10th Science Unit 9 Solutions Book Back Answers

1. a) homogeneous
2. b) heterogeneous
3. c) homogeneous and heterogeneous
4. d) non homogeneous

1. A solution is formed when the solute dissolves and gets distributed uniformly throughout the solvent, so its composition is the same everywhere.

2. Such a uniform mixture (like salt in water) is homogeneous.

1. a) 2
2. b) 3
3. c) 4
4. d) 5

1. A binary solution is made of one solute and one solvent.

2. So it contains 2 components, e.g. copper sulphate dissolved in water.

1. a) Acetone
2. b) Benzene
3. c) Water
4. d) Alcohol

1. Most substances dissolve in water, so water is called the 'universal solvent'.

2. Acetone, benzene and alcohol are non-aqueous solvents.

1. a) Saturated solution
2. b) Un saturated solution
3. c) Super saturated solution
4. d) Dilute solution

1. When no more solute can dissolve in a definite amount of solvent at a given temperature, the limit of dissolution is reached.

2. Such a solution is a saturated solution, e.g. 36 g NaCl in 100 g water at 25°C.

1. a) sodium chloride in water
2. b) glucose in water
3. c) copper sulphate in water
4. d) sulphur in carbon-di-sulphide

1. A non-aqueous solution has a solvent other than water.

2. Sulphur dissolved in carbon disulphide uses CS$_2$ as solvent, so it is non-aqueous; the other three are aqueous solutions.

1. a) No change
2. b) increases
3. c) decreases
4. d) no reaction

1. Pressure affects only the solubility of a gas in a liquid.

2. By Henry's law, solubility of a gas is directly proportional to the pressure of the gas over the solution, so on increasing pressure solubility increases.

1. a) 12g
2. b) 11g
3. c) 16g
4. d) 20g

1. Salt needed for saturation in 100 ml water = 36 g.

2. Salt already dissolved = 25 g.

3. Extra salt required = $36 - 25 = 11$ g.

1. a) 25 ml alcohol in 100 ml of water
2. b) 25 ml alcohol in 25 ml of water
3. c) 25 ml alcohol in 75 ml of water
4. d) 75 ml alcohol in 25 ml of water

1. Volume percentage = volume of solute in 100 ml of solution.

2. So 25% alcohol means 25 ml alcohol in 100 ml solution, i.e. 25 ml alcohol mixed with 75 ml of water.

1. a) Strong affinity to water
2. b) Less affinity to water
3. c) Strong hatred to water
4. d) Inertness to water

1. Deliquescent substances are so strongly attracted to water that they absorb enough moisture from air to dissolve completely.

2. Hence deliquescence is due to strong affinity to water.

1. a) ferric chloride
2. b) copper sulphate penta hydrate
3. c) silica gel
4. d) none of the above

1. Hygroscopic substances absorb moisture from air without changing their physical state and are used as drying agents.

2. Silica gel (SiO$_2$) is such a substance; ferric chloride is deliquescent and CuSO$_4$.5H$_2$O is a hydrated salt.

1. In a solution, the component present in a larger amount by weight is the solvent.

2. The component present in lesser amount by weight is the solute.

1. Here the solute is a liquid and the solvent is a solid.

2. From the table of binary solutions, mercury dissolved in sodium forms an amalgam.

1. Solubility is the number of grams of a solute dissolved in 100 g of solvent to form a saturated solution at a given temperature and pressure.

2. Solubility $=\dfrac{\text{Mass of the solute}\times100}{\text{Mass of the solvent}}$

1. The rule is 'like dissolves like'.

2. Common salt, a polar compound, dissolves readily in water, a polar solvent.

1. Volume percentage is calculated using the volume of solute and solution.

2. On heating, liquids expand, so the volume of the solution increases and the volume percentage value falls.

1. Blue vitriol is the common name of copper (II) sulphate pentahydrate, $\mathrm{CuSO_4 \cdot 5H_2O}$.

2. Gypsum is the common name of calcium sulphate dihydrate, $\mathrm{CaSO_4 \cdot 2H_2O}$.

3. Deliquescent substances absorb enough moisture from air and dissolve completely — caustic soda, $\mathrm{NaOH}$, is an example.

4. Hygroscopic substances absorb moisture without changing their physical state — quick lime, $\mathrm{CaO}$, is an example (used as a drying agent).

1. A solution with two components (one solute and one solvent) is called a binary solution.

2. A solution containing three components is called a ternary solution.

1. The component present in lesser amount (by weight) is the solute.

2. The component present in larger amount (by weight) is the solvent, which acts as the dissolving medium.

1. Here water is the solvent.

2. A solution in which water is the solvent is an aqueous solution; non-aqueous solutions use solvents like benzene, ether or carbon disulphide.

1. Green vitriol is iron (II) sulphate heptahydrate, $\mathrm{FeSO_4 \cdot 7H_2O}$.

2. $\mathrm{MgSO_4 \cdot 7H_2O}$ is Epsom salt (magnesium sulphate heptahydrate).

1. Silica gel ($\mathrm{SiO_2}$) is a hygroscopic substance.

2. Hygroscopic substances absorb moisture from the air without changing their physical state, so they are used as drying agents.

1. A solution is a homogeneous mixture of two or more substances.

2. It has two parts: the solute (present in lesser amount by weight) and the solvent (present in larger amount by weight).

3. The solute is uniformly distributed throughout the solvent, so the mixture looks the same all over. e.g. common salt dissolved in water.

1. A solution must consist of at least two components — one solute and one solvent.

2. A solution made of only one solute and one solvent (two components) is called a binary solution.

3. Example: copper sulphate (solute) dissolved in water (solvent) forms a binary solution.

1. i) Gas in liquid – carbon dioxide dissolved in water (soda water).

2. ii) Solid in liquid – sodium chloride dissolved in water.

3. iii) Solid in solid – copper dissolved in gold (alloys).

4. iv) Gas in gas – mixture of helium and oxygen gases.

1. Aqueous solution: the solution in which water acts as the solvent. e.g. common salt in water, sugar in water, copper sulphate in water.

2. Non-aqueous solution: the solution in which any liquid other than water acts as the solvent. Alcohol, benzene, ether, carbon disulphide and acetone are non-aqueous solvents.

3. Example of non-aqueous solution: sulphur dissolved in carbon disulphide; iodine dissolved in carbon tetrachloride.

1. Volume percentage is the percentage by volume of solute (in ml) present in the given volume of the solution.

2. $\text{Volume percentage} = \dfrac{\text{Volume of the solute}}{\text{Volume of the solution}} \times 100$

3. It is expressed as v/v and is used when both solute and solvent are liquids. e.g. 10% v/v ethanol means 10 ml ethanol in 100 ml solution.

1. The solubility of a gas in a liquid decreases with increase in temperature.

2. So cold water of cold regions holds a greater amount of dissolved oxygen than warm water.

3. Aquatic animals need this dissolved oxygen to breathe, hence they live more in cold regions.

1. When ionic substances crystallize from their saturated aqueous solution, their ions attract water molecules which get chemically attached in a definite ratio (hydration).

2. The definite number of water molecules present in the crystalline substance is called water of crystallization.

3. Salts containing water of crystallization are called hydrated salts. e.g. $CuSO_4.5H_2O$ (blue vitriol), $MgSO_4.7H_2O$ (Epsom salt).

1. Solubility of a solid solute in water increases with increase in temperature, so hot water dissolves a large amount of copper sulphate.

2. On cooling, the solubility decreases and the solution can no longer hold all the dissolved copper sulphate.

3. The excess solute separates out from the saturated solution and appears as crystals of $CuSO_4.5H_2O$.

1. Deliquescent substances absorb enough moisture from air to dissolve completely: Calcium chloride.

2. Hygroscopic substances absorb moisture from air without changing their physical state: Conc. sulphuric acid and Silica gel.

3. Copper sulphate pentahydrate and Gypsum salt are neither — they are hydrated salts already containing water of crystallization.

(a)

1. MgSO$_4$·7H$_2$O is Epsom salt (magnesium sulphate heptahydrate); its water of crystallization is 7.

2. On gentle heating, it loses all its seven water molecules and becomes anhydrous magnesium sulphate.

3. Equation: $\mathrm{MgSO_4 \cdot 7H_2O} \xrightarrow{\text{Heating}} \mathrm{MgSO_4} + 7\mathrm{H_2O}$

4. The change is reversible — on adding a few drops of water or on cooling, the anhydrous salt again turns back into the hydrated salt: $\mathrm{MgSO_4} + 7\mathrm{H_2O} \xrightarrow{\text{Cooling}} \mathrm{MgSO_4 \cdot 7H_2O}$

(b)

1. Solubility is a measure of how much of a solute can be dissolved in a specified amount of a solvent.

2. Definition: Solubility is the number of grams of a solute that can be dissolved in 100 g of a solvent to form its saturated solution at a given temperature and pressure.

3. It is expressed as $\text{Solubility} = \dfrac{\text{Mass of the solute}}{\text{Mass of the solvent}} \times 100$

4. Example: 36 g of sodium chloride dissolves in 100 g of water to form a saturated solution at 25°C, so the solubility of NaCl in water is 36 g at 25°C.

1. Absorption of moisture: Hygroscopic substances absorb moisture from the atmosphere at ordinary temperature but do not dissolve in it; deliquescent substances absorb enough moisture to get completely dissolved in the absorbed water, forming a saturated solution.

2. Change of physical state: Hygroscopic substances do not change their physical state on exposure to air; deliquescent substances change their physical state (solid to solution) on exposure to air.

3. Nature: Hygroscopic substances may be amorphous solids or liquids; deliquescent substances are crystalline solids that lose their crystalline shape.

4. Examples: Hygroscopic — conc. H$_2$SO$_4$, P$_2$O$_5$, quick lime (CaO), silica gel. Deliquescent — caustic soda (NaOH), caustic potash (KOH), ferric chloride (FeCl$_3$).

5. Hygroscopic substances are used as drying agents; deliquescence is maximum when the temperature is low and the atmosphere is humid.

1. Given: mass of the solute (sugar) = 45 g; mass of the solvent (water) = 180 g.

2. Formula: $\text{Mass percentage} = \dfrac{\text{Mass of the solute}}{\text{Mass of the solute} + \text{Mass of the solvent}} \times 100$

3. Mass of the solution = $45 + 180 = 225$ g

4. Substituting: $\text{Mass percentage} = \dfrac{45}{225} \times 100$

5. Result: $= 20\%$

1. Given: volume of the solute (ethanol) = 3.5 litres; volume of the solution = 15 litres.

2. Formula: $\text{Volume percentage} = \dfrac{\text{Volume of the solute}}{\text{Volume of the solution}} \times 100$

3. Substituting: $\text{Volume percentage} = \dfrac{3.5}{15} \times 100$

4. Result: $= 23.33\%$ (v/v)

1. Sugar is a solid solute and water is the liquid solvent.

2. Generally, the solubility of a solid solute in a liquid solvent increases with increase in temperature.

3. In hot water the particles move faster and the solute dissolves more quickly and in greater amount (a greater amount of sugar dissolves in warm water than in cold water).

4. So Vinu, who uses hot water, gets faster dissolution; Sarath's sugar in cold water dissolves slowly.

1. 'A' is the blue coloured crystalline salt copper sulphate pentahydrate, $CuSO_4.5H_2O$ (blue vitriol).

2. On gentle heating it loses its five water molecules of crystallization and becomes colourless anhydrous copper sulphate, which is 'B' ($CuSO_4$).

3. When a few drops of water are added to 'B', it regains the water of crystallization and turns back into blue 'A'.

4. Equation: $CuSO_4.5H_2O \xrightarrow{\text{Heating}} CuSO_4 + 5H_2O$ and $CuSO_4 + 5H_2O \xrightarrow{\text{Cooling}} CuSO_4.5H_2O$.

1. Cool drinks are carbonated beverages — carbon dioxide gas dissolved in liquid under pressure.

2. By Henry's law, the solubility of a gas in a liquid is directly proportional to the pressure of the gas over the solution; when pressure decreases, solubility of the gas decreases.

3. At the top of a hill the atmospheric pressure is much lower than at the foot of the hill.

4. So at the top, more dissolved $CO_2$ escapes from the drink, giving more fizz.