The study of chemical equilibrium is a fundamental concept in Chemistry Topics, as it examines the balance between forward and reverse reactions.
What is a Saturated Solution – Preparation, Definition
Many substances (like salt, sugar, etc.) dissolve in water. When we dissolve a substance in water, then a solution is formed. In fact, all those substances which are soluble in water form solutions on dissolving in water. A given quantity of water cannot dissolve any amount of a substance. A given quantity of water can dissolve only a certain maximum amount of a substance (at a particular temperature). And if we go on adding more and more of the substance in a given quantity of water, then the ‘excess substance’ will remain undissolved in water. This will become more clear from the following activity.
We take a beaker and put 100 mL (100 millilitres) of water in it. Add one teaspoonful of salt to water and stir it with a glass rod until the salt dissolves completely. Again add a teaspoonful of salt and stir it well. We go on adding more and more salt in water with constant stirring to dissolve it. After adding a number of spoons of salt, we will find that some salt is left undissolved at the bottom of the beaker. This means that no more salt can be dissolved in the quantity of water which we took in the beaker. The solution is now said to be saturated. The contents of the beaker are now filtered through a filter paper arranged in a funnel. The clear solution obtained as filtrate is the saturated solution of salt in water (at that temperature).
We can now define a saturated solution as follows.
A solution in which no more substance can be dissolved at that temperature, is called a saturated solution. For example, if in a given salt solution, no more salt can be dissolved at that temperature, then that salt solution will be a saturated solution. Thus, a saturated solution contains the maximum amount of substance which can be dissolved in it at that temperature. For example, a maximum of 36 grams of salt (sodium chloride) can be dissolved in 100 grams of water at a temperature of 20°C (20 degree Celsius), so a saturated solution of salt at 20°C contains 36 grams of salt dissolved in 100 grams of water. If we add more than 36 grams of salt to 100 grams of water at 20°C, then the excess salt will remain undissolved in water.
The maximum amount of a substance which can be dissolved in 100 grams of water at a given temperature, is known as the solubility of that substance in water (at that temperature). For example, a maximum of 36 grams of salt can be dissolved in 100 grams of water at a temperature of 20°C, therefore, the solubility of salt in water is 36 grams at 20°C. It is clear that the solubility of a substance refers to its saturated solution.
The solubility of different substances in water is different. Since the solubility of a substance depends on temperature, so while expressing the solubility of a substance we have to specify the temperature also. The solubilities of some of the common substances are given below. All these solubilities are ‘per 100 grams of water’.
Substance : Solubility in water(at 20°C)
1. Copper sulphate : 21 grams
2. Potassium nitrate : 32 grams
3. Sodium chloride (Common salt) : 36 grams
4. Sugar : 204 grams
It is obvious from the above data that the same quanity of water (100 grams of water) can dissolve different amounts of different substances. For example, 100 grams of water can dissolve only 21 grams of copper sulphate but the same 100 grams of water can dissolve as much as 204 grams of sugar (at the same temperature of 20°C). Please note that the solubility of a substance in water increases on increasing the temperature (by heating). This means that larger amount of a substance (salt, sugar, etc.) can be dissolved in a given amount of water on heating it (or raising its temperature). On the other hand, the solubility of a substance decreases on lowering the temperature (by cooling). This means that lesser amount of a substance (salt, sugar, etc.) will dissolve in a given amount of water on cooling it (or on lowering the temperature).
Effect of Heating and Cooling on a Saturated Solution
A solution of the substance in water is said to be saturated if it cannot dissolve more of the substance in it at that temperature.
- If the saturated solution of a substance at a particular temperature is heated to a higher temperature, then the solubility of substance increases and more of substance can be dissolved in it.
- If the saturated solution of a substance at a particular temperature is cooled to a lower temperature, then the solubility of the substance decreases and some of the dissolved substance will separate out in the form of solid crystals (see Figure).
From the above discussion we conclude that hot water will dissolve more substance whereas cold water will dissolve less substance. We will now answer a question based on the effect of temperature on the solubility of sugar.
Lemonade is prepared by mixing sugar and lemon juice in water. You wish to add ice to cool it. Should you add ice to the lemonade before or after dissolving sugar in water ? In which case it would be possible to dissolve more sugar ?
We should add ice after dissolving sugar in water. This can be explained as follows :
- Initially, the water is at room temperature so it is warm water. Warm water can dissolve more sugar and that too quite rapidly.
- If ice is added first, then the water will become cold. Cold water will dissolve less sugar and that too quite slowly.