Question Answers For All Chapters – General Science Class 8
Exercise
1. Complete the statement by filling the gaps using appropriate term from the terms given in the bracket.
(slow, coloured, arrow, fast, smell, milky, physical, product, chemical, reactant, covalent, ionic, octet, duplet, exchange, sharing, equality sign)
(a) An arrow is drawn in between the reactants and products while writing the equation for a chemical reaction.
(b) Rusting of iron is a slow chemical change.
(c) The spoiling of food is a chemical change which is recognized from the generation of certain smell due to it.
(d) A colourless solution of calcium hydroxide in a test tube turns milky on blowing in it through a blow tube for some time.
(e) The white particles of baking soda disappear when put in lemon juice. This means that it is a chemical change.
(f) Oxygen is a reactant in respiration.
(g) Sodium chloride is an ionic compound while hydrogen chloride is a covalent compound.
(h) Electron duplet is complete in each hydrogen atom in a hydrogen molecule.
(i) Chlorine (Cl₂) molecule is formed by sharing of electrons between two chlorine atoms.
2. Explain by writing a word equation:
(a) Respiration is a chemical change.
- Word equation: Glucose + Oxygen → Carbon dioxide + Water + Energy
(b) Hard water gets softened on mixing with a solution of washing soda.
- Word equation:Calcium chloride + Sodium carbonate → Calcium carbonate + Sodium chloride
(c) Limestone powder disappears on adding to dilute hydrochloric acid.
- Word equation:Calcium carbonate + Hydrochloric acid → Calcium chloride + Carbon dioxide + Water
(d) Bubbles are seen on adding lemon juice to baking soda.
- Word equation:Citric acid + Sodium bicarbonate → Carbon dioxide + Sodium citrate + Water
3. Match the pairs:
| A | B |
|---|---|
| a. Photosynthesis | iii. Chemical change |
| b. Water | iv. Covalent bond |
| c. Sodium chloride | v. Ionic bond |
| d. Dissolution of salt in water | vi. Physical change |
| e. Carbon | ii. Reactant in combustion process |
| f. Fluorine | vii. Tendency to form anion |
| g. Magnesium | i. Tendency to lose electrons |
4. Show with the help of a diagram of electronic configuration how the following compounds are formed from the constituent atoms:
(a) Sodium chloride (NaCl) – Formed by transfer of one electron from sodium to chlorine (ionic bond).
(b) Potassium fluoride (KF) – Formed by transfer of one electron from potassium to fluorine (ionic bond).
(c) Water (H₂O) – Formed by sharing of electrons between hydrogen and oxygen (covalent bond).
(d) Hydrogen chloride (HCl) – Formed by sharing of electrons between hydrogen and chlorine (covalent bond).
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