Heat and heat technology. 8th Grade Science Worksheets and Answer ... - Free Printable

Let’s solve each problem step by step, explaining the physics behind each answer.

---

1. Complete the table: Temperature of Melting Ice and Boiling Water

We need to fill in the missing temperature values in °C and K (Kelvin) for melting ice and boiling water.

#### Key facts:
- Melting point of ice (freezing point of water) = 0°C = 32°F
- Boiling point of water = 100°C = 212°F
- Kelvin conversion: K = °C + 273

#### So:

| Temperature of Melting Ice | °C | 32°F | K |
|----------------------------|----|------|-------|
| | 0°C | 32°F | 273 K |

| Temperature of Boiling Water | °C | 212°F | K |
|------------------------------|----|-------|-------|
| | 100°C | 212°F | 373 K |

✔ Answer:
- Melting ice: 0°C, 273 K
- Boiling water: 100°C, 373 K

---

2. Heating different volumes of water with same heat energy

You apply the same amount of heat energy to 1L, 2L, and 3L of water.

> Heat energy Q = m × c × ΔT
> where:
> - m = mass (proportional to volume for water)
> - c = specific heat capacity (same for water)
> - ΔT = temperature change

Since Q is constant, and c is constant, then:

> ΔT ∝ 1/m ∝ 1/V (since density is constant)

So if 1L water heats up by 10°C, then:

- For 2L: ΔT = 10°C / 2 = 5°C
- For 3L: ΔT = 10°C / 3 ≈ 3.33°C

✔ Answer:
- 2L: ΔT = 5°C
- 3L: ΔT = 3.33°C (or write as 10/3 °C)

---

3. Mixing water at different temperatures

#### a. Equal volume of 20°C water added to 20°C water

Mixing equal volumes of water at the same temperature → no net heat transfer → final temp = 20°C

✔ Answer: (20°C)

#### b. Equal volume of 40°C water added to 20°C water

Equal masses, so final temperature is average:

> T_final = (20°C + 40°C)/2 = 30°C

✔ Answer: (30°C)

#### c. Small amount of 40°C water added to 20°C water

Since only a small amount is added, the final temperature will be slightly above 20°C, but less than 30°C.

Think: If you add a drop of hot water to a bucket of cold water, it barely warms it — so definitely between 20°C and 30°C.

✔ Answer: (between 20°C and 30°C)

---

4. Red-hot iron put into cool water — True or False?

Assume no heat loss to environment (bucket ignored).

#### a. *The decrease in iron temperature equals the increase in the water temperature.*

✘ False

Why? The iron and water have different masses and different specific heats. The *heat lost* by iron = *heat gained* by water, but because their heat capacities differ, the temperature changes are not equal.

Example: Iron has low specific heat, so its temp drops a lot; water has high specific heat, so its temp rises little.

#### b. *The quantity of heat lost by the iron equals the quantity of heat gained by the water.*

✔ True

This is conservation of energy (assuming no losses). Heat lost by hot object = heat gained by cold object.

#### c. *The iron and water both will reach the same temperature.*

✔ True

They will reach thermal equilibrium — same final temperature.

#### d. *The final temperature of the iron and water is halfway between the initial temperatures of each.*

✘ False

Only true if the iron and water have equal heat capacities (mass × specific heat). Since water has much higher specific heat and usually more mass, the final temp is much closer to the water’s initial temp than halfway.

Example: Hot iron (say 500°C) in room temp water (20°C) → final temp might be 25°C, not 260°C.

✔ Answers:
a. F
b. T
c. T
d. F

---

## ✔ Final Answers Summary:

1. Table:

- Melting ice: 0°C, 273 K
- Boiling water: 100°C, 373 K

2. ΔT for 2L and 3L:

- 2L: 5°C
- 3L: 3.33°C (or 10/3 °C)

3.

a. 20°C
b. 30°C
c. between 20°C and 30°C

4.

a. F
b. T
c. T
d. F

---

Let me know if you’d like diagrams or deeper derivations!