In this experiment, you will investigate heat transfer by mixing hot and cold water. You measures 100 cm³ of cold water at 18 °C into a polystyrene cup. You...

(a) Initial temperatures

Read directly from the question: cold water = 18 °C, hot water = 62 °C. [1]

(b) Temperature rise of the cold water

The cold water starts at 18 °C and reaches a final temperature of 39 °C:

\(\Delta T = 39 - 18 = \mathbf{21}\) °C [1]

(c) Why the final temperature is not exactly 40 °C

If no energy were lost, mixing equal masses of water at 18 °C and 62 °C would give \((18 + 62)/2 = 40\) °C. The final temperature is 39 °C, which is 1 °C lower than expected. This is because some thermal energy is lost to the surroundings during transfer and mixing, and the cup itself absorbs a small amount of heat. [1]

(d) Why a polystyrene cup is used

Polystyrene is an excellent thermal insulator. It has very low thermal conductivity, so it minimises the rate of heat transfer from the water to the surroundings. A metal beaker would conduct heat away from the water rapidly, causing greater energy loss and a less accurate result. [1]

(e) Improving reliability

Any one valid method: [1]

• Repeat the experiment several times and calculate an average final temperature.
• Use a measuring cylinder to measure the water volumes more precisely.
• Pour the hot water quickly to minimise heat loss during the transfer.

(f) What to do before recording the final temperature

Stir the mixture thoroughly to ensure the temperature is uniform throughout the liquid, then wait for the thermometer reading to stabilise before taking the measurement. Without stirring, hotter water could sit on top while cooler water remains below, giving a misleading reading. [1]

(g) Resolution of the thermometer

The resolution is the smallest change a measuring instrument can detect. If the smallest division on the thermometer is 1 °C, then the resolution is 1 °C. [1]