Solubility – Solute, Solvent, Solution

Definition of Terms

Solute
This is a substance that dissolves in a solvent to form a solution. It can be solid, liquid or gas. Examples include common salt, sugar, copper (II) tetraoxosulphate (VI) etc.

Solvent
This is a substance that dissolves a solute to form a solution. It can be liquid or gas. Examples are water, ethanol, benzene etc.

Solution
This is a mixture of solute and solvent, i.e,
Solute + Solvent = Solution
It can be homogeneous with a uniform composition, e.g, an unsaturated solution of a sodium chloride, or heterogeneous with non-uniform composition like an unsaturated solution of the same salt.

Solution is defined as a homogeneous mixture of two or more chemical substances. The state of matter of a solution may be solid, liquid or gaseous.

For example, common salt in water (liquid solution), air (gaseous solution), alloys (solid solution), etc.

The components of a solution may be classified as:

• Solute
• Solvent

Difference between solute and solvent

Thus, an intimate mixture of solute and solvent is called a solution.

Aqueous solutions and non-aqueous solutions

Aqueous solutions

Solutions that contain water as the solvent are called aqueous solutions.

For example, sugar in water, carbon dioxide in water, etc.

Non-aqueous solutions

Solutions that contain a solvent other than water are called non-aqueous solutions. Ether, benzene, petrol, carbon tetrachloride etc., are some common solvents.

For example, sulphur in carbon disulphide, naphthalene in benzene, etc.

Concentrated solutions and dilute solutions

Between two solutions, the solute quantity may be relatively more or less. The solution that has a greater proportion of solute is said to be more concentrated than the the other that has a lesser proportion. If the proportion of solute is less, the solution is said to be dilute.

Saturated Solution
A saturated solution is one that holds as much solute as it can dissolve in the presence of undissolved solute particles at a given temperature. In a saturated solution, the dissolved and undissolved solutes are in equilibrium at a given temperature.

Unsaturated Solution
An unsaturated solution is one that can still dissolve more solutes at a given temperature until it becomes saturated.

For instance, the solubility limit of sodium chloride is 36g dm^-3, which means that as long as the mass of sodium chloride in 1dm^3 of its solution is below 36g, the solution will remain unsaturated. However, immediately the mass is 36g and above, the solution will be considered to be saturated because any mass in excess of 36g will be precipitated out of solution as undissolved particles. This implies that if the mass of sodium chloride in the solution is 37g; 1g which is the excess will be precipitated out as undissolved particles.

Supersaturated Solution

A solution which contains more of the solute than it can normally hold at a given temperature is said to be supersaturated. The supersaturated solution is unstable at a given temperature and the solute particles can crystallize out of the solution at the slightest touch or addition of a dry crystal of the solute. At times, dust particles can cause the crystallization of the solute particles. Also, merely scratching the inside of the glass vessel holding the solution with a glass rod or spatula can cause the solute particles to crystallize out. This is known as seeding, and is employed in crystallization as a separation technique. Solution and solubility

Solubility of a solute

Solubility is defined as the number of grams of a solute that dissolves in 100g of a solvent to form a saturated solution at a given temperature and pressure.

Solubility is the maximum weight of a solute that can be dissolved in 100g of a solvent at a given temperature and pressure.

Factors affecting the solubility of a solid solute in water

Temperature

Increase in temperature increases the solubility.

For example, it is easier to dissolve sugar in hot milk than in cold milk.

Size of solute particles

Smaller the size of the particles greater is the solubility.

For example, it is easier to dissolve powdered sugar than granules of sugar.

Mechanical stirring

Mechanical stirring increases solubility.

For example, sugar dissolves faster on stirring with a spoon.

Factors affecting the solubility of a gas in water

Temperature

Increase in temperature, decreases the solubility of a solute. On heating, the gases dissolved in milk escape making the milk bubble and boil over.

Pressure

An increase in pressure increases the solubility of a gas.

For example, aerated water bottles contain carbon dioxide gas under pressure.

Influence of Pressure on the Solubility of Gases

Pressure directly influences the solubility of gases. Higher the pressure, higher is the solubility: lower the pressure, lower is the solubility. For instance, the solubility of carbon dioxide in water, under normal atmospheric pressure is very low. But when subjected to higher pressure it dissolves to a great extent, as in the case of aerated waters.

Remember

Speed of Dissolving of a Solid in a Liquid

From our day-to-day experience, we come to know that a solid can dissolve faster if:

1. a) The particles are smaller.
2. b) The mixture is stirred or agitated.
3. c) The solvent is at a higher temperature.

So the fastest way to dissolve a solid in a liquid is to grind the solid into very tiny particles, stir the mixture, and use a hot solvent. Dissolving Bournvita in a hot cup of milk is a good example!

Determination of the solubility of a solid in water at a given temperature

If you want to determine the solubility of a solid, say potassium nitrate in water, at any given temperature, this is how you should proceed:

1. a) Take a clean dry evaporating dish, and find its weight, as accurately as possible. Let the weight of the empty evaporating dish be ‘a’ gm.
2. b) Take a clean beaker and a glass rod. Pour about 20-25 ml of distilled water into the beaker. Warm the water to 2^oCto 3^oc above the temperature, at which you are determining the solubility (i.e., if you are determining the solubility at 40^oC, warm it upto 42^oC to 43^oC). Keep on adding small quantities of potassium nitrate with constant stirring with the glass rod, till no more of it dissolves. Now stop warming it, but keep on stirring, till the temperature comes down to the required level-say 40^o Carefully transfer about 10-15 ml of this solution into the previously weighed evaporating dish. Care has to be taken to see that no crystals are transferred.

Find the weight of the evaporating dish and the saturated solution.

Let this weight = ‘b’ gm.

1. c) Place the dish with the saturated solution, on a sand bath and carefully evaporate the water. Cool the dish to room temperature and find the weight of the dish and the solute. To make sure that all the water has been evaporated, heat the dish again for 3 to 5 minutes, then cool and weigh. Repeat the heating, cooling and weighing several times, till two consecutive weights are the same.

Let this weight be ‘c’ g.

1. d) Observations

Weight of clean dry evaporating dish = ‘a’ g

Weight of dish + saturated solution = ‘b’ g

Wight of dish + solute = ‘c’ g

1. e) Calculations

Weight of solute alone = (c – a) g

Weight of solvent alone = (b – c) g

Solubility of solute in solvent at that temperature