How does a vacuum flask keep tea hot for hours – and keep cold drinks cold? It doesn’t make heat. It just makes it very hard for energy to leave (or get in). Each part of the flask is designed to cut down one way that energy is transferred. Read the article, study the diagram, then try the 10 GCSE-style questions underneath.
Course: Combined Science + Separate Physics | Tier: Foundation + Higher | Topics: Energy, Particle model, Waves (infrared)
How a vacuum flask works
Energy spreads out from hot things to cooler surroundings in three main ways: conduction (vibrating particles passing energy to the particles next to them), convection (warm fluid rising and carrying energy with it) and radiation (infrared waves, which need no particles at all). A vacuum flask tackles all three.
1. The vacuum: no particles, so no conduction or convection
The container has two walls with the air pumped out of the gap between them. Conduction and convection both need particles. With almost no particles in the vacuum, energy cannot be conducted or convected across the gap. This is the single most important feature of the flask.
2. Silvered walls: reflecting infrared
A vacuum can’t stop infrared radiation – that is how energy from the Sun reaches us through space. So the walls facing the vacuum are silvered (shiny, like a mirror). Shiny surfaces are poor emitters and poor absorbers of infrared. The inner wall gives out very little infrared, and most of what does cross the gap is reflected back towards the drink.
3. The stopper: stopping convection and evaporation
Warm air above the drink would rise out of an open top, carrying energy away by convection, and hot water would evaporate, taking energy with it. The stopper seals the top. It is made of plastic or cork, which are thermal insulators (low thermal conductivity), so very little energy is conducted through it.
4. The support and the case
The container sits on an insulating support, so energy isn’t conducted out through the base. The outer case protects the fragile container, and the air gap between them adds more insulation, because air is a poor conductor.
Where does energy still escape?
No flask is perfect. Most of the energy that escapes is conducted through the stopper and through the top of the container, where the inner and outer walls are joined. That is why flasks have narrow necks and thick stoppers – a smaller area means a lower rate of energy transfer.
Cold drinks too
A flask works both ways. The same features stop energy from the warm surroundings getting in, so an iced drink stays cold. It doesn’t “keep the cold in” – it keeps the energy out.
💡 Did you know? The vacuum flask was invented by the Scottish scientist James Dewar in 1892 to store very cold gases in his lab. Scientists still call them Dewar flasks, and huge ones are used to store liquid nitrogen in hospitals and labs.
📌 KEY DEFINITIONS – learn them word for word
Specific heat capacity: the amount of energy required to raise the temperature of one kilogram of the substance by one degree Celsius.
Thermal conductivity: the higher the thermal conductivity of a material, the higher the rate of energy transfer by conduction across the material.
Dissipated energy: energy that is transferred to the surroundings (usually by heating) and stored in less useful ways – often described as “wasted”.
Equations you’ll use
| Equation | Quantity | Symbol | Unit |
|---|---|---|---|
| E = m c Δθ | change in thermal energy | ΔE | joules (J) |
| mass | m | kilograms (kg) | |
| specific heat capacity | c | J/kg °C | |
| temperature change | Δθ | degrees Celsius (°C) | |
| P = E ÷ t | power | P | watts (W) |
| time | t | seconds (s) |
Use FIFA for every answer
- F – Formula: write the equation as it appears on the sheet.
- I – Insert: put in the numbers (convert to standard units first).
- F – Fix: rearrange to make the unknown the subject.
- A – Answer: calculate and always give the unit.
Questions
Q1 (F) A vacuum flask has a vacuum between its two walls. Explain why the vacuum stops energy being transferred by conduction. [2 marks]
Show answer
• Conduction needs particles to pass on energy (vibrating particles collide with their neighbours). (1)
• A vacuum has (almost) no particles, so energy cannot be conducted across it. (1)
Q2 (F · S only) The walls of the flask are silvered (shiny). Which way of transferring energy does this reduce? Explain how. [2 marks]
Show answer
• Radiation (infrared). (1)
• Shiny surfaces are poor emitters of infrared and reflect infrared back towards the drink. (1)
Q3 (F) A flask holds 0.50 kg of tea. The tea cools from 90 °C to 70 °C. The specific heat capacity of tea is 4200 J/kg °C. Calculate the energy transferred from the tea.
Show solution
F: ΔE = m c Δθ
I: ΔE = 0.50 × 4200 × (90 − 70) = 0.50 × 4200 × 20
F: ΔE is already the subject
A: 42 000 J (42 kJ)
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Q4 (F/H) In an open mug, the same tea cools from 90 °C to 70 °C in 15 minutes. In the flask it takes 6 hours. Using your answer to Q3, calculate the average rate of energy transfer (power) from (a) the mug and (b) the flask. (c) How many times slower does the flask lose energy?
Show solution
Convert first: 15 minutes = 15 × 60 = 900 s; 6 hours = 6 × 3600 = 21 600 s
F: P = E ÷ t
(a) P = 42 000 ÷ 900 = 47 W (46.7 W)
(b) P = 42 000 ÷ 21 600 = 1.9 W
(c) 21 600 ÷ 900 = 24 times slower
Q5 (F/H) A flask contains 0.50 kg of tea at 95 °C. It loses energy at an average rate of 2.0 W. Calculate the temperature of the tea after 4 hours. (c = 4200 J/kg °C)
Show solution
Step 1 – energy lost: 4 hours = 4 × 3600 = 14 400 s; E = P t = 2.0 × 14 400 = 28 800 J
F: ΔE = m c Δθ
I: 28 800 = 0.50 × 4200 × Δθ
F: Δθ = 28 800 ÷ 2100 = 13.7 °C
A: 95 − 13.7 = 81 °C
Q6 (F/H) Some people think a vacuum flask “keeps the cold in”. Explain why a flask keeps a cold drink cold. [3 marks]
Show answer
• The surroundings are warmer than the drink, so energy is transferred into the drink (there is no such thing as “cold” flowing out). (1)
• The vacuum stops energy being conducted or convected in, and the shiny walls reflect infrared away from the drink. (1)
• So the rate of energy transfer into the drink is very low and its temperature rises very slowly. (1)
Q7 (H) A flask contains 0.40 kg of soup. Over one hour, the soup transfers 8400 J to the surroundings and its temperature falls by 6.0 °C. Calculate the specific heat capacity of the soup.
Show solution
F: ΔE = m c Δθ
I: 8400 = 0.40 × c × 6.0
F: c = 8400 ÷ (0.40 × 6.0) = 8400 ÷ 2.4
A: 3500 J/kg °C
Q8 (H · S only) A student says the inside of a flask should be painted matt black instead of silver. Explain why this would make the flask worse at keeping a drink hot. [3 marks]
Show answer
• Matt black surfaces are good emitters of infrared radiation, so the hot inner wall would emit infrared at a higher rate. (1)
• Matt black surfaces are also good absorbers, so the outer wall would absorb this infrared instead of reflecting it back. (1)
• So the rate of energy transfer from the drink would increase and it would cool faster. Silvered surfaces are poor emitters and reflect infrared. (1)
Q9 (H) Most of the energy lost from a vacuum flask passes through the stopper. Suggest two ways the stopper could be designed to reduce this, and explain each. [4 marks]
Show answer
• Use a material with a lower thermal conductivity (e.g. foam-filled plastic or cork) (1) – so the rate of energy transfer by conduction through it is lower. (1)
• Make the stopper thicker (1) – energy has further to travel, so the rate of conduction is lower. (1)
• Or: make the neck (and stopper) narrower (1) – a smaller area means a lower rate of energy transfer. (1)
Q10 (H) Explain how the design of a vacuum flask keeps a hot drink hot for many hours. Refer to the vacuum, the silvered walls and the stopper. [6 marks]
Show answer
Model answer – marking points:
• The drink loses energy to the cooler surroundings by conduction, convection and radiation; the flask reduces all three.
• The vacuum between the double walls has (almost) no particles.
• So energy cannot be transferred across it by conduction or convection, which both need particles.
• The silvered walls are poor emitters of infrared and reflect infrared back towards the drink, reducing energy transfer by radiation.
• The stopper stops warm air rising out (convection) and stops evaporation; it is made of a material with low thermal conductivity, so little energy is conducted through it.
• The insulating support and air gap reduce conduction to the case, so the rate of energy transfer is very low and the drink stays hot for hours.
⚠️ Where students lose marks
- Saying the vacuum “stops heat” without naming conduction and convection and explaining that they need particles.
- Saying the vacuum stops radiation. It doesn’t – infrared travels through a vacuum. The shiny walls reduce radiation.
- Writing that the flask “keeps the cold in”. Cold isn’t a type of energy – the flask stops energy getting in.
- Forgetting to convert minutes or hours into seconds before using P = E ÷ t.
- Using the final temperature instead of the change in temperature (Δθ) in ΔE = m c Δθ.
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