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A simple siphon

By David, 6 September 2018 Activity

Three cups of blue liquid, sitting on upturned glasses connected by straws. The cups are cascading from highest on the left to lowest on the right and the liquid is darkest in the left cup and palest in the right cup.Do you have a few leftover plastic cups and straws that you want to keep from turning into waste? Save them up and make a cascading waterfall!

You will need

  • 3 plastic cups
  • 3 bendy straws
  • Scissors
  • Food colouring (optional)

A simple siphon

  1. First, trim the long part of the straw. Measure 10 cm from the bend and cut the rest of the straw away.
  2. Bend the straw.
  3. Put a cup in the sink.
  4. Fill the cup until the water is right up to the lip.
  5. The next few things need to be done quickly. Don’t worry if it doesn’t work the first time – just try again.
  6. Holding a thumb over the short end of a bendy straw. Hold a fingertip over the short end of the straw.
  7. Thumb over short end of bendy straw, with long end of straw inserted into cup of blue water.Stick the long end of the straw all the way into the cup until the bend is over the lip.
  8. Cup of blue liquid with a straw bent over the rim of the cup and blue liquid pouring out.Remove your fingertip and angle the short end of the straw downwards.
  9. If everything works correctly, water should start pouring out the end of the straw!
  10. Cascading siphons

  11. Now you can set up several siphons, with each siphon filling the next cup.
  12. Take two more straws. Measure 10 cm from the bend and cut the rest of each straw away.
  13. Take three cups. Set the first cup up underneath a tap, higher than the other cups. Start the siphon as per steps 5–8 of ‘A simple siphon’.
  14. Two cups filled with clear liquid, connected by a bent straw.When the siphon is going, turn the tap on just enough to keep the water level steady.
  15. Now, set up a second cup so it gets filled by the first siphon, then start a siphon emptying that cup. You might like to elevate that cup slightly so you can get a third siphon in the cascade.
  16. Three cups in cascading order, filled with varying shades of blue water, connected by straws.Try adding some food colouring to the water so you can see it cascading through your siphons!

What’s happening?

When you put a straw in a cup, liquid doesn’t usually come out the end. So why does it start flowing in this activity?

When you put your finger over the end of the straw, you trap air inside it. When you release your finger, the air escapes and water rushes into the straw. The water gains so much speed that it doesn’t stop when it gets to the water level inside the cup. Instead it keeps going through the bend and hopefully out the other side.

The falling water pulls more water from inside the cup. But for this to work, the end of the straw outside the cup has to be below the water level inside the cup. This is because gravity is pulling water down on both sides of the bend.

The straws in this activity are acting as siphons. Siphons are very useful for getting liquids from one place to another. For example, you can use a hose to siphon water out of a pool. A hose is also really handy for siphoning water from a fish tank, to drain the tank for cleaning.

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