symbol is Au which is taken from its Latin name ‘Aurum’. It is a heavy element with an atomic number of 79. It is ductile and malleable; that is why it can be made into jewellery. It is soft and one of the least reactive chemical elements (it is resistant to many acids but does dissolve in aqua regia).Its melting point is $ 1064^{ o }{ C }$ and its boiling point is $ 2700^{ o }{ C }$.

(b) Ethanol: It is a compound with the chemical formula $ { C }_{ 2 }{ H }_{ 5 }-OH$. Since it is a compound; it is a pure substance. It is volatile and exists as a liquid at room temperature. It has a characteristic odour and is used as an active ingredient in alcoholic drinks. Its boiling point is $ 78.37^{ o }{ C }$ and its melting point is $ -114.1^{ o }{ C }$.

(c) Air: The earth’s atmosphere is composed of air. The atmosphere above the earth’s surface is an ozone layer that protects us from the harmful radiations that come from outer space. Air comprises about 78% of nitrogen, 21% of oxygen, 0.9% of argon, 0.04% of carbon dioxide along with small amounts of other gases. Since it is a mixture of different gases such that each gas has its own boiling/melting point, therefore air does not have a fixed boiling/melting point.

(d) Oxygen: It is a gas with the chemical formula $ { O }_{ 2 }$. It is a colourless and odourless gas and is a poor conductor of heat and electricity. Its boiling point is $ -183^{ o }{ C }$ and its melting point is $ -218.8^{ o }{ C }$.
So, the correct answer is “Option C”.

Note: Sometimes the melting point and the freezing point for a substance have the same value but they could also be different for other substances. The melting point is defined for a solid and is the temperature at which the vapour pressure of the liquid phase and the solid phase are at equilibrium (and equal). If the temperature of a solid is increased above its melting point then the solid will melt but if we decrease the temperature of a liquid below the melting point then the liquid could freeze or it may still exist as a liquid. If the liquid still exists in the liquid phase below its melting point then it is called supercooling and if we disturb it by shaking it or by any other method then the liquid will freeze. Hence the freezing point for a liquid could be lower than its melting point.