State how carbon occurs in the free state and in the combined state.

In the free state —

• carbon occurs free in nature in the crystalline form [i.e. crystal] as diamond & graphite & in the

• amorphous form [impure] as coal formed below the earth's surface.

In the combined state —

• Carbon occurs as carbon dioxide in the atmosphere [about 0.03% by vol.]
• It also occurs as hydrocarbons in natural gas & petroleum.
• In the solid state in wood [cellulose].
• In plants & animals as carbohydrates & in minerals as limestone - CaCO₃