Homogeneous mixtures
(a) Solid solution. Alloys, e.g., brass, steel, bronze.
(b) liquid solution. Sugar solution in water, seawater, alcohol in water.
(c) Gaseous solution. Air, natural gas.
Heterogeneous mixtures
(a) Solid + Solid Sugar and sand, iron filings and sulphur, copper sulphate and potassium permanganate
(b) Solid +  Liquid Pastes, suspensions, etc.
(c) Solid + gas Smok
(d) Liquid + Liquid Milk, emulsions.
(e) liquid + Gas Soap bubbles.

1.The concentration of a solution may be expressed in terms of percentage by mass of solute per 100 grams of the solution.
2. % Conc. of solution = \({mass\ of\ solute \over Mas \ of\ solution}\times100\)
3. Thus, if a solution is 5%, it means it is 5 gram of a solute dissolved in 100 grams of the solution or it contains 5-gram of solute and 95 grams of solvent.
4. The concentration of a solution may also be expressed as the 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.

Air is a homogeneous mixture of several gases. 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 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.

(a) Mass percentage of NaCl \({15\over215}\times=6.98\) % 
(b) Solute: Co₂; solvent Water.

Amount of alcohol in 100 mL solution = 12 mL
Amount of water = 100 - 12 = 88 mL
88 mL water and 12 mL alcohol are to be mixed with to get 12% of alcohol.

(a) If at given temperature, more salt is added to water, it settles down then it is saturated solution. If on adding more salt, a clear solution is obtained, then the given solution is unsaturated.
(b) We can increase the solubility of saturated solution by raising temperature.

(a)Saturated solution. A solution in which no more of the solid (solute) can be dissolved at a given temperature is called a saturated solution,
Unsaturated solution. A solution in which more of the solid can be dissolved at the given temperature is called an unsaturated solution.
(b) A saturated solution becomes unsaturated on heating.

(a) Hydrogen gas
(b) Hydrogen sulphide
(c) In (a) iron reacts with dil. H2SO4 gas.
In (b) iron sulphide compound formed reacts with dil. H₂SO₄ to give H₂S gas.

Water is a compound because:
(i) The composition of hydrogen and oxygen in water taken from any source is the same.
(ii) The properties of water are different from that of its constituents, i.e., H₂ or O₂.

The following changes are chemical changes:
(a) Growth of a plant
(b) Rusting of iron
(d) Cooking of food
(e) Digestion of food
(g) Burning of candle.