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Drinking straws

Air and fluids · level 3, lesson 3 of 6

A question to start

When you drink through a straw, it feels as if you are pulling the drink up. But you cannot pull a liquid. What really pushes it up?

The idea

When you suck on a straw, you take some of the air out of the straw. Now the air inside the straw pushes down on the drink less than before.

Outside the straw, the air still presses down hard on the top of the drink. That push is now bigger than the push inside the straw, so it pushes the drink up the straw and into your mouth.

air pushesdown on the drinkless air in here

The air can only push the drink up so far. With all the air taken out of a very long straw, the air outside could push water up about 10 metres, as high as a three-storey house, and no higher.

Worked examples

1. A hole in the straw. Make a small hole near the top. Now air leaks in as you suck, the push inside does not drop, and the drink will not come up.

2. A sealed bottle. Push a straw through a tight lid on a full bottle, with no gap. When you suck, no air can get in to push down on the drink, and very little comes up.

3. A 12-metre straw. Even the strongest pump could not lift water up it from the bottom: the air can only push water about 10 metres up.

Try it

  1. When you suck on a straw, what do you take out of it?
  2. What pushes the drink up the straw?
  3. Why does a straw with a hole near the top not work?
  4. About how high can air push water up a straw on Earth?
Answers
  1. Some of the air in it.
  2. The air outside, pressing down on the drink.
  3. Air leaks in, so the push inside the straw stays as strong as outside.
  4. About 10 metres.

Common mistake

"Sucking pulls the drink up." Sucking only takes air out of the straw. The air outside does the pushing.

Where it leads