Question Answers For All Chapters – General Science Class 8
1. A. Whom should I pair with?
| Group A | Group B |
|---|---|
| a. Temperature of a healthy human body | ii. 98.6°F |
| b. Boiling point of water | iv. 212°F |
| c. Room temperature | i. 296 K |
| d. Freezing point of water | iii. 0°C |
1. B. Who is telling the truth?
| Statement | True / False |
|---|---|
| a. The temperature of a substance is measured in Joules. | False (Temperature is measured in °C, °F, or K) |
| b. Heat flows from an object at higher temperature to an object at lower temperature. | True |
| c. Joule is the unit of heat. | True |
| d. Objects contract on heating. | False (Objects expand when heated) |
| e. Atoms of a solid are free. | False (Atoms in a solid are tightly packed) |
| f. The average kinetic energy of atoms in a hot object is less than in a cold object. | False (Hot objects have higher kinetic energy) |
1. C. You will find if you search.
a. A thermometer is used to measure temperature.
b. The apparatus used to measure heat is called a calorimeter.
c. Temperature is the measure of the average kinetic energy of the atoms in a substance.d. The heat contained in a substance is the measure of the total kinetic energy of atoms in the substance.
2. Nishigandha and Shivani’s Tea Preparation
- Shivani’s tea will be ready first because a stove produces more heat quickly than a solar cooker.
- A solar cooker depends on sunlight, so it takes longer to heat the water compared to a stove’s direct flame.
3. Write Brief Answers
(a) Describe a clinical thermometer. How does it differ from a laboratory thermometer?
- A clinical thermometer is used to measure body temperature and has a range of 35°C to 42°C.
- A laboratory thermometer is used in experiments and has a wider temperature range (up to 110°C).
(b) What is the difference between heat and temperature? What are their units?
- Heat is the total energy of all molecules in a substance, while temperature is the average energy of molecules.
- Heat is measured in Joules (J) or calories (cal), while temperature is measured in °C, °F, or K.
(c) Explain the construction of a calorimeter. Draw the necessary figure.
- A calorimeter has two vessels (inner and outer) with an air gap to prevent heat loss.
- It contains water, a thermometer, and a stirrer to measure heat transfer during chemical reactions.
(d) Explain why rails have gaps at specific distances.
- Rails expand in summer due to heat, and gaps are provided to prevent bending.
- Without these gaps, rails could expand and cause accidents.
(e) Explain with formulae the expansion coefficients of liquid and gas.
- Expansion of liquids:V₂ = V₁ (1 + β ΔT) (β = coefficient of volume expansion)
- Expansion of gases at constant pressure:V₂ = V₁ (1 + β ΔT) (Gases expand more than liquids when heated)
4. Solve the Following Examples
(a) What must be the temperature in Fahrenheit so that it will be twice its value in Celsius?
Using the formula F = (9/5 × C) + 32, solving for F = 2C:
- Answer: 320°F
(b) A bridge is made from 20 m long iron rods. At 18°C, the distance between two rods is 0.4 cm. Up to what temperature will the bridge be in good shape?
- Answer: 35.4°C
(c) At 15°C, the height of the Eiffel Tower is 324 m. If it is made of iron, what will be the increase in length in cm at 30°C?
- Answer: 5.6 cm
(d) Two substances A and B have specific heats c and 2c respectively. If A and B are given Q and 4Q amounts of heat respectively, the change in their temperatures is the same. If the mass of A is m, what is the mass of B?
- Answer: 2m
(e) When a substance having mass 3 kg receives 600 cal of heat, its temperature increases by 10°C. What is the specific heat of the substance?
- Using the formula: Q = m × c × ΔT
- c = Q / (m × ΔT) = 600 / (3 × 10) = 0.02 cal/g°C
- Answer: 0.02 cal/(g°C)
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