The vacuum and silvered wall surfaces of a vacuum flask walls are respectively

Analyze the question: A vacuum flask (thermos) is built to keep contents hot or cold. The question asks what the vacuum and the silvered walls each do, in that order.

Explain the concept: Heat travels in three ways:

• Conduction needs a material medium (particles) to pass energy along.
• Convection needs a fluid (liquid or gas) that can move.
• Radiation is infrared waves that need no medium.

Solution:

1. The vacuum (the space between the double glass walls) has almost no particles. With no medium, conduction and convection cannot occur, so the vacuum prevents conduction and convection. In practice the vacuum is never perfect, and a little heat still goes through the stopper and the glass at the neck, but the vacuum is the part that stops these two.

2. The silvered surfaces are shiny, so they are poor emitters and good reflectors of radiant heat. They reflect heat back into the liquid (if hot) or keep heat out (if cold). Reflection is never 100%, so they only minimize radiation. A vacuum does not stop radiation, since radiation travels through a vacuum (as sunlight does), so the silvering is needed to reduce it.

So the order is:

• Vacuum → prevents conduction and convection
• Silvered walls → minimize radiation

This matches option B.

Why the others are wrong:

• A: says the vacuum only minimizes conduction and convection, and that silvering prevents radiation. Silvering cannot fully prevent radiation.
• C: says the vacuum minimizes convection, and treats the vacuum as dealing with radiation.
• D: says radiation is prevented, which is not possible.

Exam tip: Remember "Vacuum stops the two that need matter (conduction and convection); silver only reduces the one that doesn't (radiation)." In WASSCE wording, prevent goes with the vacuum and minimize goes with the silvering.