Understanding Solutions, Solubility, and Water Properties
Q. 1. Explain the terms
(i) Solution
(ii) Solute
(iii) Solvent
Ans.
(i) A solution is a homogeneous mixture of two or more components whose composition may be gradually changed by changing the relative amounts of components.
(ii) A solute is the substance that dissolves in the solvent.
(iii) A solvent is the medium that allows one or more components to dissolve in it to form a solution.
Q. 2. Explain any three factors which affect the solubility of a solid solute in a solvent.
Ans. The factors that affect the solubility are:
(a) Size of solute particles: Smaller the size of solute particles, greater is the total surface area exposed to the solvent, hence, there will be an increase in the solubility of that solute.
(b) Stirring: This causes the solvent to get closer to the solute and increase the solubility of that solute.
(c) Temperature: The solubility of a gas in liquid decreases with an increase in temperature while the solubility of most solids in water increases with an increase in temperature.
Q. 3. (a) If you are given some copper sulphate crystals, how would you proceed to prepare its saturated solution at room temperature?
(b) How can you show that your solution is really saturated?
Ans. (a) Preparation of saturated copper sulphate solution:
Take 100 g of distilled water in a beaker. To this add one gram of copper sulphate crystals. Stir this mixture with the help of a glass rod and dissolve the copper sulphate crystals. In the same way, go on dissolving more copper sulphate with constant and vigorous stirring. At one point in time, no more copper sulphate dissolves. It is called a saturated solution at that temperature.
(b) Take some saturated solution of copper sulphate prepared in a test tube and add some more copper sulphate crystals. The crystals do not dissolve but settle down. This shows that the solution is saturated as a saturated solution cannot dissolve any more of the solute without changing the temperature.
Q. 1. Normally, the 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 a rise in temperature.
Ans. The solubility of most solids in water increases with an increase in temperature. But it does not increase uniformly in all cases.
The substance whose solubility:
(a) increases rapidly with temperature – Potassium nitrate
(b) increases gradually with temperature – Potassium chloride
(c) increases slightly with temperature – Sodium chloride
(d) initially increases then decreases with a rise in temperature – Calcium sulphate
Q. 4. State any three observations when crystals of copper (II) sulphate are heated in a test-tube strongly.
Ans. We observe the following when crystals of copper (II) sulphate are heated in a test-tube strongly:
(i) Initially, drops of water are condensed at the mouth of the test tube (cooler part). An anhydrous residue is seen which is non-crystalline in nature.
(ii) The crystals lose their blue colouration on further heating.
(iii) On further heating, the residue changes to white powder and the evolution of steam is stopped.
Q. 1. Table salt becomes sticky on exposure to humid air during the rainy season. Explain.
Ans. This is because table salt contains impurities like magnesium chloride and calcium chloride. These impurities are deliquescent in nature, thus when exposed to humid air, they absorb the moisture from the air, become moist, lose their crystalline structure and finally dissolve in the absorbed water, forming a saturated solution.
Q. 2. What is the effect of temperature on the solubility of KNO3 and CaSO4 in water?
Ans. The solubility of potassium nitrate (KNO3) in water increases with an increase in temperature.
The solubility of calcium sulphate (CaSO4) in water decreases with an increase in temperature.
Q. 3. Explain why:
(a) A solution is always clear and transparent.
(b) The solute cannot be separated from a solution by filtration.
Ans. (a) Water-soluble substances dissolve completely in water and disappear. Water retains its property and remains clear and transparent in a solution.
(b) As the solute gets dissolved in the solvent, it cannot be separated by filtration. However, if the solute is partially dissolved it can be separated by filtration.
Q. 4. Explain why:
(a) Fused CaCl₂ or conc. H₂SO₄ is used in a desiccator.
(b) Effervescence is seen on opening a bottle of soda water.
Ans. (a) Fused CaCl₂ or conc. H₂SO₄ are deliquescent by nature and they absorb moisture. Hence, they are used in a desiccator.
(b) The solubility of carbon dioxide in water is increased when the water surface is subjected to higher pressure. Thus, on opening a soda water bottle, the dissolved gas immediately bubbles out or shows effervescence since the pressure on the surface of the water suddenly decreases.
Q. 5. What are drying or desiccating agents? Give two examples.
Ans. Drying or desiccating agents are substances that readily absorb moisture from other substances without chemically reacting with them. e.g., Conc. sulphuric acid, alumina Al₂O₃, phosphorus pentoxide P2O5.
Q. 5. Explain why a hot saturated solution of potassium nitrate forms crystals as it cools.
Ans. When a saturated solution of potassium nitrate cools, the excess of nitrate separates from the solution to form crystals.
Q. 7. The solubility of NaCl at 40°C is 36.5 g. What is meant by this statement?
Ans. The solubility of NaCl at 40°C is 36.5 g means 36.5 g of NaCl dissolves in 100 g of water at the temperature of 40°C.
Q1. State three different states of water. Justify.
Ans. Water exists in all three states, as a solid (ice), liquid (water) and as a gas (water vapour). Normally, water exists in a liquid state. It freezes at 0°C and gets converted to ice which is a solid-state. Water boils at 100°C and gets converted to a gaseous state (water vapour or steam).
Q. 3. Why is water considered a compound?
Ans. Water is formed by the covalent bonding (sharing of electrons) of two elements hydrogen and oxygen in the ratio of 1:8 by mass. Thus, water is considered a compound.
Q. 4. ‘Water is a universal solvent’. Comment.
Ans. Water dissolves many substances like solids, liquids, gases and even inorganic substances as it has a high dielectric constant.
Q. 2. State any three importance of dissolved salts in water?
Ans. Importance of dissolved salts in water are as follows:
- They are required for the plant’s growth and development.
- They are responsible for the taste of water.
- They provide required minerals needed by our body.
Q. 2. State the importance of the solubility of CO2 and O₂ in water.
Ans. Marine life utilizes oxygen from the air dissolved in water for their respiration. Aquatic plants utilize dissolved carbon dioxide for preparing their food (photosynthesis).
Q. 3. How is air dissolved in water different from ordinary air?
Ans. The composition of air dissolved in water is 33% oxygen, 66% nitrogen and 1% carbon dioxide. But the composition of ordinary air is 21% oxygen, 78% nitrogen and 0.01% carbon dioxide. Oxygen is more soluble in water compared to nitrogen.
Q. 1. Explain why:
(a) Boiled or distilled water tastes flat.
(b) Ice at 0° C is cooler than water at the same temperature.
(c) Steam causes serious burns than boiled water.
Ans. (a) Boiled or distilled water does not have any minerals, salts or dissolved gases in it thus it tastes flat.
(b) Due to the high specific latent heat of fusion of ice, ice at 0 degrees Celcius has a greater cooling effect than water at 0°C.
(c) Due to the high specific latent heat of condensation, steam causes far more serious burns than water.
3. Explain why:
(a) Rainwater does not leave behind concentric rings when boiled.
(b) Air dissolved in water contains a higher proportion of oxygen.
(c) If distilled water is kept in a sealed bottle for a long time, it leaves etchings on the surface of the glass.
(a) Rainwater does not contain dissolved salts and thus does not form concentric rings when boiled.
(b) Among the two main compositions of air, oxygen is more soluble in water compared to nitrogen. Thus, air dissolved in water contains a higher proportion of oxygen.
(c) Substances considered insoluble in water dissolve in water in traces. Due to this reason, distilled water kept in a sealed bottle for a long time, leaves etchings on the inside surface of the glass.
3. (a) Why does the temperature in Mumbai and Chennai not fall as low as it does in Delhi?
(b) Give the properties of water responsible for controlling the temperature of our body.
Solution
(a) The temperature in coastal cities like Mumbai and Chennai do not fall as low as in Delhi because these cities are situated in the coastal areas. Due to high specific heat capacity, the presence of a large amount of water is able to modify the climate of the nearby land areas making them warmer in winter and cooler in summer. So, the temperature does not fall as low as it does in Delhi.
(b) Our body is almost 65% water, and it has the property of specific heat. Due to high specific heat capacity, the presence of a large amount of water is able to modify the climate of the body and control the temperature of our body, which is warm in winter and cool in summer.
5. What causes the violence associated with torrential rain?
Solution
The sudden release of the latent heat of condensation causes the violence associated with torrential rain.
6. (a) Which property of water enables it to modify the climate?
(b) The density of water varies with temperature. What are its consequences?
(c) What is the effect of impurities present in the water on the melting point and boiling point of water?
Solution
(a) Due to the high specific heat capacity, the presence of a large amount of water is able to modify the climate.
(b) The property of anomalous expansion of water enables marine life to exist in the colder regions of the world because even when water freezes on the top, it is still liquid below the ice layer, as the density of water is greater than that of ice.
(c) The boiling point of water increases due to the presence of dissolved impurities.
The freezing point of water decreases due to the presence of dissolved impurities.
7. How do fishes and aquatic animals survive when the pond gets covered with thick ice?
Solution
Even though ponds are covered with thick ice, beneath there will be water because of the maximum density level of water which helps the fishes and aquatic animals survive.
5. (a) Define
(i) Henry’s law
(ii) Crystallisation
(iii) Seeding.
(b) State the different methods of crystallisation.
Solution
(a) (i) Henry’s law: At any given temperature, the mass of a gas dissolved in a fixed volume of a liquid or solution is directly proportional to the pressure on the surface of a liquid.
(ii) Crystallisation: It is the process by which crystals of a substance separate out on cooling its hot saturated solution.
(iii) Seeding: This process of inducing crystallisation by adding a crystal of a pure substance into a saturated solution is called seeding.
(b) In the laboratory, crystals may be obtained by the following methods:
- By cooling a hot saturated solution gently
- By cooling a fused mass
- By sublimation
- By slowly evaporating a saturated solution
7. Give the names and formulae of two substances in each case:
(a) Hydrated substance
(b) Anhydrous substance
(c) Liquid drying agent
(d) A basic drying agent
Solution
(a) (i) Washing soda crystals: Na2CO3.10H2O
(ii) Blue vitriol: CuSO4.5H2O
(b) (i) Table salt: NaCl
(ii) Nitre: KNO3
(c) Sulphuric acid: H2SO4
(d) Quick lime: CaO
12. State the term:
(a) A solution where the 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 compound.
Solution
(a) Non-aqueous Solution
(b) Deliquescence
(c) Hydrated substance
(d) Hygroscopic
(e) Efflorescence
(f) Dehydrating agent