The inside of a vacuum flask is usually coated with silver to reduce heat loss by

Analyze the question: A vacuum flask (thermos) keeps hot liquids hot and cold liquids cold. The question asks which mode of heat loss is reduced by the silvered inner walls.

Concept: The three ways heat is transferred

• Conduction: heat passes through a material by particle-to-particle contact (needs a medium).
• Convection: heat is carried by moving fluids (liquids or gases).
• Radiation: heat travels as infrared electromagnetic waves (needs no medium).

Parts of a vacuum flask and what each does

1. Double-walled glass with a vacuum between the walls: with no particles in between, this prevents conduction and convection.
2. Silvered (shiny) surfaces: shiny surfaces are poor absorbers and poor emitters of radiant heat. They reflect heat radiation back into the liquid (for hot contents) or reflect it away from the contents (for cold ones). This reduces heat transfer by radiation.
3. Cork or plastic stopper: reduces conduction and convection through the mouth of the flask.

Eliminating the options

• Condensation and evaporation are not the main modes of heat transfer here. The stopper limits them.
• Conduction and convection are already stopped by the vacuum, not by the silver.
• Radiation is the one the silver coating addresses.

Answer: E (radiation)

Exam tip: Remember "Vacuum stops conduction and convection, silver stops radiation." Whenever you see "shiny/silvered/polished" in a heat question, think radiation.