How does a vacuum flask keep a beverage warm for long periods? (6 Marks)

Heat is lost in the fluid through Conduction convection radiation and evaporation. The silvered surfaces reflect infrared rays back into the liquid keeping heat in the fluid reducing heat lost by radiation. the plastic cap prevents heat lost by evaporation as the particles cannot escape thus their energy doesn't escape as well as reduce heat transfer from the evaporated particles to the outside as plastic is an insulator so the energy cannot easily pass through the cap. Furthermore the double walled glass bulb reduces conduction heat loss as it is an insulator the hollow reduces heat transferring to the outer side by conduction the vacuum prevents heat transfer conduction and convection currents between the glass walls .