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COMPOUNDS AND MIXTURES

    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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