Question

4 the metal tungsten, symbol w, is used to make wire filaments in light bulbs. the wire glows when electricity passes through it. in a light bulb, the tungsten wire may get so hot that it melts and breaks.
the figure below shows the heating curve for tungsten
temperature /°c
time / min
(a) write down part of the graph, a, b, c, d or e, at which tungsten
3
(i) exists as a mixture of solid and liquid
(ii) exists as a gas only
(iii) has a definite shape and volume
(b) write down the time taken for tungsten to melt completely.
1
(c) what is the maximum temperature tungsten can be heated to before it starts to break?
1
(d) explain, in terms of kinetic particle theory, why the temperature remains constant at d even though heat is taken in during this period.
1
ensway secondary school science department, 2026

Explanation:

Step1: Analyze heating curve regions

Heating curve regions: A (solid heating), B (solid-liquid phase change), C (liquid heating), D (liquid-gas phase change), E (gas heating).

Step2: Answer part (a) sub-questions

(i) Solid-liquid mixture = phase change (B)
(ii) Gas only = post-vaporization (E)
(iii) Definite shape/volume = solid (A)

Step3: Calculate melting time

Melting starts at 2 min, ends at 5 min. Time = $5 - 2 = 3$ min

Step4: Find max temp before breaking

Flat region D = boiling, max temp before boiling is ~5700°C

Step5: Explain constant temp at D

Heat used for phase change, not kinetic energy.

Answer:

(a)
(i) B
(ii) E
(iii) A

(b) 3 minutes

(c) 5700 °C

(d) The heat energy supplied is used to overcome the intermolecular forces between tungsten liquid particles to convert it to gas, rather than increasing the kinetic energy of the particles. Since temperature is a measure of average kinetic energy, the temperature remains constant during this phase change.