For a common man, pure substance that it is not adulterated. for example, for a common man, pure milk means the liquid material that is given by cow or buffallo and it is not mixed with water or other materials to thicken it. But for a chemist, any milk is not a pure substance. Milk is a mixture of water, fats and proteins. These materials are different in milk obtained from different source. Similarly, pure fruit juice for a common man means that it is not mixed with fruit juice of other fruits water or salt. But for a chemist, any fruit juice is a mixture of water and several other substances. For a common man pure gold means gold of a given composition. But a chemist known that no ornaments can be made from pure gold. All ornaments of gold contain gold and copper.

Two components of a solution: Solute and solvent. Two properties of a solution:
(i) homogeneous
(ii) Transparent.

True solution obtained in water are aqueous soliution, e.g., vinegar. true solutions in organic liquids like alcohol, acetone, etc. are non-aqueous solution, e.g., amino acids dissolved in acetone etc.

Take water in a beaker. Heat the water gently to 50⁰ C and slowly add copper sulphate powder, stirring the solution continuously with the help of a glass rod. Continue adding the compound, keeping the temperature of solution at 50⁰ C, till some compound remains undissolved and settle down. Then quickly filter the solution. The solution so obtained is a saturated solution of copper sulphate at 50⁰ C.On cooling, the solubility of copper sulphate will decrease e and it will start settling down at the bottom of the beaker.

A pure substance always has the same colour, taste or texture at a given temperature and pressure. Also it has fixed point or boiling point at constant pressure. Also it has a fixed melting point or boiling point at constant pressure. For example, pure water boils at 373k at 1 atmospheric pressure. But water containing some invisible dissolved substance boils at a temperature above 373k.

Given, mass of solute = 110 g
Mass of solution = 550 g
% composition = \({110 \over 550}\times 100 \) 
Concentration = 20% by mass.

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.

Comparison of properties of a true solution, colloidal solution, and suspension

property	True solution	Colloidal solution	Suspension
(1) Appearance	homogeneous and transparent	Heterogeneous and translucent	heterogeneous and opaque
(2) particle size	< 1 nm (10-7 cm)	1 nm - 100 nm	> 100 nm (10-5 cm)
(3) Visibility	particles are not visible even with a powerful microscope	Particles can be seen with a high power microscope	particles can be seen with naked eyes
(4) Stability	Stable	Stable	Unstable
(5) Diffusion	Diffuse rapidly	Diffuse slowly	Do not diffuse
(6) Filterability	Pass through filter paper	passes through filter paper	can be separated by filter paper
Example	NaCI dissolved in water	Blood	Mud water.

Test of a colloidal solution or a suspension.
(i) A colloidal solution is turbid and the particle settles down on adding salt. In a suspension, particles settle down on keeping under the influences of gravity.
(ii) If the particles in a heterogeneous and opaque solution can be seen with naked eyes and get settled on keeping, then it is a suspension.

Mass of silver (Ag) ornament = mg
Mass of gold used for polishing = \(\frac{1}{100}\times\) mg = 0.01 mg
Atomic mass of Ag = 108 u
\(\therefore\) 1 mole of Ag = 108 g = 6.022 x 10²³ atoms
Thus, 108 g of Ag have atoms = 6.022 x 10²³
\(\therefore\) mg of Ag have atoms = \(\frac{6.022\times 10^{23}}{108}\)
Similarly, atomic mass of gold (Au) = 197 u
1 mole of Au = 197 g = 6.022 x 10²³ atoms
Thus, 197 g of Au have atoms = 6.022 x 10²³
\(\therefore\) 0.01 mg of Au will have atoms = \(\frac{6.022\times 10^{23}\times 0.01\ m}{197}\)
\(\therefore\) Ratio of the number of atoms of gold and silver = \(\frac{6.022\times 10^{23}}{197}\times 0.01\ m; \frac{6.022\times 10^{23}}{108}m\)
= \(\frac{1}{19700}:\frac{1}{108}\) 
= 108: 19700