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Experiment Topic: Air Pressure

Boiling Water with Ice

Boiling Water with Ice

Can you boil water using ice?

Learn about how pressure affects the boiling point of water!

Equipment

◊ A clear glass flat bottomed flask with a bung

◊ A hot plate or cooker top

◊ Some water

◊ Some ice

◊ Heat proof gloves and a heat proof mat

◊ A ring standBoiling water with ice

Method

1. Bring some water to boil in the flask.

2. Remove the flask from the heat and immediately put the bung tightly in.

3. Leave the flask for a minute, then suspend it upside down on the ring stand.

4. Place some ice cubes on the flat bottom of the flask. What happens?

You should see the water start to boil again! As the water and water vapour in the flask cools because of the ice, the pressure in the flask falls. Water boils at lower temperatures at lower pressures.

So how does this relate to atmosphere?

This is why it is not possible to make a good cup of tea at high altitudes where the pressure is lower! The boiling point of water is 100°C at standard pressure (sea level). On top of Mount Everest the pressure is about 330 mb and so the boiling point of water is about 70°C.

Where can I find more information?

Watch this experiment:

Make a Cartesian Driver

a cartesian diver

Cartesian Diver

Learn about buoyancy

Equipment

◊ A 2 litre plastic drinks bottle, with a lid

◊ A pipette

Method

1. Fill the 2 litre bottle to the very top with water.

2. Fill the pipette ¼ full with water (so that it will be light enough to float)a cartesian diver

3. Put the pipette in the bottle, make
sure it floats and then screw the
bottle lid back on.

4. Squeeze the sides of the bottle. The
pipette (known as the diver) sinks.
Release your squeeze and it will float back
up to the top.a bottle and a pipette

How does it work?

Initially, the weight of the pipette is balanced by the weight of the water it is displacing (Archimedes principle). The air in the pipette compresses more easily than the water, and so when you squeeze the bottle, increasing the pressure, the air is compressed. At this point, water is pushed into the pipette, increasing its mass. The float sinks. When the bottle stops being squeezed the pressure is decreased and the diver floats back to the surface.

Possible extension: turn it into a competition by adding a hook to the bottom of your float. Place something heavy with an upward hook at the bottom of the bottle. Can you anchor the float?

In the atmosphere, if air is forced to rise over something like a mountain, it will either end up more or less buoyant than the air around it – it will either carry on rising or sink back to the level it came from. If it carries on rising, you can get clouds.

Where can I find more information?

An article about using Cartesian divers to teach physics and the Institute of Physics’ Marvin and Milo version.

More about atmospheric stability

Alternatively

If you don’t have a pipette, you could use some condiment sachets (e.g. tomato ketchup) or a ball point pen lid, weighted with paper clips or blue tack. Select a sachet that just floats/ add paper clips or blue tack to the pen lid until it just floats, open end down, and then carry on as above.

More experiments and demonstrations

Inflate a Balloon

a balloon inflated over a bottle

How to inflate a balloon the easy way

Learn about air pressure

Equipment

◊ An empty plastic 2 litre bottle

◊ A balloon

◊ Some ice

◊ Some hot watera balloon over the mouth of a bottle

Method

1. Put the empty and open bottle in a freezer or on some ice for long enough to let it really cool down.

2. Now quickly put the balloon over the mouth of the bottle.

3. Pour very hot water over the sides of the bottle and see what happens! Cold air is more dense than warm air. As the air in the bottle warms, it expands, inflating the balloon.a balloon inflated over a bottle

How does this relate to the atmosphere?

When air is heated – maybe because it is in contact with warm ground, it expands and rises. As it rises the pressure falls and the air cools. If the air rises far enough, clouds form.clouds in the sky

What else can I try doing?

Have a look at some of our other experiments – make a barometer, see if you can get an egg into a bottle, ask how heavy is air and see whether you can crumple a can. All these explore what we mean by air pressure.

More experiments and demonstrations

Candle and Water Trick

candles under glasses on plates

Candle and Water Trick

As the temperature falls, so does the pressure

Equipment

◊ A glass

◊ Matches

◊ Candle

◊ Food colour

◊ Water

◊ Saucer or platecandles under glasses on plates

Method

1. Put a very little water on a plate, and mix in a couple of drops of food colour.

2. Place a candle in the middle of the plate, and light it. Slowly bring a glass down on top of the candle until it is standing in the water, on the plate.

3. Watch what happens next!

The burning candle heats the air above it, including the air that goes into the glass. Once the glass is standing on the plate, the burning candle uses up all the available oxygen in the glass, then goes out. As it does so, the air in the glass cools, and as it cools, the air pressure in the glass falls below atmospheric pressure. Water is drawn into the glass until the pressure is equalised. You can turn this experiment into a competition by placing a small coin on the plate under the water and, offering students a variety of possible tools, seeing who can retrieve the coin without getting their fingers wet.

So how does this relate to atmosphere?

When we measure the air pressure at the surface of the Earth, we are literally measuring how much air is above us. If the air pressure falls, there is less air above us, if the air pressure rises, there is more air above us. The relationship between temperature and pressure is very important – as the temperature falls, so does the pressure and as the temperature rises, so does the pressure. That means that as air moves up in the atmosphere and the pressure falls (because there is less remaining atmosphere above) its temperature has to fall as well. Typically, the temperature of the atmosphere falls about 6°C for each 1000m you go up –so the tops of mountains are always much colder than the valleys below. This experiment also demonstrates how storm surges work – when the air pressure is low over a sea or ocean, the water level can rise. This can have devastating consequences – for example the North Sea flood of 1953.

Another experiment

For another experiment looking at the relationship between temperature and pressure, all you need is a plastic syringe (the sort sold in pharmacies for administering medicine to babies). With your finger over the nozzle, pour a little very hot, but not boiling, water into the syringe. There will be a bubble of air at the bottom, so you won’t scald your finger! Now use the plunger to push all but 3ml of the water out, then put your finger over the nozzle again, and pull the plunger out. As the pressure in the syringe falls, the temperature falls but so does the boiling point of water – you should see the water starting to boil!

More experiments and demonstrations

Turn Water Upside Down

a full bottle of water turned upside down

Can you turn water upside down?

Learn about air pressure! This is a variation on the classic glass and piece of paper trick.

Equipment

◊ An empty 500ml fizzy drink bottle

◊ Water

◊ A piece of net curtain or similar

◊ An elastic band

◊ Toothpicksa full bottle of water turned upside down

 

Method

1. Stretch the netting over the top of the bottle and hold in place with the elastic band.

2. Fill the bottle with water right till it’s almost overflowing.

3. Place your hand over the top of the bottle and turn it upside down. Remove your hand – no water should come out!

Try pushing a toothpick through one of the holes in the net. It should go through and rise to the top of the bottle. Try adding some more!

To let the water out of the bottle, just tilt it gently to one side.

 

What’s going on?

Atmospheric pressure acts in all directions, not just down. In this case the pressure due to the weight of the air above us, pushes up on the water in the bottle and stops it from coming out.

Atmospheric pressure (around 1000mb) can support a column of water about 10.3m high – the depth of
water in the bottle is a lot less than that. A barometer made with water, rather than mercury, would need a column of water 10.3 m high!

 

How does this relate to the atmosphere?

The large scale circulation of the atmosphere and oceans, and smaller scale weather systems, are all driven by pressure differences. Air, and water, will always tend to move from areas of high pressure to areas of lower pressure.

At any one time the pressure at sea level may vary around the world from as little as 870mb (inside a hurricane) to 1083.8mb (recorded in Siberia, 31st December 1968).
In the UK, the air pressure varies between about 970-1040mb as weather systems pass over. Pressure also falls with height. This is why aeroplanes must have pressurized cabins. At the top of Mount Everest the pressure is only about 330mbar.

More Pressure Related Experiments

1. Get a plastic 1.5ltr lemonade bottle and drink the contents. Then immerse it up to the neck in hot water. Don’t let water get inside. Wait for a couple of minutes while the air inside warms up – this makes some of the air leave the bottle (a partial vacuum). Quickly replace the top and remove from the hot water. Soon the bottle will start to buckle as the air inside cools down and contracts, allowing the outside pressure to act on it. This works with plastic milk bottles too.

2. Use any plastic bottle and a ‘wine saver’ device (they extract the air from opened wine bottles to stop the rest spoiling). Pump out the air and watch the bottle collapse as flat as a pancake!

3. Put a ruler on the table, leaving a bit of it sticking out over the edge. Put a sheet of paper over the ruler, but not over the bit that’s sticking out. Now give a smart hit to the end of the ruler. You should find that it’s quite difficult to make the ruler move. Why? It’s because that sheet of paper has many kilos of air on it, so when you hit that ‘lever’ you’re trying to move much more than a sheet of paper!

4. Put a clean drinking straw in a clear cup of water. Hold a second clean straw next to the first, but with the end outside the cup, and try sucking through both straws at once. Can you suck water up? It is impossible to drink if one of the straws is outside the glass. In order for water to be forced into your mouth, the pressure outside (atmospheric pressure) needs to be greater than the pressure inside your mouth (a partial vacuum), but, with two straws, you suck air up as well and cannot maintain low pressure in your mouth. Now make a small hole with a pin in one of the straws, about 3cm from the top. Can you drink through it? No matter how hard you suck, a straw won’t work if air can get into your mouth and stop you from maintaining low pressure there.

5. Place a well-stretched balloon inside a 1 litre clear plastic bottle and spread its neck over the top of the bottle. Do the same with another bottle and balloon, but, this time, put a small hole in the side of the bottle, near its base. Can you blow up the balloons inside the bottles? It should only be possible with the holey bottle. Quickly put your thumb over the hole when the balloon is inflated, and it should stay inflated when you stop blowing into it. It should also be possible to inflate the balloon by sucking air through the hole in the bottle! Think about where the pressure is the same as, greater or less than atmospheric pressure.

More experiments and demonstrations

Crumple Can

a drinks can

Can you crumple a can?

Learn about air pressure

Equipment

◊ A hot plate, or electric hob

◊ An empty drinks can

◊ A shallow pan filled with cold water

◊ Tongs or oven glovesa drinks can

Method

1. Put a spoonful of water in the can, and place it on the hotplate. Let it get hot.

2. Quickly, pick up the can using the tongs or oven gloves, and plunge it opening down in the water in the pan.

As the temperature of the can rises, it fills with water vapour. At this stage, the air inside the can is warmer than the surrounding air, but the pressure inside the can is the same as the surrounding air. When you put the hot can upside down in the pan of cold water, after a few moments, the can should crumple with a ‘pop’ sound. This is because, as the temperature inside the can falls and the water vapour condenses, the pressure inside the can becomes lower than the pressure of the surrounding air. If you had used something sturdier than an aluminium can, you would find that water would have time to flood into the container to equalize the pressure.

 

So how does this relate to atmosphere?

The large scale circulation of the atmosphere and oceans, and smaller scale weather systems, are all driven by pressure differences. Air, and water, will always tend to move from areas of high pressure to areas of lower pressure.

The pressure of the air above us at the surface of the Earth is usually about 1000mb – that’s 100,000Pa. That is the equivalent of 3 adult elephants on each square metre! At any one time the pressure at sea level may vary around the world from as little as 870mb (inside a hurricane) to 1083.8mb (recorded in Siberia, 31st December 1968).

In the UK, the air pressure varies between about 970-1040mb as weather systems pass over. Pressure also falls with height. This is why aeroplanes must have pressurized cabins. At the top of Mount Everest the pressure is only about 330mbar.

 

Where can I find more?

Find out more by reading chapter 6 of Bohren’s book “clouds in a glass of beer”.

Some more experiments

1. Get a plastic 1.5ltr lemonade bottle and drink the contents. Then immerse it up to the neck in hot water. Don’t let water get inside. Wait for a couple of minutes while the air inside warms up – this makes some of the air leave the bottle (a partial vacuum). Quickly replace the top and remove from the hot water. Soon the bottle will start to buckle as the air inside cools down and contracts, allowing the outside pressure to act on it. This works with plastic milk bottles too.

2. Use any plastic bottle and a ‘wine saver’ device (they extract the air from opened wine bottles to stop the rest spoiling). Pump out the air and watch the bottle collapse as flat as a pancake!

3. Put a ruler on the table, leaving a bit of it sticking out over the edge. Put a sheet of paper over the ruler, but not over the bit that’s sticking out. Now give a smart hit to the end of the ruler. You should find that it’s quite difficult to make the ruler move. Why? It’s because that sheet of paper has many kilos of air on it, so when you hit that ‘lever’ you’re trying to move much more than a sheet of paper!

4. Put a clean drinking straw in a clear cup of water. Hold a second clean straw next to the first, but with the end outside the cup, and try sucking through both straws at once. Can you suck water up? It is impossible to drink if one of the straws is outside the glass. In order for water to be forced into your mouse, the pressure outside (atmospheric pressure) needs to be greater than the pressure inside your mouth (a partial vacuum), but, with two straws, you suck air up as well and cannot maintain low pressure in your mouth. Now make a small hole with a pin in one of the straws, about 3cm from the top. Can you drink through it? No matter how hard you suck, a straw won’t work if air can get into your mouth and stop you from maintaining low pressure there.

5. Place a well-stretched balloon inside a 1 litre clear plastic bottle and spread its neck over the top of the bottle. Do the same with another bottle and balloon, but, this time, put a small hole in the side of the bottle, near its base. Can you blow up the balloons inside the bottles? It should only be possible with the holey bottle. Quickly put your thumb over the hole when the balloon is inflated, and it should stay inflated when you stop blowing into it. It should also be possible to inflate the balloon by sucking air through the hole in the bottle! Think about where the pressure is the same as, greater or less than atmospheric pressure.

More experiments and demonstrations

How Heavy is Air

A plunger

How Heavy is Air?

Learn about air pressure

Equipment

◊ Two plungersA plunger

 

Method

1. Get 2 people to put the 2 plungers together. Pulling them apart is easy.

2. Now get them to push the plungers together, forcing the air out. Without twisting the plungers try to pull them apart. It is impossible!

As there is little air left between the plungers, the pressure of air on the outside is much greater than the pressure inside, so they are hard to pull apart. We don’t normally feel the weight of the air because it pushes on us in all directions.

(To free the plungers, peel the rubber of one back a bit)

How does this relate to the Atmosphere?

The large scale circulation of the atmosphere and oceans, and smaller scale weather systems, are all driven by pressure differences. Air, and water, will always tend to move from areas of high pressure to areas of lower pressure.

At any one time the pressure at sea level may vary around the world from as little as 870mb (inside a hurricane) to 1083.8mb (recorded in Siberia, 31st December 1968).Sea level pressure on a world map

More experiments and demonstrations

Marshmallows in a Bottle

Marshmallows in a bottle

Marshmallows in a bottle

Learn about air pressure

Equipment

  • Marshmallows in a bottleSmall marshmallows
  • A bottle (a clear wine bottle would be best)
  • A bottle pump such as is used to keep the fizz in fizzy drinks, or to keep an open
    bottle of wine good.

Method

Pumping air out of the bottle

1. Tip some of the marshmallows into the bottle.

2. Attach the pump to the top of the bottle, making sure there are no air leaks; (use blue tac if necessary).

3. Start to pump, stopping to shake the marshmallows occasionally.

4. If you are pumping air out of the bottle, the air trapped in the 

marshmallows will expand as the pressure of the air in the bottle falls. The marshmallows will look like they are growing! If you are pumping air into the bottle, the air trapped in the marshmallows will be compressed (squashed) as the pressure of the air in the bottle falls. The marshmallows will look like they are shrinking!

What is the pressure in the atmosphere?
Marshmallows expandingThe pressure at the Earth’s surface is usually approximately 1000mbar/100,000Pa. That is the equivalent of about 3 elephants on each square metre! When weather systems pass over the pressure varies between 970-1040mbar. Pressure also falls with height; near the surface it falls by about 1mb for every 10m we go up. For example at the top of Mount Everest the pressure is only about 330mbar, so climbers need oxygen. Aeroplanes must have pressurized cabins for this reason.

More experiments and demonstrations

Egg in a Bottle

eggs

Can you get an egg in and out of a bottle?

Learn about air pressureeggs

Equipment

◊ An empty milk bottle (or other glass bottle)

◊ A hard boiled egg with the shell removed, or a small balloon

◊ Scrap paper

◊ Matches

matches

 

Method

 

Getting the egg into the bottle

1.Twist up the piece of scrap paper, light it and quickly put it in the bottle.

2. Immediately put the egg on the top of the bottle. As the paper burns, the air inside the bottle heats up, it expands and becomes less dense than the air outside the bottle. The fire goes out as the oxygen inside the bottle is used up. The air cools and the egg is forced into the bottle due to the pressure difference between the inside and outside of the bottle.

 

Getting the egg out of the bottle

1. Hold the bottle underside down.

2. Blow into the bottle sealing over the end with your mouth. This forces extra air into the bottle past the egg. The pressure inside the bottle increases and the egg will be pushed out of the bottle.

So how does this relate to atmosphere?

The large scale circulation of the atmosphere and oceans, and smaller scale weather systems, are all driven by pressure differences. Air, and water, will always tend to move from areas of high pressure to areas of lower pressure.

The pressure of the air above us at the surface of the Earth is usually about 1000mbar – thats 100,000Pa. That is the equivalent of 3 adult elephants on each square metre! At any one time the pressure at sea level may vary around the world from as little as 870mb (inside a hurricane) to 1083.8mb (recorded in Siberia, 31st December 1968).

The air pressure varies between about 970-1040mbar as weather systems pass over. Pressure also falls with height. This is why aeroplanes must have pressurised cabins. At the top of Mount Everest the pressure is only 330mbar.an elephant

More experiments and demonstrations

More experiments

Drinking bird

More simple experiments and demonstrations from around the web:

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