OS7 How A Siphon Works

Water runs uphill in a straw. Think about how you drank through a straw. First you pulled all the air out of the straw. Second the liquid replaced the air in the straw so you got a drink.

If a straw was open to the air, you couldn’t get a drink. You had to get and keep the air out of the straw.

What is a siphon? It is a long flexible tube used to move a liquid from one place to another.

Can a siphon make water run uphill like a straw can?

siphon moving water

Question: How does a siphon work?


2 Gallon Jars, clear

3 – 4 feet of clear plastic tubing (At least ¼ in. diameter, ½ in. is better)

1 ½ gallon water (Add a drop of food coloring to make it easier to see.)

Large measuring cup

Large pan or bowl to set a jar in in case the water spills

Chair or steps as high as a gallon jar

Note: It is easy to spill water in this project so working outside is a good idea.


Hold both ends of the tubing in one hand

Pour in water to half fill the tubing

Hold an end in each hand

Lift one end of the tubing and see what the water does (be careful not to spill)

Lower that end and lift the other end

Lower that end so the ends are even

Hold a thumb or finger over one end of the tubing

Lift one end of the tubing and observe the water

Lift the other end and observe the water

Even the ends of the tubing and block both ends with your thumbs or fingers

Lift one end of the tubing and observe the water

Lift the other end and observe the water

Fill one gallon jar almost full with water

Set the jar on the step or chair

empty siphon tube

The siphon tube is full of air when it’s put into the jars. The water can’t push the air out so no water flows through the tube.

Set the empty jar in the large pan on the floor next to the step or chair

Put the tubing into the empty jar

Pull enough tubing out to put into the jar of water all the way to the bottom

Observe what the water does

Take the tubing out of the water

Hold both ends of the tubing and pour water into it until the tubing is full

siphon moves water from high to low jar

Once the air is out of the siphon tube, water runs quickly from the full jar into the empty jar.

Close off one end tightly with a thumb

Put the open end back into the empty jar

Put the closed end into the jar of water half way to the bottom

Release the end of the tubing and push it to the bottom of the jar

Observe what the water does

siphoning water from lower step

Especially when the full jar is on the low step, the siphon loop rises high above the jars. Yet the water still flows from the top jar into the bottom jar.

Pulling some of the tubing up from the bottom jar, make the loop between the two jars higher until the tubing only goes to the bottom of each jar

Observe what the water does

If the top jar is empty, switch the jars and start again

Put the full jar on a lower step or chair and do this again

Observe what the water does

siphon works for two jars on the ground

It’s easy to see why water would move from a jar set higher than the lower jar yet water still moves from the full jar to the empty one when both are on the ground.

Put the two jars on the same level and start again

Observe what the water does


Describe what the water in the tube does

With both ends open

With one end open

With both ends blocked

Describe what happens with the siphon tube

When put into the jars filled with air

When put in the jars filled with water

Describe what happens when the loop is lifted up

Describe how the siphon works with the jars closer together in height

When the siphon starts

When the loop is lifted

Describe what happens when the jars are beside each other

When the siphon starts

When the loop is lifted


When both ends of the tubing are open, what can go in and out of the tubing besides water?

Is this still true when you block one or both ends?

How does this change how the water acts?

Why do you close one end of the filled tubing to put it into the jar of water?

Why does water move through the tubing from the full to the empty jar when it is filled with water but not when it is filled with air?

How does the movement of water change as the height of the loop changes?

Can the loop be too high for the water to keep moving?

How does the height difference between the jars affect how the water moves?

What causes the water to move from one jar to the other? Is this the same as how water comes up a straw?

Does water really move uphill by itself? Why do you think so?

siphon almost done

The siphon continues to move water as long as the water is higher than the bottom jar until the level is so low air gets into the tube.

What I Found Out:

Making the water blue really helped me to see where the water was in the tube. I put enough in to half fill the tube.
When both ends of the tube were open, the water moved up and down as I moved the ends of the tube. The two surfaces stayed level no matter how fast or slowly I moved the tube ends.
Blocking one end of the tube changed things. The water moved only a little ways toward that end then stopped. When I lifted the blocked end up, the water didn’t move down very far until bubbles of air started moving up into that end.
Once both ends were blocked, air bubbles had to move from one end to the other to make the water levels change.
Air controls the water levels in the tube. When an end is not blocked, air can move in and out easily. Blocking one or both ends keeps the air at that level unless the open end is low enough for more air to move into the blocked end.
I used a step stool with two steps on it to set my jars on. The first time I set the full jar on the top step and the empty jar on the ground. One end of the tube went in the top jar. The other end went in the bottom jar. Nothing happened.
Leaving the two jars where they were, I took the tube out and poured water in it until it was full. Blocking one end keeps air from getting into the tube to push the water out. If the end in the lower end is open, air can bubble up into the tube before the other end gets into the top jar.
When the tube is full of air, the water doesn’t get pulled in just like when the one straw was outside the glass. A straw only works when all the air is pulled out. Having the tube full lets gravity pull the water down from the full jar to the empty jar. It works like a siphon. Raising the loop doesn’t stop the water from moving. It can slow the water down especially when the two jars were both on the ground.
The water moved differently when the top jar was placed on the different levels. Using the top step let the water move fast from the full to the empty jar. The top jar had only a little water left in it when air got into the tube and pushed the water out of the tube.
Using the lower step slowed the water down. It still moved from the top to the bottom jar, just not as fast. The top jar ended up almost empty at the end.

siphon stops when water levels are equal

When both jars are on the ground, the siphon stops working when the water levels in both jars are the same leaving the tube filled with water.

Placing both jars on the ground changed everything. The water moved very slowly from the full jar to the empty one until the water level was the same in both. then it stopped. The tube was still full of water but it did not move. The water only moved when one jar had more water in it than the other, then ran downhill in a way to level it up.
Water does not move uphill on its own. It can appear to do so through the siphon loop but it is really ending up lower than at the beginning.