How do you establish that air is a mixture and not a pure substance?
Air is a homogeneous mixture of several gases.
mostly of nitrogen (~78%) and oxygen(~20%). There are other gases as well; argon slightly less than 1%, and of course, carbon dioxide, (~.04%). Therefore it can be established as follows:
(i) The amounts of gases present in air at different places varies only marginally.
(ii) No definite formula can be assigned to air.
(iii) The different gases present in air are not separable by a physical process : fractional distillation of liquid air.
(iv) The properties of air are the properties of its constituents. Air supports combustion because of oxygen present in it. Air turns lime water milky due to the presence of carbon dioxide in it.How will you prepare a saturated solution of copper sulphate in water at 50°C? What will happen if this solution is allowed to cool?
Take water in a beaker. Heat the water gently to 50°C and slowly add copper sulphate powder, stirring the solution continuously with the help of a glass rod. Continue adding the compound, keeping constant temperature of solution 50°C, till some compound remains undissolved and settles down. Then quickly filter the solution. The solution so obtained is a saturated solution of copper sulphate at 50°C. On cooling, the solubility of copper sulphate will decrease and it will start settling down at the bottom of the beaker.
How do you express concentration of a solution?
Concentration of a solution may be expressed in terms of percentage by mass of solute per 100 gram of the solution. 
Thus, if a solution is 5%, it means it is 5 gram of a solute dissolved in 100 gram of the solution or it contains 5 gram solute and 95 gram solvent.
Concentration of a solution may also be expressed as mass of solute dissolved in 100 cm3 of the solution. Thus, 10% sugar solution by volume means 10 g sugar dissolved in 100 cm3 of solution.
List the various types of homogeneous and heterogeneous mixtures with examples.
(i) Homogeneous mixtures
(a) Solid solution. Alloys, e.g., brass, steel, bronze.(b) Liquid solution. Sugar solution in water, sea water.
(c) Gaseous solution. Air, natural gas.
(ii) Heterogeneous mixtures:
(a) Solid + Solid : sugar and sand, iron filing and sulphur, copper sulphate and potassium permanganate.
(b) Solid + Liquid : Pastes, suspensions etc.
(c) Solid + Gas : Smoke.
(d) Liquid + Liquid : Milk, emulsions.
(e) Liquid + Gas : Soap bubbles.
Give the distinguishing features of a pure substance or a compound and a mixture.
Various points of distinction between a chemical compound and a mixture are summarised below:
|
Characteristics |
Pure substance or compound |
Mixture |
|
1. Composition
|
The elements in a compound are present in definite proportion by weight.
|
The ingredients of a mixture may be present in varying ratio. |
|
2. Homogeneity
|
A compound is always homogeneous.
|
A mixture may be homogeneous (solution) or heterogeneous. The constituents of a mixture can be separated by simple methods.
|
|
3. Properties |
.A compound has entirely different properties from those of its constituents.
|
Properties of a mixture are an average of the properties of its constituents.
|
|
4. Separation
|
The constituents of a compound cannot be separated by simple separation techniques |
The constituents of a heterogeneous mixture can be separated by simple physical separation techniques. |
|
5. Energy changes
|
Energy in the form of heat, light or electricity is either evolved or absorbed when a compound is formed. |
There is generally no energy change when a mixture is formed from its constituents. |
|
6. Volume change |
At constant temperature and pressure, formation of a compound may involve either no change or large change in volume. |
At constant temperature and pressure, formation of a mixture involves either very little or no change in volume. |
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