Carbon and Its Compounds
ICSE Class 8 · Chemistry · Chapter 9
What this chapter covers
- Carbon: an element found everywhere
- Allotropes of carbon
- Diamond and graphite: the crystalline forms
- Coal, coke and soot: the amorphous forms
- Uses of carbon allotropes
- Carbon dioxide: laboratory preparation
- Properties and uses of carbon dioxide
- Test for carbon dioxide
- Carbon monoxide
Key definitions
Allotropy is the property of an element by which it exists in two or more forms that have different physical properties but the same chemical properties.
Allotropes are the different forms of the same element, which differ in physical properties such as hardness, colour and conductivity but show the same chemical properties.
Crystalline allotropes are forms of carbon in which the atoms are arranged in a regular, repeating pattern that gives the substance a definite shape.
Amorphous allotropes are forms of carbon in which the atoms have no regular pattern, so the substance has no definite crystal shape.
Coal is a black, hard, amorphous form of carbon formed from the remains of plants that were buried deep under the earth for millions of years.
Coke is a grey, hard and porous form of carbon obtained by strongly heating coal in the absence of air.
Soot is a fine, black, powdery form of carbon that forms when substances rich in carbon, such as kerosene, oil or wood, burn in a poor supply of air.
Carbonic acid is a weak acid formed when carbon dioxide dissolves in water.
Lime water is a clear, colourless solution of calcium hydroxide in water.
Incomplete combustion is the burning of a fuel in a limited supply of air, in which carbon monoxide and soot are formed along with or instead of carbon dioxide.
Asphyxia is a condition in which the body does not get enough oxygen, leading to suffocation.
Key points
- Carbon is a versatile element present in all living things and in a huge number of compounds.
- Allotropes are forms of the same element with different physical properties but the same chemical properties.
- Diamond and graphite are crystalline allotropes; coal, coke and soot are amorphous forms of carbon.
- Every form of carbon burns in oxygen to give carbon dioxide; pure forms such as diamond and graphite give only carbon dioxide.
- In diamond each carbon atom is joined to four others in a rigid network, so diamond is very hard and does not conduct electricity.
- In graphite each carbon atom is joined to three others in sliding layers, so graphite is soft, slippery and conducts electricity.
- Diamond is used for jewellery and cutting; graphite for pencils, lubricants and electrodes; coke for extracting iron; soot for inks, polish and tyres.
- Carbon dioxide is made in the laboratory from marble chips and dilute hydrochloric acid and collected by upward displacement of air.
- Carbon dioxide is colourless, heavier than air, fairly soluble in water and does not support burning.
- Carbon dioxide is an acidic oxide: it turns blue litmus pale red and reacts with basic oxides to form carbonates.
- Carbon dioxide turns lime water milky, and the milkiness disappears with excess gas.
- Carbon monoxide forms when fuels burn in a poor supply of air; it burns with a blue flame and is a neutral oxide.
- Carbon monoxide is a reducing agent used to extract iron from its oxide.
- Carbon monoxide is a poisonous, odourless gas that causes asphyxia, so rooms with fires need air and no one should stand behind a running engine.