Explain why:

(a) Boiled or distilled water tastes flat.

(b) Ice at zero degree centigrade has greater cooling effect than water at 0°C.

(c) Burns caused by steam are more severe than burns caused by boiling water.

(d) Rivers and lakes do not freeze easily?

(e) Air dissolved in water contains a higher proportion of oxygen.

(f) If distilled water is kept in a sealed bottle for a long time, it leaves etchings on the surface of the glass.

(g) Rain water does not leave behind concentric rings when boiled.

(a) Pure water is tasteless. The taste in water is due to the gases and solids dissolved in it i.e., impurities present in it. As boiled and distilled water are pure containing no impurities hence they taste flat i.e., are tasteless.

(b) The latent heat of fusion of ice is 336 J g^-1 so it absorbs 336 J of heat and changes to water at 0°C and then water at 0°C absorbs heat and temperature is raised. Therefore, ice at 0°C absorbs extra heat in comparison to water at 0°C (due to latent heat of fusion). Hence, ice at 0°C has greater cooling effect than water at 0°C.

(c) Steam has a higher heat content on account of high specific latent heat of condensation that is 2268 J g^-1. Hence, steam at 100°C carries more heat than water. Therefore, burns caused by steam are more severe than burns caused by boiling water.

(d) Rivers and lakes do not freeze easily because the specific latent heat of fusion of ice is sufficiently high (= 336 J g^-1). The water in lakes and rivers will have to liberate a large quantity of heat to the surrounding before freezing. The layer of ice formed over the water surface, being a poor conductor of heat, will also prevent the loss of heat from the water of lake, hence the water does not freeze all at once.

(e) Air dissolved in water contains a higher proportion of oxygen because oxygen is more soluble in water compared to nitrogen. The composition of air dissolved in water is 33% oxygen compared to 21% in ordinary air.

(f) Substances that are apparently insoluble in water actually dissolve in it in traces. Even, when we put water in a glass vessel, an extremely small amount of glass dissolves in it. It is for this reason that when distilled water is kept in a sealed bottle for a long time, it leaves etchings on the inside surface of the glass.

(g) As rainwater does not contain dissolved solids hence, it does not leave behind concentric rings when boiled.