Form 1:
Classification of substances

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METHODS OF SEPARATING MIXTURES

Mixtures can be separated by applying the following methods:

(a) Decantation

Sediments can be separated from a liquid by pouring out the liquid. This process is called decantation.

Experiment

Put some sand in a beaker. Add about 200cm3 of water. Allow sand to settle. Pour off water carefully into another beaker.

Observation

Sand settles at the bottom as sediments.

Less clean water is poured out.

Explanation

Sand does not dissolve in water. Sand is denser than water and thus settles at the bottom as sediment. When poured out, the less dense water flows out.

(b)Filtration

Decantation leaves suspended particles in the liquid after separation. Filtration is thus improved decantation.Filtration is the method of separating insoluble mixtures/particles/solids from a liquid.

Experiment:

To separate soil and water using filtration:

Fold a filter paper to fit well into a filter funnel.

Place the funnel in an empty 250 cm3 beaker.

Put one spatula end full of soil into 50cm3 of water. Stir.

Put the soil/water mixture into the filter funnel.

Observations

Clean water is collected below the filter funnel.

Soil remains above the filter paper.

Explanation

A filter paper is porous which act like a fine sieve with very small holes.

The holes allow smaller water particles to pass through but do not allow bigger soil particles. The liquid which passes through is called filtrate.

The solid which do not pass through is called residue.

Set up of apparatus

In industries, filtration is used in engine filters to clean up air

(c)Evaporation

Evaporation is a method of separating a solute/solid from its solution.

This involves heating a solution (solvent and solute) to vapourize the solvent out of the solution mixture leaving pure solute/solid. If a mixture contain insoluble solid, they are filtered out.

Experiment:

To separate a mixture of soil and salt (sodium chloride).

Procedure:

Put one spatula end full of soil on a filter paper.

Put one spatula full of common salt/sodium chloride into the same filter paper.

Mix well using the spatula,.

Place about 200cm3 of water into a beaker.

Put the contents of the filter paper into the water.

Stir thoroughly using a glass/stirring rod for about one minute.

Fold a filter paper into a filter funnel.

Pour half portion of the contents in the beaker into the filter funnel. Put the filtrate into an evaporating dish.

Heat on a water bath.

Observation

(i)On mixing, colourless crystals and brown soil particles appear on the filter paper.

(ii)On adding water common salt dissolves in water. Soil particles do not dissolve in water.

(iii)On filtration, a colourless liquid collected as filtrate below the filter funnel/paper. Brown residue collected above the filter funnel/paper.

(iv)On evaporation, colourless crystals collected after evaporation

Explanation

Solid mixture of sand and common salt take the colors of the two.

On adding water, common salt dissolves to form a solution.

Soil does not because it is insoluble in water and thus forms a suspension.

On filtration, a residue of insoluble soil does not pass through the filter paper.

It is collected as residue.

Common salt solution is collected as filtrate.

On heating the filtrate, the solvent/water evaporate/vaporize out of the evaporating dish leaving common salt crystals.

Vapourization/evaporation can take place even without heating.

This is the principle/process of drying wet clothes on the hanging line.

Set up of apparatus
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