Chemistry
Notes
Chemistry
Notes
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Chapter 1 - Solids, Liquids and Gases. |
Matter
Matter is defined as anything that has mass and occupies space. It can exist in three states: solid, liquid and gas. A solid has a definite shape and volume; a liquid has definite volume but no definite state; and a gas has neither a definite shape nor definite volume.
1.1 Physical and Chemical changes.
Matter can undergo physical or chemical changes.
Physical changes
A physical change is one which is easily reversible and in which no new substances are formed.
Chemical changes
A chemical change is one which is not easily reversible and in which a new substance is formed.
The characteristics of physical and chemical changes are summarized in the following table:
Physical Change |
Chemical Change |
It is easily reversible |
It is not easily reversible |
No new substances are formed |
Entirely new substances are always produced |
There is no change in the mass of the substances involved |
The mass of each individual new substance formed differs from that of the original individual substances |
It does not involve any great heat changes except the latent heat changes which occur during changes of state, e.g. latent heat of fusion and vapourization |
A considerable amount of heat change is normally involved |
Some examples of physical changes are :
The change of state of matter as in the melting of solids to liquids, the freezing of liquids to solids, the vaporization of liquids to gases.
The separation of mixtures by (a) distillation, (b) evaporation, (c) fractional distillation, (d) crystallization and (e) sublimation.
The magnetization of iron rods.
The expansion and contraction of a substance when it is heated or cooled respectively.
Some examples of chemical changes are :
The burning of substances in air.
The dissolution of metals in acids.
The rusting of iron.
The addition of water to quicklime.
The fermentation and decay of substances.
1.2 Elements, Compounds and Mixtures.
Elements :
An element is a substance which cannot be split up into simpler units by any ordinary chemical process.
Compounds :
A compound is a substance which contains two or more elements chemically combined together.
Mixtures :
A mixture contains two or more constituents, which can easily be separated by physical methods.
The differences between mixtures and compounds are summarized as follows :
Mixture |
Compound |
It may be homogeneous or heterogeneous. |
It is always homogeneous. |
The constituents are not chemically bound together and can therefore be easily separated and recovered by physical means. |
The component elements are chemically bound together and cannot be separated by physical means. |
The constituents can be added together in any ratio by mass. Hence a mixture cannot be represented by a chemical formula. |
The components are present in a fixed ratio by mass. Hence, a compound can be represented by a chemical formula. |
The properties of a mixture are the sum of those of its individual constituents. |
The properties of a compound differ entirely from those of its component elements. |
There is no heat change when a mixture is made. |
Formation of a compound is usually accompanied by considerable heat change. |
1.3 Methods of purification.
Some of the methods use for separating mixtures into their components are :
Sieving
This method is employed to seoarate solids particles of different sizes. The mixture is put into a mesh of a particular size. Passage through the mesh is based purely on the size of the particles. Thus smaller particles can be separated from the larger ones.
Mining industries such as those involving diamond and gold employ this method extensively.
Filtration
This is used to separate insoluble particles from liquids.
Many industries use filtration to remove solid particles from their liquid components; e.g. water purification systems and breweries.
Evaporation to dryness
Evaporation may be used to recover a solute from a solution. The solvent is usually sacrificed.
In the laboratory, a water bath or sand bath is usually utilized to bring about a steady rate of evaporation. the solute required is left behind in the dish while the solvent escapes as vapour. This method is not suitable for salts which are easilly destroyed by dry heating.
Sublimation
Sublimation is the phenomenon whereby some solids when heated, change directly to the gaseous state whithout passing throung the liquid state. Examples of such solids are iodine and ammonium chloride. These substances can therefore be separated from other solids by sublimation.
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