物理物態變化 DSE、CE 選擇題（第 3 頁）

21.

Questions 34 and 35 : The following set-up is used to measure $\ell_v$ , the specific latent heat of vaporization of a liquid.

Beaker of liquid with immersion heater connected to a kWh meter and power supply, standing on an electronic balance

The result of the experiment is as follows : Initial reading of the balance $= 1.60$ kg Final reading of the balance $= 1.45$ kg Energy supplied as measured by the kWh meter $= 0.10$ kWh Find the measured value of $\ell_v$ .

22.

Questions 34 and 35 : The following set-up is used to measure $\ell_v$ , the specific latent heat of vaporization of a liquid.

How will the value of $\ell_v$ obtained in the experiment be affected if the following measures are taken separately ? I. replacing the beaker with a polystyrene container which is a better insulator II. adding more liquid into the beaker until the heater is completely immersed

	I	II
	decreases	increases
	decreases	decreases
	increases	increases
	increases	decreases

23.

Figure (a): beaker of liquid with immersion heater on an electronic balance; Figure (b): balance reading against time, constant m2 until t1 then decreasing to m1 at t2

As shown in Figure (a), some liquid in a beaker is heated by a $1000$ W immersion heater. Figure (b) shows the variation of the reading of the electronic balance with time $t$ . Which of the following statements about the liquid is incorrect ?

24.

Temperature against time graph for crushed solid: rising, flat melting section, then rising again

A certain amount of crushed solid is heated and the variation of its temperature $T$ with time $t$ is shown above. If the same heater is used to heat a smaller amount of the solid, which of the following graphs (in dash lines) best shows the variation of the temperature of the solid ?

Four graphs A to D, each with the original curve (solid line) and a new curve (dashed line)

25.

A melting ice block of mass $0.05$ kg is mixed with $x$ kg of water at $0^\circ\mathrm{C}$ in a well-insulated container. If $25\,000$ J of energy is supplied to the mixture, the mixture changes to water at $4^\circ\mathrm{C}$ . Find the value of $x$ . (Given : specific latent heat of fusion of ice $= 3.34\times10^5\ \mathrm{J\,kg^{-1}}$ , specific heat capacity of water $= 4200\ \mathrm{J\,kg^{-1}\,{}^\circ C^{-1}}$ .)

26.

The figure shows the cooling curve of a substance which is initially in the liquid state. The temperature of the substance remains unchanged during the period $PQ$ . Which of the following statements about the substance during the period $PQ$ is/are correct ?

(1)The substance is not losing any energy to the surroundings.

(2)Latent heat is absorbed by the substance.

(3)The average potential energy of the molecules of the substance is decreasing.

Cooling curve of temperature against time with a flat section between points P and Q

27.

Beaker of water with immersion heater connected to joulemeter and power supply, standing on an electronic balance

The above apparatus is used to find the specific latent heat of vaporization of water, $\ell_v$ . Which of the following factors will cause the result obtained to be larger than the true value of $\ell_v$ ?

(1)Some energy is lost to the surroundings.

(2)Some steam condenses and drips back into the beaker.

(3)Some boiling water inside the beaker splashes out of the beaker.

28.

Beaker of liquid with immersion heater connected to a power supply, standing on an electronic balance

The above apparatus is used to find the specific latent heat of vaporization of a liquid. Which of the following can improve the accuracy of the experiment ?

29.

Two identical set-ups with ice in funnel X (heater connected to power supply) and funnel Y (heater not connected), each with a beaker below

The above set-up is used to find the specific latent heat of fusion of ice. Which of the following is not an essential precaution to ensure an accurate result ?

30.

A block of melting ice with mass $0.02$ kg is put into a polystyrene cup containing $0.3$ kg of water with initial temperature $20^\circ\mathrm{C}$ . After the mixture is stirred well, the ice block melts completely and the final temperature of the water becomes $14^\circ\mathrm{C}$ . Which of the following equations can be used to find the specific latent heat of fusion of ice, $L$ ? (Given : Specific heat capacity of water $= 4200\ \mathrm{J\,kg^{-1}\,{}^\circ C^{-1}}$ )