Re: Will water boil faster on a stove-using hot water?

Hi and Hello

Yes, it does seem a strange supposition that water will boil in an equal 
time, irrespective of its initial temperature.

I would assert that it is wrong!

First of all, how could you carry out a practical to determine that? Well, 
I guess that you already did that.

Just to check your observations with the microwave, I got my 11 year olds 
to carry out a class experiment.

Method
1.We set up identical 250 ml beakers (open topped) with 200 ml of water.
2.Identical temperature probes were placed in the outer lower quadrant of 
the beaker, so as not to touch the glass. They are placed here, instead of 
in the centre, so that they are recording the water temperature as it 
finishes its 'convection cycle' in the beaker. I believe this to be a 
better measure of temperature for our purposes.
3.Matched Bunsens were used to heat these. The bunsen used for heating the 
water starting at the higher initial temperature was slightly more 
efficient than the other. (See the increased slope gradient in the results. 
Note also that I've chosen to use a conventional heating source, rather 
than a microwave, since this is much easier to stick the probes in! My 
microwave at home is sufficiently powerful that it heats smaller quantities 
of water like this so fast hat the top of the water may be boiling and the 
bottom of the container still cold!!
4.We heated one to ca 50oC (see results below), and then started heating 
the other one from room temperature (ca. 19oC).


Observations
1.The hotter one rose to ca.99.5oC first (e.g. boiling point)
2.When the water with the lower initial temperature was heated, there was 
no initial change of temperature. Remember what I said about the 
'convection cycle' and the position of the temp probe?

Results
Have a look at the first graph, sorry about the snapshot quality! 'Water 
boiling'

Explaining these results is also reasonably easy. 
The Kinetic Theory states that
1.All matter is made up of tiny invisible moving particles;
2.All particles move all of the time.
3.Heavier particles move more slowly than lighter ones at a given 
temperature.

The one thing that it doesn't actually state is the attraction between 
particles. This is very strong, and therefore particles with low levels of 
energy, eg those in a solid, do not have enough energy to overcome these 
strong forces of attraction, and move freely over each other (eg as in a 
liquid) or indeed move away from each other (eg as in a gas).

How would you decide when a liquid is boiling?

Either you could suggest that it is boiling whenever it has reached it's 
boiling point, or as many people do, suggest that it is boiling whenever 
bubbles of gas appear.

It is probably better to consider whenever the boiling point has been 
reached, since gas bubbles will start being released long before a liquid 
reaches its boiling point.

This being the case, then according to the kinetic theory, the liquid 
particles must have a certain energy level when they reach the boiling 
point. Therefore particles of water at 50oC must require less energy than 
particles at 20oC, to raise their energy level to that of particles of 
water at 100oC

I hope that this answers your question.


R Jonathan Stewart

PS Have a look at the water boiling graph and see if you can repeat it, 
preferably with two thermometers!