Water

Question 1

GR: Water is known as the universal solvent

Answer 1

1. Water has the ability to dissolve a large number of substances eg: salts, sugar, etc.
2. It also dissolves a large number of gases eg: hydrogen chloride, ammonia, etc.
3. Hence water dissolves almost every common substance and is therefore known as universal solvent.

Question 2

Solvent

Answer 2

Liquid or medium of dissolution which allows the solute to dissolve in it so as to form a solution is called a solvent.

Question 3

Solute

Answer 3

Substance which dissolves or disappears in the solvent to form a solution is called a solute.

Question 4

Solution

Answer 4

A homogeneous mixture of a solute in a solvent whose composition may be gradually changed by changing the relative amount of its components.

Question 5

Dilute solution

Answer 5

A solution in which the amount of solute is relatively small compared to the amount of solvent in a given mass of the solution is called a dilute solution.

Question 6

Concentrated solution

Answer 6

A solution in which the amount of solute is relatively large as compared to the amount of solvent in a given mass of the solution is called a concentrated solution.

Question 7

Characteristics of a true solution

Answer 7

1. Nature: it is clear, transparent and homogenous in nature.
2. Particle size: Particle size is molecular and the particles
   - Can pass through the pores of filter paper.
   - Cannot be seen under a microscope.
   - Do not settle down.
3. Separation:
   - Solute can be recovered from the solution by physical means and not by chemical means.

Question 8

GR: A true solution is a mixture and not a compound

Answer 8

1. Solute can be recovered from a true solution by physical means and not by chemical means.

Question 9

Unsaturated solution

Answer 9

A solution which can dissolve more of the solute at a given temperature is called an unsaturated solution at that temperature.

Question 10

Saturated solution

Answer 10

A solution which cannot dissolve more of the solute at a given temperature is called the saturated solution at that temperature.

Question 11

Conversion from saturated to unsaturated solution

Answer 11

By heating the solution slowly or by increasing the amount of solvent

Question 12

Supersaturated solution

Answer 12

A solution which contains more of the solute at a given temperature than that present in a saturated solution is known as a super saturated solution.

Question 13

How are supersaturated solutions prepared?

Answer 13

1. A saturated solution of a solute is prepared in boiling water.
2. If the above solution is cooled excess solute separates out from the hot saturated solution.
3. The hot solution now contains more solute dissolved in it than it can hold at that given temperature.

Question 14

Exceptions to the preparation of supersaturated solutions

Answer 14

Substances like Na2SO4.10H2O do not form crystals after their hot saturated solutions are slightly cooled.

Question 15

Solubility

Answer 15

The solubility of a solute in a solvent at a particular temperature is the maximum amount of the solute in grams that will saturate 100g of the solvent at that temperature.

Question 16

Solubility curve

Answer 16

If the solubility of a solute in a given solvent is plotted against its respective temperature, a graph showing the effect of temperature on solubility of the substance is obtained. This graph is called the solubility graph or curve.,

Question 17

Qualitative effect of temperature on solubility

Answer 17

With rise in temperature, solublilty:

1. Increases - KNO3, other examples - KClO3, NaNO3, CuSO4, NH4Cl
2. increases slightly - NaCl, other examples: KCl, Ca(OH)2[below 70C]
3. Decreases: CaSO4 , other examples - Ca(OH)2[above 70 degrees],

Question 18

Crystals

Answer 18

When a hot saturated solution is cooled, the excess solid is generally thrown out in the form of particles having definite geometric shape. These particles are called crystals.

Question 19

Characteristics of Crystals

Answer 19

1. Crystals are homogenous solids, arranged symmetrically.
2. Crystals are bounded by plane surfaces, meeting at sharp edges at definite angles to each one another. They also have a regular definite shape.
3. Some shapes that they have are (any 4):
   - Cubic
   - Rhombic
   - Tetragonal
   - Monoclinic
   - Triclinic
   - Hexagonal

Question 20

Crystallisation

Answer 20

The process by which crystals are separated or deposited from a hot saturated solution of a substance on cooling gently is called crystallisation.

Question 21

Conditions for crystallisation

Answer 21

1. The hot saturated solution should be cooled slowly followed by a slow evaporation of the same.
2. Rapid cooling of a hot saturated solution would form an amorphous mixture, which is not well defined and does not have a fixed geometric shape. Slow evaporation enhances the concentration of the solute causing formation of well defined crystals.

Question 22

Water of Crystallisation

Answer 22

The fixed number of water molecules which enters into a loose chemical combination with the substance when the substance is crystallised from its hot saturated solution is known as water of crystallisation.

Question 23

Hydrated substances

Answer 23

Substances which contain a fixed number of water molecules in loose chemical combination with itself are known as hydrated substances.

Question 24

Anhydrous substances

Answer 24

Substances which do not contain fixed number of water molecules in loose chemical combination with itself are known as anhydrous substances.

Question 25

Efflorescence

Answer 25

The phenomenon by which crystalline hydrated salts on exposure to the atmosphere lose their moisture[water of crystallisation] partially or completely to the atmosphere and turn into the amorphous state.

Question 26

Efflorescent substance

Answer 26

Crystalline hydrated salts which lose their moisture[water of crystallisation] partially or completely on exposure to the atmosphere and change into the amorphous state.

Question 27

Deliquescence

Answer 27

The phenomenon by which water soluble salts on exposure to the atmosphere absorb moisture from the atmosphere and dissolve in the absorbed moisture to turn into the liquid state is known as deliquescence.

Question 28

Deliquescent substances

Answer 28

Water soluble salts which on exposure to the atmosphere absorb moisture from the atmosphere and dissolve in the absorbed moisture to turn into the liquid state are known as deliquescent substances.

Question 29

GR: Efflorescence and Deliquescence occur

Answer 29

1. Efflorescence occurs when the vapour pressure of the hydrated crystals exceeds the vapour pressure of the atmospheric humidity[hence it is minimum in humid days]. 2. Deliquescence occurs when the vapour pressure of the deliquescent crystal solution is very low compared to the atmospheric humidity[hence it is minimum on dry days]/

Question 30

GR: Common salt turns moist on exposure to air.

Answer 30

1. Common salt is not deliquescent but it contains a small amount of calcium and magnesium chloride impurities which are deliquescent. 2. Hence the impurities absorb moisture from air, thereby turning common salt sticky and wet(especially during humid conditions).

Question 31

Name a liquid hygroscopic substance which acts as a drying agent and dehydrating agent

Answer 31

Concentrated Sulphuric acid

Question 32

State 3 examples of Efflorescent substances and the resulting substances for these 3

Answer 32

1. Washing Soda(Na2CO3.10H2O) -> Na2CO3.H2O(monohydrate) 2. Copper sulphate(CuSO4.5H2O) -> CuSO4(anhydrate) 3. Glauber's salt(Na2SO4.10H2O) -> Na2SO4(anhydrate)

Question 33

State 4 examples for Deliquescent substances

Answer 33

1. Iron[iii] chloride(FeCl3) 2. Calcium chloride(CaCl2) 3. Sodium Hydroxide(NaOH) 4. Potassium Hydroxide(KOH) 5. Magnesium Chloride(MgCl2)

Question 34

Differentiate between Hygroscopic substances and Deliquescent substances

Answer 34

- Hygroscopic substances like deliquescent substances absorb moisture from the atmosphere when exposed to it but unlike deliquescent substances do no change their state. - Deliquescent substances on the other hand are crystalline solids while hygroscopic substances may be amorphous solids or liquids.

Question 35

State 4 examples of Hygroscopic substances

Answer 35

1. Quicklime [CaO] 2. Anhydrous calcium chloride [CaCl2] 3. P2O5 4. Silica Gel 5. Concentrated sulphuric acid [H2SO4]