Question Answers For All Chapters – General Science Class 8
Exercise
1. Choose the appropriate option and rewrite the following statements.
a. The intermolecular force is maximum in the particles of solid.
b. Solids retain their volume even when external pressure is applied. This property is called incompressibility.
c. Matter is classified into the types mixture, compound, and element, by applying the criterion chemical composition of matter.
d. Matter that contains two or more constituent substances is called mixture.
e. Milk is an example of a type of matter called colloid.
f. Water, mercury, and bromine are similar to each other because they are liquids.
g. Valency of carbon is 4 and that of oxygen is 2. From this, we understand that there are 2 chemical bonds between the carbon atom and one oxygen atom in the compound carbon dioxide.
2. Identify the odd term out and explain.
a. Gold, silver, copper, brass
- Odd term: brass
- Explanation: Gold, silver, and copper are elements, but brass is a mixture (alloy) of copper and zinc.
b. Hydrogen, hydrogen peroxide, carbon dioxide, water vapor
- Odd term: hydrogen
- Explanation: Hydrogen is an element, while the others are compounds.
c. Milk, lemon juice, carbon, steel
- Odd term: carbon
- Explanation: Carbon is an element, while the others are mixtures.
d. Water, mercury, bromine, petrol
- Odd term: mercury
- Explanation: Mercury is an element, while water and bromine are compounds, and petrol is a mixture.
e. Sugar, salt, baking soda, blue vitriol
- Odd term: sugar
- Explanation: Sugar is an organic compound, while the others are inorganic compounds.
f. Hydrogen, sodium, potassium, carbon
- Odd term: carbon
- Explanation: Hydrogen, sodium, and potassium have valency 1, while carbon has valency 4.
3. Answer the following questions.
a. Plants synthesize glucose in sunlight with the help of chlorophyll from carbon dioxide and water and give away oxygen. Identify the four compounds in this process and name their types.
- Compounds:
- Glucose (C₆H₁₂O₆) – Organic compound
- Carbon dioxide (CO₂) – Inorganic compound
- Water (H₂O) – Inorganic compound
- Oxygen (O₂) – Element
b. In one sample of brass, the following ingredients were found: copper (70%) and zinc (30%). Identify the solvent, solute, and solution from these.
- Solvent: Copper (major component, 70%)
- Solute: Zinc (minor component, 30%)
- Solution: Brass (the mixture of copper and zinc)
c. Sea water tastes salty due to the dissolved salt. The salinity (the proportion of salts in water) of some water bodies: Lonar lake – 7.9%, Pacific Ocean – 3.5%, Mediterranean Sea – 3.8%, Dead Sea – 33.7%. Explain two characteristics of mixtures from the above information.
- Characteristic 1: Mixtures can have varying proportions of their components (salinity differs in each water body).
- Characteristic 2: Properties of mixtures depend on their components (salty taste comes from dissolved salts).
4. Give two examples each.
(a) Liquid element: Mercury (Hg), Bromine (Br).
(b) Gaseous element: Oxygen (O₂), Hydrogen (H₂).
(c) Solid element: Iron (Fe), Carbon (C).
(d) Homogeneous mixture: Sugar solution, Air.
(e) Colloid: Milk, Fog.
(f) Organic compound: Glucose (C₆H₁₂O₆), Methane (CH₄).
(g) Complex compound: Hemoglobin, Chlorophyll.
(h) Inorganic compound: Sodium chloride (NaCl), Carbon dioxide (CO₂).
(i) Metalloid: Silicon (Si), Boron (B).
(j) Element with valency 1: Hydrogen (H), Sodium (Na).
(k) Element with valency 2: Oxygen (O), Magnesium (Mg).
5. Write the names and symbols of the constituent elements and identify their valencies from the molecular formulae given below.
| Compound | Elements | Symbols | Valency |
|---|---|---|---|
| KCl | Potassium, Chlorine | K, Cl | 1, 1 |
| HBr | Hydrogen, Bromine | H, Br | 1, 1 |
| MgBr₂ | Magnesium, Bromine | Mg, Br | 2, 1 |
| K₂O | Potassium, Oxygen | K, O | 1, 2 |
| NaH | Sodium, Hydrogen | Na, H | 1, 1 |
| CaCl₂ | Calcium, Chlorine | Ca, Cl | 2, 1 |
| CCl₄ | Carbon, Chlorine | C, Cl | 4, 1 |
| HI | Hydrogen, Iodine | H, I | 1, 1 |
| H₂S | Hydrogen, Sulfur | H, S | 1, 2 |
| Na₂S | Sodium, Sulfur | Na, S | 1, 2 |
| FeS | Iron, Sulfur | Fe, S | 2, 2 |
| BaCl₂ | Barium, Chlorine | Ba, Cl | 2, 1 |
6. Chemical composition of some matter is given in the following table. Identify the main type of matter from their.
| Name of Matter | Chemical Composition | Type of Matter |
|---|---|---|
| Sea water | H₂O + NaCl + MgCl₂ | Mixture |
| Distilled water | H₂O | Compound |
| Hydrogen gas in a balloon | H₂ | Element |
| LPG gas | C₄H₁₀ + C₃H₈ | Mixture |
| Baking soda | NaHCO₃ | Compound |
| Pure gold | Au | Element |
| Gas in an oxygen cylinder | O₂ | Element |
| Bronze | Cu + Sn | Mixture (Alloy) |
| Diamond | C | Element |
| Heated white powder of blue vitriol | CuSO₄ | Compound |
| Limestone | CaCO₃ | Compound |
| Dilute hydrochloric acid | HCl + H₂O | Mixture |
7. Write scientific reason.
(a) Hydrogen is combustible, oxygen supports combustion, but water extinguishes fire.
- Hydrogen (H₂) burns, and oxygen (O₂) helps in burning, but when they combine to form water (H₂O), it loses these properties.
(b) Constituents of a colloid cannot be separated by ordinary filtration.
- The particles in a colloid are too small to be filtered out but large enough to scatter light.
(c) Lemon sherbet has sweet, sour, and salty tastes and can be poured into a glass.
- It is a mixture of sugar (sweet), lemon juice (sour), and salt (salty), and it has fluidity like a liquid.
(d) A solid has a definite shape and volume.
- The particles in a solid are tightly packed, so solids maintain their shape and volume.
8. Deduce the molecular formulae of the compound obtained from the following pairs of elements by the cross multiplication method.
(a) Carbon (C) (valency 4) & Chlorine (Cl) (valency 1) → CCl₄
(b) Nitrogen (N) (valency 3) & Hydrogen (H) (valency 1) → NH₃
(c) Carbon (C) (valency 4) & Oxygen (O) (valency 2) → CO₂
(d) Calcium (Ca) (valency 2) & Oxygen (O) (valency 2) → CaO
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