### Theory:

Matter is further classified into two types as shown below:

Note: In science, homo means 'the same', hetero means 'different'.

1. Pure substance:
The pure substance consists of one kind of particles, which are either elements or compounds.
An element consists of the same kind of atoms, while the compound is formed by a chemical combination of two or more elements.

Example for elements:
• Pure gold
• Copper
• Silver
• Platinum
• Aluminium
Example for compounds:
• Water ($$H_2O$$)
• Backing soda ($$NaHCO_3$$)
• Common salt or table salt ($$NaCl$$)
A molecule is the combination of two or more atoms.
2. Mixture:
A mixture is an impure substance, which consists of more than one kind of particles with any proportion.
Note: A substance formed by chemical combination is called a compound, while the substance formed by a physical combination is called a mixture.

Formation of compounds and mixtures:

Elements are chemically combined to form compounds-For example, pure gold ($$22$$-carat), air, water, common salt.

Mixture is formed by the physical combination of
• two or more elements (for example, $$22$$-carat gold, which is the combination of gold with other metals such as copper, cadmium, zinc). This is also called a homogeneous mixture.

• two or more compounds (for example, aerated drink, which is made up of $$CO_2$$, water, sweetening agent and colouring agent). This is also called a heterogeneous mixture.
• an element and compounds (for example, tincture of iodine, which is made up of iodine in alcohol). This is also called a heterogeneous mixture.

$$24$$k gold means $$99.9$$% of pure gold while $$22$$k means approximately $$91.6$$% of gold and the remaining $$9$$% containing other metals or metal alloys such as copper, silver, zinc. On the other hand, that is called $$916$$ gold, which comprises $$91.6$$ gold. Pure gold easily breaks; hence, we cannot use it for making ornaments. Instead, to make the gold hard, we use gold with metal alloys (copper, zinc, silver). The purity of gold is mentioned by "carat" or "k".