• Question: If heat energy is produced by movement in the molecules, shouldn't it be theoretically possible to boil water just by stirring it hard enough?

    Asked by bridget to Amy, Drew, Julia, Kimberley, Sara on 21 Jun 2011.
    • Photo: Drew Rae

      Drew Rae answered on 20 Jun 2011:

      Bridget. That’s very insightful. Essentially, this is what a microwave does (they are microwaves, so they are micro-stirs). All the energy when you stir something has to go somewhere, right? Stirring is a very slow way to put energy into something, and with water a lot of the energy just spins the water faster. Also, the hotter the water gets, the quicker it will lose heat to the atmosphere, so you would have to be stirring fast enough to replace the lost energy as well as adding new energy.

      So you are right that it is theoretically possible. What you might be able to do as a science experiment is use a blender for the stirring, and see if you can measure a temperature change in the water. Start with water at room temperature (leave it standing in the blender for a few hours) so you don’t spoil the experiment by the water warming or cooling naturally.

      You could even work out what power the blender is, and measure its efficiency as a heater!

    • Photo: Julia Griffen

      Julia Griffen answered on 20 Jun 2011:

      mmmm interesting thought… how to explain…??mm

      By stirring water you are moving the bulk as a whole and not moving each of the indicudual moecules of water separately. Mixing water will help water boil more quickly though.. but for different reasons. I’ll excplain further down..

      So two water molecules are held together by very strong bonds, called hydrogen bonds. For water to boil these bond need to break. mixing alone does not do this.

      Water molecules can be brought to the boil by providing energy though heat, e.g directly via flame or in the microwave. This heat vibrates the water molecules and causes the hydrogen bonds to break and individual water molecules can then form steam and escape the liquid as a gas.

      Mixing helps water to boil by increasing the surface area of the boilng liquid. Which means there is more area for the excited energised water molecule to escape from as steam..

      Hope this makes sense And i;ve explained it ok…:)

    • Photo: Amy MacQueen

      Amy MacQueen answered on 20 Jun 2011:

      Am I right in thinking that at any one time there will be molecules in water that are essentially at boling point and others much cooler…even if the water itself is at room temperature? but I see Julia’s point – mixing alone won’t get you up to boiling unless you can mix hard enough and quick enough to compensate for all the heat lost to the atmosphere (as Drew said).

      Interesting thought though – theoretically possible, but not practically probable – unless your mixing skills are super-human!!

    • Photo: Sara Imari Walker

      Sara Imari Walker answered on 21 Jun 2011:

      Hi Bridget! You’ve started a great discussion! This is certainly a fun one to think about. I would say that I definitely agree with Drew. It is theoretically possible – stirring hard enough you could transfer enough kinetic energy (i.e. heat) to the water for the bonds in the water to start breaking. But it would take A LOT of stirring! Stirring is a really inefficient means of transferring your energy to the water. Better to just turn the stove on 😉

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