Classification of matter
Let's look at the classification scheme of matter and examine the following terms: pure substances and mixtures.
Contents
Pure substances
Pure substances are subdivided into two categories: elements and compounds.
Elements
An element is a substance that cannot be separated into simpler substances by chemical reactions. In the periodic table, there are over 100 of these elements. Most of them can be found naturally on Earth. Others are produced synthetically by nuclear reactions.
Elements are indicated by symbols. It is important to note that the symbol for an element is case-sensitive. The convention is as follows:
- The first letter of the symbol of an element is always capitalized.
- The second and third letters are never capitalized.
Compounds
A compound is a substance that is composed of two or more elements chemically held together in fixed proportions. An example of a compound is water.
Water
Every unit of water has two hydrogen atoms and one oxygen atom. This unit has a 'fixed proportion' of hydrogen atoms to oxygen atoms. We can refer to this fixed proportion as:
- water is 11% hydrogen and 89% oxygen by mass
- OR
- water is made up of 2 hydrogen atoms and 1 oxygen atom
Inherited by this fixed proportion of hydrogen and oxygen unit are all the physical and chemical properties of water as we know it.
Some physical and chemical properties of water include:
- melting point is 0°C
- boiling point is 100°C
- an excellent solvent for ionic compounds
- capable of forming hydrogen bonds with other water molecules
- solid form of water is less dense than liquid water
Mixtures
A mixture is a combination of two or more substances in which each substance's identity is retained. Mixtures are subdivided into two categories: homogeneous mixtures and heterogeneous mixtures.
Homogeneous mixtures
Homogeneous mixtures are solutions which have uniform composition and appearance throughout. A solution may be gaseous (eg - air), solid (eg - brass) or liquid (eg - seawater). If a spoonful of sugar is dissolved in a glass of water, the composition of the sugar solution becomes the same in the glass. The sugar will not settle out and every spoonful of sugar solution that is removed from the glass will have the same composition. If the water in the glass is evaporated to dryness, the sugar retains its identity and can be recovered.
Heterogeneous mixtures
Heterogeneous mixtures are mixtures that do not have uniform composition and appearance throughout. The individual components which make up the mixture remain physically separated and can be seen as separate components. If a spoonful of sand is introduced in a glass of water, even after considerable stirring, the sand will settle to the bottom of the glass. Visually, the sand will separate out and the composition of sand in the glass will be the greatest at the bottom of the glass. Any attempts to withdraw spoonfuls of sand and water will have varying composition of each substance.
Overall classification scheme
Content suitability
BCIT courses: CHEM 0011
