Explain how the features labelled in Figure 5 of a vacuum flask reduce heat transfer.

Here are the answers to the questions based on Figure 5:

i) The labelled feature of the vacuum flask that reduces heat transfer by both conduction and convection is the vacuum.

ii) The vacuum reduces heat transfer by conduction because there are very few particles in a vacuum to transfer thermal energy through vibrations. It reduces heat transfer by convection because convection requires the movement of fluid (gas or liquid) particles, and a vacuum contains no such fluid.

iii) The labelled feature of the vacuum flask that reduces heat transfer by radiation is the silvered inner surfaces.

iv) The silvered inner surfaces reduce heat transfer by radiation because they are highly reflective. They reflect thermal radiation (infrared waves) back into the flask, preventing heat loss from a hot liquid, or reflect external radiation away from the flask, preventing heat gain by a cold liquid. They are also poor emitters of thermal radiation.