Time / min Glass / °C Stainless steel / °C Plastic / °C 0 85 85 85 5 78 75 80 10 72 66 76 15 66 58 72 20 60 52 67 25 55 48 62 A student compares the cooling...

(a) The cooling curves for all three containers are plotted below. [2] for correct points, [1] for smooth labelled curves.

All three start at 85 °C. Stainless steel cools fastest (steepest curve), plastic cools slowest (shallowest curve), and glass is in between.

(b) Total temperature drop for each container after 25 minutes:

\[ \text{Glass: } 85 - 55 = \mathbf{30} \text{ °C} \]

\[ \text{Stainless steel: } 85 - 48 = \mathbf{37} \text{ °C} \]

\[ \text{Plastic: } 85 - 62 = \mathbf{23} \text{ °C} \] [1]

(c) Stainless steel loses heat most quickly, with the largest temperature drop of 37 °C after 25 minutes (compared to 30 °C for glass and 23 °C for plastic). [1]

(d) Stainless steel is a metal, which has high thermal conductivity due to its free (delocalised) electrons. Heat is conducted rapidly through the metal walls to the outer surface, where it is lost to the surroundings. [1] Glass and plastic are non-metals and poor thermal conductors (insulators), so heat passes through their walls much more slowly, resulting in smaller temperature drops. [1]

(e) Two variables the student must control (any two from): [1]

• Same volume of water (250 cm³)
• Same starting temperature (85 °C)
• Same type of lid
• Same wall thickness of the containers
• Same room temperature

(f) The student should choose plastic. [1] Plastic is the poorest thermal conductor of the three materials, so it has the smallest temperature drop (23 °C in 25 minutes) and keeps the drink hotter for longer. [1]

(g) One additional step to slow cooling (any one): [1]

• Wrap the container with an insulating material (e.g. foam or bubble wrap)
• Use a better-insulated lid
• Wrap the container with reflective foil to reduce heat loss by radiation