ICSE Class 9 Chemistry Chapter 03 Elements, Compounds and Mixtures

Elements, Compounds And Mixtures

Scope Of Syllabus

(i) General characteristics and differences between elements, compounds and mixtures.

Reasons for considering a substance as an element, compound or mixture may be given to make the concepts clear.

(ii) Types of mixtures: of two solids, a solid and a liquid, two liquids, liquid and gas, two gases. Definition of mixture; each type of mixture should be shown to the students (including both homogeneous and heterogeneous - true solution, suspension and colloidal solution to make the concepts clear.

(iii) Separation of mixtures involving - use of a solvent, filtration, evaporation and distillation, fractional distillation, simple paper chromatography, centrifugation, immiscible liquid. The following examples should be used to illustrate the principles of separation of mixtures by using following methods.

(a) Use of solvent and filtration (e.g., sodium chloride - sand (water as solvent), carbon and sulphur (carbon tetrachloride as solvent).

(b) Evaporation e.g., sodium chloride from its aqueous solution.

(c) Distillation e.g., purification of water containing dissolved solids.

(d) Fractional distillation involves the difference in boiling points of liquids e.g., benzene - toluene.

(e) Simple paper chromatography (limited to separation of colouring matter in ink).

(f) Centrifugation (involving separation of cream from milk).

(g) Immiscible liquids (separating funnel e.g., water - carbon tetrachloride).

Important Points To Remember

1. Matter can be easily classified as

2. Pure substances are one which contains particles of only one kind. Pure substances have a definite set of properties.

3. Pure substances which are made up of particles or atoms of only one kind are called elements. The element cannot be made or broken down into simpler substances by any physical or chemical means.

4. These elements are placed in the increasing order of their atomic number in periodic table. There are 116 elements discovered.

5. Elements are classified as

(i) Metals

(ii) Non-metals

(iii) Metalloids

(iv) Inert gases or Noble gases.

6. The differences between metals and non-metals are:

7. The elements having certain properties of metals and non-metals are called metalloids. For example, Arsenic, Germanium, Antimony, Bismuth.

8. The elements having eight electrons in their outermost shell or valence shell are called inert gases or noble gases. Except helium, all the inert gases have eight electrons in their valence shell.

He - Helium complete duplet i.e., two electrons in valence shell.

Ne - Neon

Ar - Argon

Kr - Krypton

Xe - Xenon

Rn - Radon.

9. Compounds are the pure substances in which two or more elements combined chemically in fixed proportion by weight.

Teacher's Note

Understanding elements, metals, and non-metals helps us appreciate why we use copper for electrical wiring and aluminum foil for food storage - each element has unique properties perfectly suited for everyday applications.

10. Characteristics of Compounds are:

(i) Compounds are homogeneous.

(ii) In compounds, two or more elements combined chemically in fixed proportion by weight.

(iii) The different constituents of compounds cannot be separated by simple physical means. They can only be separated by chemical means, for example electrolysis of water.

\[2\text{H}_2\text{O} \xrightarrow{\text{electrolysis}} 2\text{H}_2 + \text{O}_2\]

Water breaks down to give hydrogen and oxygen on passing electric current whereas water cannot be separated into hydrogen and oxygen by simple physical means like filtration, distillation, etc.

(iv) The constituents of compounds loose their identical properties. For example, water is a compound of hydrogen and oxygen. Hydrogen is a combustible gas whereas oxygen is a supporter of combustion whereas water is a colourless liquid which is neither combustible nor is a supporter of combustion.

(v) Formation of compounds involves energy changes, i.e., the energy is either released or absorbed, i.e., either the reaction is exothermic or endothermic in nature.

11. When two or more elements or compounds combine physically in any proportion by weight mixtures are formed.

12. Mixture may be homogeneous or heterogeneous in nature.

13. The mixture having the same composition and same property in its every part is called homogeneous mixture.

14. The mixture having different properties and different compositions in different parts is called heterogeneous mixture.

15. Characteristics of Mixtures are:

(i) In mixture the different constituents combine physically in any proportion by weight.

(ii) During the formation of mixture no energy changes take place.

(iii) The constituents of mixture do not loose their identical property. For example, in a mixture of iron and sulphur, iron gets attracted by the magnet and sulphur dissolves in carbon disulphide.

(iv) The different constituents of mixture can be easily separated by simple physical means like distillation, filtration, etc.

(v) Mixture may be homogeneous or heterogeneous in nature.

16. The comparative study of compounds and mixtures are as follows by taking the example of mixture of iron and sulphur and the compound of iron and sulphur, i.e., iron (II) sulphide.

Teacher's Note

The distinction between a mixture (like iron and sulfur powder) and a compound (like iron sulfide) shows why we can sometimes reverse a process (separate mixtures) but cannot simply unmix a chemical reaction - this is why cooking permanently changes ingredients but organizing a mixed salad doesn't.

17. The differences between compound and mixture can be summarized as:

18. The homogeneous mixture of solute and solvent is called as solution.

19. Solids, liquids and gases can be mixed together in different proportions to get the mixtures. The mixtures formed may be either homogeneous or heterogeneous. Depending upon the physical property of mixture, they can be classified as follows:

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