HKDSE Physics

A polystyrene cup contains boiling water of mass 450 g. Crushed ice of mass 300 g at 0°C is added to the boiling water and well mixed. What is the final temperature of the mixture if no heat is exchanged with the surroundings ? [CW]

Given:

specific heat capacity of water is 4200 J kg^-1 °C^-1

specific latent heat of fusion of ice is 3.34 x 10⁵ J kg^-1,

specific latent heat of vaporization of water is 2.26 x 10⁶ J kg^-1

• 14.1 °C

• 28.2 °C

• 42.5 °C

• 100 °C

Answer explanation

Assume the final temperature of the mixture is Θ.

heat gained by the ice = heat loss by the hot water

∴ m_i l_f + m_i c ΔT_i = m_w c ΔT_w

∴ (0.3)(3.34 x 10⁵) + (0.3) (4200) (Θ-0) = (0.45) (4200) (100-Θ)

∴Θ = 28.2 °C

Which of the following can be the unit(s) of energy? [NSS]

(1) W s

(2) J s^-1

(3) kg °C

• (1) only

• (2) only

• (1) and (3) only

• (2) and (3) only

When the temperature of a metal block increases, [NSS]

• the size of its particles increases.

• its particles move more rapidly.

• the average kinetic energy of its particles remains unchanged.

• the number of its particles increases.

A heater immersed in water is connected to a kilowatt-hour meter. It is found that

0.15 kW h is used by the heater. If half of the energy is lost to the surroundings,

what is the total energy gained by the water? [NSS]

• 75 J

• 150 J

• 4500 J

• 270 kJ

Which of the following is a unit of heat capacity? [NSS]

• J

• J kg^-1 C^-1

• J °C^-1

Which of the following are related to the high specific heat capacity of water? [NSS]

(1) Water is used as a coolant in motor cars.

(2) Coastal areas have a smaller temperature change throughout a day.

(3) The temperature of our body changes more slowly than the temperature of

the surroundings.

• (1) and (2) only

• (1) and (3) only

• (2) and (3) only

• (1), (2) and (3)

An immersion heater of power 150 W is switched on for 2 minutes. How much

energy is given out by the heater? [NSS]

• 150 J

• 300 J

• 18 kJ

• 30 kJ

A metal block of mass 3 kg is supplied with 36 kJ of energy. Its temperature rises

from 30 °C to 60 °C. What is the heat capacity of the block in J °C^-1? [NSS]

• 30

• 400

• 1200

• 12000

A heater is used to heat up a 5-kg metal block. The temperature of the block rises

from 25 °C to 35 °C. The heat capacity of the block is 100 J °C^-1. Assume that

there is no energy loss to the surroundings. How much energy is transferred to the

block from the heater? [NSS]

• 100 J

• 500 J

• 1000 J

• 5000 J

An ice block of 50 g is taken out from a freezer of –15 °C. Its temperature then

rises from –15 °C to –7 °C. Which of the following expressions shows the energy

(in J) absorbed by the ice block? [NSS]

(Given: specific heat capacity of ice = 2060 J kg^-1 °C^-1)

• 50 × 2060 × (–7 – 15)

• 50 × 2060 × (–7 + 15)

• 0.05 × 2060 × (–7 – 15)

• 0.05 × 2060 × (–7 + 15)