A Compound is a substance which contains two or more elements chemically combined together. A compound is formed as a result of chemical change.
Examples of compounds are
Compound Constituent Elements
Water Hydrogen, oxygen
Sand silicon, oxygen
Limestone calcium, carbon, oxygen
Common salt sodium, chlorine
Ethanol carbon, hydrogen, oxygen
PROPERTIES OF A COMPOUND
- It has properties different from those of its component elements.
- Its formation often requires large amount of heat.
- It cannot be separated by physical means.
- The components of a compound have a fixed ratio by mass.
- Compounds are homogenous.
- Define a compound.
- Give three examples of a compound.
FORMULAE OF COMPOUNDS
When an element exists as a molecule, a number is written as a subscript after the symbol of that element. For example, hydrogen is written as H2 and oxygen as 02.
A compound contains whole numbers of atoms of the component elements. Its molecular formula is written as follows.
- The symbols of all the component elements are written close together as a group.
- The number of atoms of each component element is written as a subscript after the symbol of that element.
Hydrochloric acid HCl
Lead II chloride PbCl2
Calcium trioxonitrate(V) Ca(NO3)2
WRITING FORMULA FROM VALENCIES
Formulae of compounds can be deduced from the valencies of the component elements or radicals, following the rules below.
- Write the symbols of the element or radicals in a compound
- Write their valencies below the symbols of elements/radicals
- Exchange their valencies.
- Now write the formula of the compound bringing the symbols of the element or radicals together
- Write the formula of sodium tetraoxosulphate(VI)
Rule 1 Na S04
Rule 2 & 3 1 2
Rule 4 Na2S04
- Write the formula of calcium chloride
Rule 1 Ca Cl
Rule 2 & 3 2 1
Rule 4 CaCl2
- Write the formulae of; (i) tetraoxosulphate(vi) acid (ii) Magnesium Chloride
- State three properties of a compound
A mixture contains two or more constituents which can easily be separated by physical methods.
Examples of mixtures with their constituents are outlined below:
Air Oxygen, Carbon (iv)oxide, nitrogen, rare gases, dust, moisture
Soil Sand, clay, humus, water, air, mineral salts
Urine urea, water, mineral salt
Palm wine water, sugar, alkanol, mineral salts, vitamins, yeast, protein, fat
Coca-cola water, sugar, cola, CO2
Milk water, sugar, fat, protein, mineral salts, vitamin
Sea water water, mineral salts, bacteria, remains of organic matter
Brass copper and zinc
DIFFERENCES BETWEEN MIXTURES AND COMPOUNDS
- It may be homogenous It is always homogenous.
- It can be separated into It cannot be separated into its
its constituents by physical components by physical means.
- The constituents can be The components are present in a
added in any ratio by mass fixed ratio by mass. Hence a
Hence a mixture cannot be compound can always be repre-
represented by a chemical sented by a chemical formula.
- The properties of a mixture The properties of a compound
are the sum of those of its are entirely different from those
individual constituents. of its components.
- List five (5) compounds and their formulae
- What is a mixture?
- State four differences between compound and mixture
- State the valency of the following elements and radicals: Na, K, S, O, SO42-, NO3–, CO32-
- Write the formula of: a) Lead (ii) tetraoxosulphate (vi) b) Hydrochloric acid c) Sodium trioxocarbonate (iv) d) Calcium hydroxide
- Which of the following is a mixture? (a) water(b) sugar(c) milk (d) starch
- Which of the following is a compound? (a) water (b) soil (c) diamond (d) graphite
- Which of these formulae represents ammonia? (a) NH3 (b) NH4+ (c) NH2 (d) CH4
- The formula for sand is (a) C02 (b) SO2 (c) NO2 (d) SiO2
- Compounds are always (a) heterogeneous (b) homogeneous (c) homogeneous or heterogeneous (d) chemogeneous
- a. Define (i) Compound (ii) Mixture
- Give two examples each of compound and Mixtures
- a. State four differences between compound and mixture
- What is the formula of
- i) tetraoxosulphate (vi) acid
- ii) Ammonium sulphide
iii) Sodium tetraoxophosphate