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:
(a)Solutions/solid-liquid dissolved mixture
Experiment:
To make a solution of copper (II) sulphate (VI)/Potassium magnate(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 to each beaker. Stir for about two minutes.
Observation
Copper (II) sulphate (VI) crystals dissolve to form a blue solution
Potassium manganate (VII) crystals dissolve to form a purple solution
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 solvent.
A substance /solid /gas which dissolves in a solvent is called solute.
When a solute dissolves in a solvent it forms a uniform mixture called solution.
A solute dissolved in water as the solvent exists in another state of matter called aqueous state. Water is referred as the universal solvent because it dissolves many solutes. A solute that dissolves in a solvent is said to be soluble. Soluble particles 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 e. g.
1. Sodium chloride solution is a solution formed after dissolving sodium chloride crystals/solid in water. Sodium chloride exists in 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 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 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 manganate(VII)exist in aqueous state after dissolving.
Potassium manganate(VII) + Water -> Potassium manganate(VII) solution
KMnO4(s) + (aq) -> KMnO4 (aq)
(b)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 to each beaker. Stir for about two minutes.
Observation
Some soil, maize and lead (II) Iodide float in the water
A brown suspension/precipitate/particles suspended in water containing soil
A white suspension/precipitate/particles suspended in water containing flour
A yellow suspension/precipitate/particles suspended in water containing Lead (II) iodide. Some soil, maize and lead (II) Iodide settle at the bottom after some time.
Explanation
Some solid substances do not dissolve in a liquid. They are said to be insoluble in the solvent .When an insoluble solid is put in liquid:
(i) Some particles remain suspended/floating in the liquid to form a suspension /precipitate.
(ii) Some particles sink/settle to the bottom to form sediments after being allowed to stand.
An insoluble solid acquire 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 has some fine blue particles suspended /floating in the liquid.
3. A green suspension /precipitate has some fine green particles suspended /floating in the liquid.
4. A brown suspension /precipitate has some fine brown particles suspended /floating in the liquid.
4. A yellow suspension /precipitate has some fine yellow particles suspended /floating in the liquid.
(c) (i) Miscibles /Liquid-liquid mixtures
To form water-ethanol and Kerosene-turpentine miscibles
Procedure
(i)Measure 50cm3 of ethanol into 100cm3 beaker. Measure 50cm3 of water. Place the water into the beaker containing ethanol. Swirl for about one minute.
(ii)Measure 50cm3 of kerosene into 100cm3 beaker. Measure 50cm3 of turpentine oil. Place the turpentine oil into the beaker containing kerosene. Swirl for about one minute.
Observation
Two liquids do not form layers.
Ethanol and water form a uniform mixture.
Kerosene and turpentine oil form uniform mixture
Explanation
Ethanol is miscible in Water. Kerosene is miscible in turpentine oil. Miscible mixture form 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.
- Immiscibles /Liquid-liquid mixtures
To form water-turpentine oil and Kerosene-water miscibles
Procedure
(i)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.
(ii) 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
Two liquids form layers.
Turpentine and water do not form a uniform mixture.
Water and kerosene do not form uniform mixture
Explanation
Kerosene is immiscible in Water. Water is immiscible in turpentine oil. Immiscible mixtures do not form uniform mixtures. They form layers. The size of the particles of one liquid is almost equal to the particles of the other. The particles of one liquid cannot occupy the spaces between the particles of the other. The heavier particles settle at the bottom. The less dense particles settle on top.
(d)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 alloy. In the alloy, one metallic particle occupies the spaces between the metallic particles of the other.
c) Common alloys of metal.
| Alloy name | Constituents of the alloy | Uses of the alloy |
| Brass | Copper and Zinc | Making screws and bulb caps |
| Bronze | Copper and Tin | Making clock springs, electrical contacts and copper coins |
| Soldier | Lead and Tin | Soldering, joining electrical contacts because of its low melting points and high thermal conductivity |
| Duralumin | Aluminum, Copper and Magnesium | Making aircraft, utensils, and windows frames because of its light weight and corrosion resistant. |
| Steel | Iron, Carbon ,Manganese and other metals | Railway lines, car bodies girders and utensils. |
| Nichrome | Nichrome and Chromium | Provide resistance in electric heaters and ovens |
| German silver | Copper, Zinc and Nickel | Making coins |
