Solubility

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Solubility is the ability of a solute to dissolve in a solvent. To explain this, consider the following case. Three students A, B and C are given 3 beakers, each containing 100 g of water at 20^oC. They are also given one salt each. Student A is given potassium nitrate, Student B is given sodium chloride and student C is given sodium nitrate. Each of them adds small quantities of the given salt to the water, and stirs it. Finally, they succeed in making the saturated solution. Then they calculate the mass of salt that is required to saturate the solutions. They get the following data (Table 6.5).

Table 6.5

Student	A	B	C
Salt	KNO3	NaCl	NaNO3
Temperature	200C	200C	200C
Mass of water	100g	100g	100g
Mass of salt used to saturate	31.6g	36g	88g

 What is the conclusion?

The same mass of a solvent, at the same temperature, has the ability to dissolve, different masses of different solutes. In the above illustration, the solutes with the highest solubility (ability of dissolving to the maximum) in descending order are: NaNO₃ > NaCl > KNO₃.

Thus, "the solubility of a solute in a solvent, is the number of grams of the solute that can dissolve in 100 grams of that particular solvent, to saturate it at that temperature".

Temperature and Solubility

Temperature plays a very important role in deciding the solubility of solutes.

1. Increase in temperature decreases the solubility of gases in liquids.

If the solute is a gas and the solvent is a liquid, the solubility of the gas decreases with the increase in temperature. For instance, fish can breathe the oxygen dissolved in ordinary water. Take some water from an aquarium, boil it and then cool it back to the temperature of the aquarium and then place a small fish in it for a short while. If it is not shifted back to the aquarium water, it will die soon. Why? As the temperature was raised, boiling expelled the dissolved air. The solubility of air in water decreased by escaping in the form of bubbles. Hence the fish would have almost no dissolved air to breathe in.

Remember:

Consider this example: Two friends go to a restaurant and order two bottles of soda. One asks for a very chilled one, but the other asks for one that is not cooled. Both the bottles are opened almost at the same time. But instead of drinking the soda, they are busy talking. After some time they notice that bubbles of carbon dioxide gas are still coming out of the chilled soda, but hardly any gas coming out of the other bottle.

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2. The solubility of solids increases with increase of temperature.

However, the case for solids is different. Generally, the solubility of solids, like copper sulphate, potassium bromide, potassium chloride, potassium nitrate, potassium sulphate, sodium nitrate etc., increases with increase of temperature.

Six students are asked to find out the solubility of potassium nitrate in water at different temperatures. The temperatures are 0^oC, 20^oC, 40^oC, 60^oC, 80^oC and 100^oC. They tabulate their findings as shown in the table (Table 6.6).

Table 6.6

Temperature in C	0	20	40	60	80	100
Solubility in grams per 100 grams of water	13.5	32	68	106	174	226

 

But there are exceptions to the above, such as calcium hydroxide, calcium sulphate, cerium sulphate, etc. Their solubilities at four different temperatures are given in table (Table 6.7).

Table 6.7

Solute/Solubility	0oC	20oC	60oC	100oC
Calcium hydroxide	1.19	0.17	0.12	0.08
Calcium sulphate	0.18	0.19	0.20	0.16
Cerium sulphate	17.35	9.16	3.73	-

If the solute and solvent are both soluble liquids, like alcohol and water, there is no limit to solubility i.e., any amount of alcohol or water can be mixed with each other. Temperature changes for such solutions are usually not of primary importance.

It is important to remember that for a mixture of liquids, the terms miscible and immiscible are frequently used, rather than soluble and insoluble. If alcohol is added to water, the two liquids form one liquid phase and therefore these liquids are said to be miscible. A process of fractional distillation will be required to separate them. On the other hand water and oil remain separate when mixed together and thus these liquids are said to be immiscible with one another. A separating funnel is needed to separate them.

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:

a) The particles are smaller.

b) The mixture is stirred or agitated.

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!

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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:

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.

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^oC. 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.

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.

d) Observations

Weight of clean dry evaporating dish = 'a' g

Weight of dish + saturated solution = 'b' g

Wight of dish + solute = 'c' g

e) Calculations

Weight of solute alone = (c - a) g

Weight of solvent alone = (b - c) g

Solubility of solute in solvent at that temperature