jeudi 19 juillet 2018

Homework Help - Counterintuitive Results

My son has a simple (he's 9) science experiment for homework. Boil a measurement of water in a tea kettle for X number of minutes (we chose 5 min.)

1. What do you think will happen?
(The assignment is called "Evaporation" so we got this right.)

We're not in a lab so precision is not possible. It's "a tea kettle". Aluminum and something mixed. Holds the liter of water we chose, with room for a bunch more so I'm reckoning it's a 2l kettle. It's a whistling kettle.

We boiled the water and left the whistle-tap closed so it was forcing steam out of just that little hole. But in surprisingly high volume.

2. Evaporation loss: 125ml or 12.5%

Gas hob on high. No better measurement available.

>>>>>>

As a secondary experiment, on my suggestion, we put the same amount of water in a 1.5l (fairly small) sauce pan. My (very bad) intuition was that because it had no lid and vapour was free to exit across the entire area of the circle there would be much greater evaporation loss.

Again, gas hob on "high", five minutes rolling boil. But the loss of fluid was less than for the five minutes in the tea kettle. Material? Some kind of Tefal clad aluminum or alloy.


Is there a scientific reason for this other than the obvious.... that we mis-measured. I know that liquids forced through a smaller space at the same initial flow will simply speed up so that the same amount passes through the smaller space at a higher speed. This would perhaps explain if the evaporation rates were the same, but we lost 125ml of water in the tea kettle test and only about 68 in the sauce pan test.

Pressure? The kettle is fairly tightly closed. The saucepan, obviously, is not.


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