Classification of Substances

Substances are either pure or impure. A pure substance is one which contains only one substance. An impure substance is one which contains two or more substances.

A pure substance is made up of a pure solid, pure liquid or pure gas. A mixture is a combination of two or more pure substances which can be separated by physical means. The three states of matter in nature appear mainly as mixtures of one with the other.

Common mixtures include:

SOLUTIONS/SOLID-LIQUID DISSOLVED MIXTURE

Experiment:

To make a solution of copper (II) sulphate (VI)/Potassium manganate (VII)/sodium chloride

Procedure

Put about 100 cm³ of water in three separate beakers. Separately place a half spatula end full of copper(II) sulphate(VI), Potassium manganate(VII) and sodium chloride crystals into each beaker. Stir for about two minutes.

Observation

1. Copper(II) sulphate(VI) crystals dissolve to form a blue solution
2. Potassium manganate (VII) crystals dissolve to form a purple solution
3. Sodium chloride crystals dissolve to form a colourless solution

Explanation

Some solids, liquids and gases dissolve in some other liquids. A substance/liquid in which another substance dissolves is called a solvent.

A substance/solid /gas which dissolves in a solvent is called a solute. When a solute dissolves in a solvent, it forms a uniform mixture called a solution.

A solute dissolved in water as the solvent exists in another state of matter called the aqueous state. Water is referred to as the universal solvent because it dissolves many solutes.

A solute that dissolves in a solvent is said to be soluble. Soluble particles are uniformly spread between the particles of water/solvent and cannot be seen.

Solute + Solvent -> solution

Solute + Water -> Aqueous solution of solute

The solute dissolved in water gives the name of the solution

1. Sodium chloride solution is a solution formed after dissolving sodium chloride

crystals/solid in water. Sodium chloride exists in an aqueous state after dissolving.

Sodium chloride + Water -> Sodium chloride solution

NaCl(s) + (aq) -> NaCl(aq)

2. Ammonia solution is a solution formed after dissolving ammonia gas in water. Ammonia exists in an aqueous state after dissolving.

Ammonia gas + Water -> Aqueous ammonia

NH3(g) + (aq) -> NH3(aq)

3. Copper(II)sulphate(VI) solution is a solution formed after dissolving Copper(II) sulphate(VI) crystals/solid in water. Copper (II) sulphate (VI) exists in the aqueous state after dissolving.

Copper (II) sulphate (VI) + Water -> Copper (II) sulphate (VI) solution

CuSO4(s) + (aq) -> CuSO4 (aq)

4. Potassium manganate (VII) solution is a solution formed after dissolving Potassium manganate (VII) crystals/solid in water.

Potassium manganates (VII) exist in an aqueous state after dissolving.

Potassium manganate (VII) + Water ->Potassium manganate (VII) solution

KMnO4(s) + (aq) -> KMnO4 (aq)

SUSPENSION/ PRECIPITATES/SOLID-LIQUID MIXTURE WHICH DO NOT DISSOLVE

Experiment: To make soil, flour and Lead (II) Iodide suspension/precipitate

Procedure

Put about 100 cm³ of water in three separate beakers. Separately place a half spatula end full of soil, maize and lead (II) Iodide into each beaker. Stir for about two minutes.

Observation

1. Some soil, maize and lead (II) Iodide float in the water
2. A brown suspension/precipitate/particles suspended in water containing soil
3. A white suspension/precipitate/particles suspended in water containing flour
4. A yellow suspension/precipitate/particles suspended in water containing Lead (II) iodide.
5. Some soil, maize and lead (II) Iodide settle at the bottom after some time.

Explanation

1. Some solid substances do not dissolve in a liquid.
2.  They are said to be insoluble in the solvent.

When an insoluble solid is put in a liquid:

1. Some particles remain suspended/floating in the liquid to form a suspension/precipitate.
2. Some particles sink/settle to the bottom to form sediments after being allowed to stand.

An insoluble solid acquires the colour of the suspension/precipitate, e.g.

1. A white suspension /precipitate have some fine white particles suspended/floating in the liquid. Not “white solution”
2. A blue suspension /precipitate have some fine blue particles suspended/floating in the liquid.
3. A green suspension /precipitate have some fine green particles suspended/floating in the liquid.
4. A brown suspension /precipitate has some fine brown particles suspended/floating in the liquid.
5. A yellow suspension /precipitate has some fine yellow particles suspended/floating in the liquid.

MISCIBLES/LIQUID-LIQUID MIXTURES

To form water-ethanol and Kerosene-turpentine miscible

Procedure

1. Measure 50cm₃ of ethanol into a 100cm³ beaker. Measure 50cm₃ of water. Place the water into the beaker containing ethanol. Swirl for about one minute.
2. Measure 50cm₃ of kerosene into a 100cm³ beaker. Measure 50cm³ of turpentine oil. Place the turpentine oil into the beaker containing kerosene. Swirl for about one minute.

Observation

1. Two liquids do not form layers.
2.  Ethanol and water form a uniform mixture.
3. Kerosene and turpentine oil form uniform mixture

Explanation

Ethanol is miscible in Water. Kerosene is miscible with turpentine oil. A miscible mixture forms a uniform mixture.

They do not form layers.

The particles of one liquid are smaller than the particles of the other.

The smaller particles occupy the spaces between the bigger particles.

(ii) Immiscibles /Liquid-liquid mixtures

To form water-turpentine oil and Kerosene-water miscibles

Procedure

1. Measure 50cm₃ of water into 100cm₃ beaker. Measure 50cm₃ of turpentine oil. Place the oil into the beaker containing water. Swirl for about one minute.
2. Measure 50cm₃ of water into 100cm₃ beaker. Measure 50cm₃ of kerosene. Place the kerosene into the beaker containing water. Swirl for about one minute.

Observation

1. Two liquids form layers.
2. Turpentine and water do not form a uniform mixture.
3. Water and kerosene do not form uniform mixture

Explanation

1. Kerosene is immiscible in Water. Water is immiscible in turpentine oil. Immiscible mixtures do not form uniform mixtures.
2. They form layers. The size of the particles of one liquid is almost equal to the particles of the other.
3. The particles of one liquid cannot occupy the spaces between the particles of the other.
4. The heavier particles settle at the bottom. The less dense particles settle on top.

SOLID-SOLID MIXTURES/ALLOYS

Before solidifying, some heated molten/liquid metals dissolve in another metal to form a uniform mixture of the two.

On solidifying, a uniform mixture of the metals is formed. A uniform mixture of two metals on solidifying is called an alloy. In the alloy, one metallic particle occupies the spaces between the metallic particles of the other.

1. c) Common alloys of metal.