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Chemistry

COMPOUND

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

  1. It has properties different from those of its component elements.
  2. Its formation often requires large amount of heat.
  3. It cannot be separated by physical means.
  4. The components of a compound have a fixed ratio by mass.
  5. Compounds are homogenous.

 

EVALUATION

  1. Define a compound.
  2. 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.

  1. The symbols of all the component elements are written close together as a group.
  2. The number of atoms of each component element is written as a subscript after the symbol of that element.

 

Examples

Compound                             Formula

            Hydrochloric acid                      HCl

Water                                       H20

Ammonia                                  NH3

Carbon(IV)oxide                       C02

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.

  1. Write the symbols of the element or radicals in a compound
  2. Write their valencies below the symbols of elements/radicals
  • Exchange their valencies.
  1. Now write the formula of the compound bringing the symbols of the element or radicals together

 

Examples

  1. Write the formula of sodium tetraoxosulphate(VI)

Rule 1                          Na                                S04

Rule 2 & 3                    1                                  2

Rule 4                          Na2S04

 

  1. Write the formula of calcium chloride

Rule 1                          Ca                                Cl

Rule 2 & 3                    2                                  1

Rule 4                          CaCl2

 

EVALUATION

  1. Write the formulae of; (i) tetraoxosulphate(vi) acid (ii) Magnesium Chloride
  2. State three properties of a compound

 

MIXTURES

A mixture contains two or more constituents which can easily be separated by physical methods.

Examples of mixtures with their constituents are outlined below:

 

Mixture                                                              Constituents

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

Mixture                                                       Compound

  1. It may be homogenous It is always homogenous.

or heterogeneous.

  1. It can be separated into It cannot be separated into its

its constituents by physical                   components by physical means.

means

  1. 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.

Formula.

  1. 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.

 

EVALUATION

  1. List five (5) compounds and their formulae
  2. What is a mixture?
  3. State four differences between compound and mixture

 

GENERAL EVALUATION/REVISION

  1. State the valency of the following elements and radicals: Na, K, S, O, SO42-, NO3, CO32-
  2. Write the formula of: a) Lead (ii) tetraoxosulphate (vi) b) Hydrochloric acid c) Sodium trioxocarbonate (iv) d) Calcium hydroxide

 

WEEKEND ASSIGNMENT                               

  1. Which of the following is a mixture? (a) water(b) sugar(c) milk (d) starch
  2. Which of the following is a compound? (a) water (b) soil (c) diamond (d) graphite
  3. Which of these formulae represents ammonia? (a) NH3 (b) NH4+ (c) NH2 (d) CH4
  4. The formula for sand is (a) C02 (b) SO2 (c) NO2 (d) SiO2
  5. Compounds are always (a) heterogeneous (b) homogeneous (c) homogeneous or heterogeneous (d) chemogeneous

 

THEORY

  1. a. Define (i) Compound (ii) Mixture
  2. Give two examples each of compound and Mixtures
  3. a. State four differences between compound and mixture
  4. What is the formula of
  5. i) tetraoxosulphate (vi) acid
  6. ii) Ammonium sulphide

iii) Sodium tetraoxophosphate

 

See also

ELEMENTS

NATURE OF MATTER

LABORATORY FAMILIARIZATION

INTRODUCTION TO CHEMISTRY

ELECTROLYSIS

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