Upside down water trick

by Fellowship Agency July 18, 2018

You will need:

• 1 Gratnells shallow (F1) tray
• 1 hand-sized, flat rimmed, plastic pot, cup or jar
• Sufficient water to mostly fill the pot
• Optional: Food colouring – we used green for this activity, but you can use any colour, it just makes the water a little easier to see
• 1 stirrer
• 1 piece of card ~4 cm wider than the diameter of the top of the pot

What to do:

1. Place all the equipment in to the tray and work over the tray to catch any drips or spills.
2. Add a small amount of food colouring to the pot.
3. Add water to the pot, being sure to leave at least a ~2cm gap at the top.
4. Stir well to fully mix in the food colouring.
5. Place the piece of card on top of the pot and place your hand flat down on top, ensuring the card is in contact with the rim of the pot all the way around.
6. Confidently turn the pot all the way over, keeping your hand on the card with firm pressure the whole time.
7. Allow the water to come in to contact with the card then slowly and gently move your hand away.
8. The card should stay in place and the water should not fall out of the pot!
9. Place your hand back on the card and return the pot to its upright position. Phew!

What is happening?

It’s all about air pressure and gravity. Before you put the card ‘lid’ on the pot, the air pressure inside the pot is the same as the surrounding air. When you put the card on and turn the pot over, an airtight seal is formed around the rim by the water but a tiny bit of water escapes because of the force of gravity pushing down on it. This provides a little more space (volume) in the pot for the air so the air molecules can spread out a little bit further. The same number of air molecules in a larger volume results in the air pressure inside the pot decreasing and becoming slightly less than the air pressure in the surrounding air. Because the pressure of the surrounding air is greater it ‘pushes’ the card lid on to the pot, stopping it from falling away when you remove your hand. This is the same principle that is used to form a vacuum.

If the airtight seal is broken, even just a tiny bit, air rushes into the cup equalising the pressure. The effect of gravity takes over, the card falls and the water spills out of the pot.

Other things to try…

• Repeat the activity with a fresh piece of card. Use a stopwatch to time how long the water stays in the pot before it falls out – this is where the tray becomes essential!
• Investigate the effect of adding less water to the pot, does the water stay in for more or less time before it falls out?
• Does the temperature of the water make any difference?
• Does the material the pot is made from or the shape of the pot effect the results? You could try using glass instead of plastic.
• Will it work with paper instead of card? How does this effect the time it takes for the water to fallout of the pot?
• Remember to only change one variable at a time as you experiment.

If you liked this activity, check out our Thirsty Candle Experiment.

Health & Safety

As with all Gratnells Learning Rooms What’s In My Tray Activities you should carry out your own risk assessment prior to undertaking any of the activities or demonstrations.