Katli and Kulfi: Experiment #5: Why does water from an inverted glass not fall when it is covered with a piece of cardboard?

We are back with another famous experiment I have done in school. It was in my physics lab. I went to a Central Board School and I am proud to say that science experiments which were part of my syllabus has made me love science more than any other subject.

As a part of our syllabus, we attend Prayoga (a non profit school science education project by Stree Seva Mandir) in Grade 8. Prayoga continues to motivate the students to enjoy science experiments and I am really thrilled that my school was part of it.

Now coming to our experiment, we used tea coaster as a cardboard base and one of Katli's tumbler to fill water.

Aim:

To see why water from an inverted glass doesn't not spill when it is covered with a piece of cardboard?

Things Needed:

1 plastic cup
1 piece of cardboard ( little larger than the size of tumbler's mouth)
Water to fill it half
A parent to help

Directions

Prepare the working area.Make sure it can stand all the water spills. I do the experiments in kitchen island.
Place a large tray below and place cup inside. Fill it with water. Hold the cup in one hand.
Place the cardboard on top of the cup. Make sure one of the parent is performing the experiment with the child. Hold the cardboard with your left palm assuming your holding the cup in right hand.
Turn it upside down, then back and again invert it.
Now slowly remove your left hand and hold the cup firmly. The cardboard will stay intact and will not fall down. Tell the child that its a magic. And repeat it so many times as children see the experiment like a magic show.

What actually happens?

When the cup is inverted with a tightly fit cardboard at its mouth, the water doesn't leak. The cardboard along with the surface tension of water forms a tight seal at the mouth of the cup. Remember even if a single drop drips out of the tumbler, the cardboard will not stay. The absence of atmospheric pressure creates a vacuum. The cardboard stays intact as atmospheric pressure outside is greater than the one created inside the cup. That pressure holds the cardboard and prevents water to leak.