Why water boils cooler in the mountains
Boil a pot of water at the seaside, in Kathmandu, Shimla, Leh and at Everest base camp. See how the boiling point falls as you climb, and why dal takes so much longer to cook up high.
Core concept
Water boils when its vapour pushes as hard as the air above it; higher up the air pressure is lower, so water boils at a lower temperature (about 1 °C lower for every 300 m) and food cooks more slowly
Topic: Chemistry in the kitchen
Water boils when bubbles of steam can form inside it, and that happens when the steam pushes as hard as the air pressing down on the water. At sea level the air presses hard, so water must reach 100 °C. As you climb, there is less air above you and the pressure falls, so the bubbles form sooner: water boils at about 1 °C lower for every 300 m of height. In Kathmandu (about 1400 m) it boils at about 95 °C, and at Everest base: The opposite of an acid, like soap or baking soda solution. It feels slippery and turns red litmus blue. All key words camp at about 82 °C. Boiling water up high is not as hot, so rice, dal and potatoes take longer to cook: roughly twice as long for every 10 °C lower. That is why people in the mountains use a pressure cooker, which raises the pressure and so the boiling point.
What you will learn
- Explain why water boils at a lower temperature at high altitude.
- Estimate the boiling point at a place from its height.
- Explain why cooking takes longer up high, and how a pressure cooker helps.
Before you start
- Know that air presses on everything (air pressure) and that water boils at 100 °C at sea level.
Materials
- A pot of water and a stove (with an adult)
Equipment
- A kitchen thermometer that reads to 110 °C
Procedure
- Record boiling water in Kathmandu.
- Move to sea level and record.
- Move to Leh and then to Everest base camp, recording each.
How this simulation is simplified
- The boiling point falls about 1 °C for every 300 m of height, a good guide below 6000 m.
- Cooking time roughly doubles for every 10 °C lower; dal takes 20 minutes at sea level.
- Heights: sea level 0 m, Kathmandu 1400 m, Shimla 2200 m, Leh 3500 m, Everest base camp 5364 m.