Chemistry
Explain why :
(a) water is an excellent liquid to use in cooling systems.
(b) a solution is always clear and transparent.
(c) lakes and rivers do not suddenly freeze in the winters.
(d) the solute cannot be separated from a solution by filtration.
(e) fused CaCl2 or conc. H2SO4 is used in a desiccator.
(f) effervescence is seen on opening a bottle of soda water.
(g) Table salt becomes sticky on exposure to humid air during the rainy season.
Answer
(a) Water is an effective coolant. By allowing water to flow in pipes around the heated parts of a machine, heat energy from such parts is removed. Water in pipes can extract more heat from the surroundings without much rise in it's temperature because of it's high specific heat capacity. This is why radiators in car and generator use water for cooling.
(b) Solutions are homogeneous in nature. The solute particles are dispersed uniformly throughout the solvent. The size of particles of a true solution is very small about 10-10 m. Due to homogeneity and small size, they do not scatter light significantly. Hence, a solution is always clear and transparent.
(c) Rivers and lakes do not freeze suddenly 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.
(d) The size of particles of a true solution is about 10-10 m. In a solution, the solute particles and the solvent molecules cannot be distinguished even under a microscope. Due to such small size of particles, the solute cannot be separated from a solution by filtration.
(e) Fused CaCl2 is deliquescent in nature, absorbs moisture and hence used as drying agent or desiccating agent, similarly, conc. sulphuric acid is hygroscopic in nature and can remove moisture from other substances; Hence, they are used as drying agents.
(f) The solubility of carbon dioxide in water under normal atmospheric pressure is rather low, but when the water surface is subjected to higher pressure, a lot more carbon dioxide gas dissolves in it, because, an increase in pressure on the surface of water increases the solubility of the gas in water. On opening the soda water bottle, the dissolved gas rapidly bubbles out since pressure on the surface of the water suddenly decreases. Hence, effervescence is seen on opening a bottle of soda water.
(g) Table salt [sodium chloride] contains impurities like magnesium chloride and calcium chloride, which are deliquescent. Hence, table salt absorb moisture in rainy season and turns sticky.
Related Questions
State the term : (Do not give examples)
(a) A solution where solvent is a liquid other than water.
(b) When a substance absorbs moisture on exposure to moist air and dissolves in the absorbed water and turned to solution.
(c) A substance which contains water of crystallisation.
(d) When a substance absorbs moisture from the atmosphere, but does not form a solution.
(e) When a compound loses its water of crystallisation on exposure to dry air.
(f) The substance that can remove hydrogen and oxygen atoms in the ratio of 2 : 1 (in the form of water) from the compounds.
Complete the following table :
Common Name Chemical Name Formula Acid, base or salt Efflorescent, hygroscopic or deliquescent substance Solid caustic potash Quick lime Oil of vitriol Washing soda Solid caustic soda Blue vitriol Normally, solubility of a crystalline solid increases with temperature. Does it increase uniformly in all cases ? Name a substance whose solubility :
(a) increases rapidly with temperature.
(b) increases gradually with temperature.
(c) increases slightly with temperature.
(d) initially increases then decreases with rise in temperature.
What are drying or desiccating agents. Give examples.