Separation of substances - Class 5 Notes & Olympiad Questions

Soluble and Insoluble Substances in Water

Substances can be classified as soluble and insoluble depending on whether they can dissolve in a liquid or not.

Soluble Substances

a) A soluble substance is something that can dissolve in water. It means that when you mix the substance with water, it will disappear and spread out evenly in the water.
b) Imagine adding sugar to a glass of water and stirring it. The sugar will disappear and become part of the water, making it sweet. This is because sugar is a soluble substance in water.
c) Other examples of soluble substances are salt, lemonade powder, or instant coffee.
d) These substances can mix well with water and create a solution where you can no longer see the solid particles because they are dissolved.

Insoluble Substances

a) On the other hand, an insoluble substance is something that does not dissolve in water. It means that when you mix it with water, it will not disappear or spread out in the water. Instead, it will remain separate and visible in the water.
b) For example, mixing sand or pebbles with water. No matter how much you stir, the sand or pebbles will not dissolve. They will settle at the bottom or float on the surface because they are insoluble substances in water.
c) Other examples of insoluble substances are oil, chalk, or iron nails. These substances do not mix well with water and will not dissolve, forming separate layers or remaining as visible particles.

Separation of Substances

a) Separating substances means taking a mixture and finding a way to separate its different parts or components.
b) One important thing to consider when separating substances is whether they are soluble or insoluble in a particular liquid, like water.
c) To separate soluble and insoluble substances in water, we use different methods.

Separating Insoluble Substances

1. Filtration

a) Filtration is the most commonly used method for separating insoluble substances from water.
b) The mixture is passed through a filter medium, such as filter paper or a porous material, which allows the water to pass through while trapping the insoluble particles.
c) The solid residue collected on the filter is the separated insoluble substance.

Example: Filtering a mixture of sand and water. The sand particles are trapped on the filter paper, while the water passes through, resulting in the separation of sand from water.

2. Sedimentation and Decantation

a) If the insoluble substance is heavier than water, sedimentation and decantation can be utilised.
b) The mixture is allowed to stand undisturbed for a period of time, during which the insoluble particles settle to the bottom due to gravity.
c) Once the settling has occurred, the clear water can be carefully poured off, leaving behind the separated insoluble substance.

Example: Allowing a mixture of muddy water to stand undisturbed. Over time, the heavier mud particles settle at the bottom, and the clear water can be carefully poured off, separating the mud from the water.

Separating Soluble Substances

1. Evaporation

a) Evaporation is a straightforward method to separate soluble substances from water.
b) The mixture is heated, causing the water to evaporate and leave behind the dissolved substance. The resulting solid residue can then be collected.
c) This method is commonly used to obtain salts or other solid substances from a water solution.

Example: Evaporating a saltwater solution. Heating the solution causes the water to evaporate, leaving behind salt crystals, which can be collected as the separated substance.

2. Crystallisation

a) Crystallisation is a technique used to separate a dissolved substance from water by allowing it to form crystals.
b) The solution is heated or left undisturbed, promoting the evaporation of water and the gradual formation of crystals. The crystals can then be collected through filtration or decantation, resulting in the separation of the dissolved substance.

Example: Dissolving sugar in water and allowing it to evaporate slowly. As the water evaporates, sugar crystals form, which can be collected through filtration or decantation, separating the sugar from the water.

3. Distillation

a) Distillation can be used to separate a soluble substance from water if the substance boils at a different temperature than water upon heating.
b) The mixture is heated, and the vapour produced is then cooled and condensed back into liquid form, leaving the dissolved substance behind.

Example: Distilling a mixture of water and alcohol. By heating the mixture, the alcohol vaporizes first, and upon cooling and condensation, it can be collected as a separate liquid from the water.