Is water temperature increase uniform?
Discussion
Key Stage 3 Science criteria says kids should know it takes longer to heat water, the hotter the water gets.
Does it?!
Everything I can find online suggests that temperature increase is uniform after 0 and up to 100. All the graphs I've seen are a nice straight (diagonal) line between those values before vapour comes into play.
The experiment (which we don't have the kit to do) is for between 20 and 80, but the example values given show a significant slowing down of temperature increase as you go above 50 degrees.
Not often KS3 Science gets me but I've drawn a complete blank on this one.
Does it?!
Everything I can find online suggests that temperature increase is uniform after 0 and up to 100. All the graphs I've seen are a nice straight (diagonal) line between those values before vapour comes into play.
The experiment (which we don't have the kit to do) is for between 20 and 80, but the example values given show a significant slowing down of temperature increase as you go above 50 degrees.
Not often KS3 Science gets me but I've drawn a complete blank on this one.
Water (in fact all most molecules) can adiabatically (ie without temperature change) absorb or release energy as they change phase. For water, it freezes at 0 degC and boils at 100 degC. Those are both phase changes, and so the rate of temperature rise for any fixed mass of water with a fixed thermal flux (heat) input will be linear between those points and change as the temperature crosses those points (at 1 bar abs pressure for pure water)
Heat flux (thermal energy) flows between objects of different temperature, like say water flows between resevoirs of different height. The flow of heat is proportional to the difference in temperature and the thermal impedance (as the flow of water is depended on the pressure (height difference) and the size of the pipe between them).
In a practical experiment, heat will also be "lost" to the environment. You cannot heat say a pan of boiling water to above ambeint temperature without some heat flowing out and away into that ambient, and that flow is also driven by the temperature difference and the thermal resistance. Numerous other factors come into play, for example, the interelationships between conduction, convection and radiation all change the heat paths and the total heat flow.
So, broadly, you are in a 25 degC room, with a pan of 25degC water on a gas stove.
You light the gas, and this burns (at about 2,000 degC at it's hottest point) heating the air around the flame, that drives convection upwards, and radiation also comes from that hot flame. Both impinge on the bottom of the pan, which because it is colder (at 25degC to start) absorbs some of the heat released by the burning gas. The metal of the pan warms, and conduction carried heat to the water within. Some heat is lost from the outside surface of the pan, as air currents convect it away and into the bulk of the 25 degC air in the room. The hotter the pan gets, the more heat convects away, but there is not much radiation because the temeprature of the pan itself is reasonably low (lets say 150 degC on the outside)
As the water in the pan warms, then it too starts to loose heat to the environment. Some water also evapourates off even before the bulk water volume starts to boil off. The hotter the water, the higher the deltaT to the room, the more heat is lost.
So at first, the rate of rise of the water is slow, because the pan must heat first
Then the rate of rise of the water is fast because the biggest thermal graident exists between the flame and the water, and the water is still close to room temp so little heat is lost to the rom
Then the rate of rise slows as the water gets hotter and hotter and the thermal gradient to the room grows, and of course the deltaT to the flame falls.
Ultimately the shape of the temperature increase asymptotically approaches that of the pan itself. However, at some point, the water starts to boil, and here, the latent heat of vapourisation takes the heat flux, and uses that to boil the water to steam, and the temperature of the water is stuck at 100 degC (assuming 1 bar abs pressure and pure water)
Heat flux (thermal energy) flows between objects of different temperature, like say water flows between resevoirs of different height. The flow of heat is proportional to the difference in temperature and the thermal impedance (as the flow of water is depended on the pressure (height difference) and the size of the pipe between them).
In a practical experiment, heat will also be "lost" to the environment. You cannot heat say a pan of boiling water to above ambeint temperature without some heat flowing out and away into that ambient, and that flow is also driven by the temperature difference and the thermal resistance. Numerous other factors come into play, for example, the interelationships between conduction, convection and radiation all change the heat paths and the total heat flow.
So, broadly, you are in a 25 degC room, with a pan of 25degC water on a gas stove.
You light the gas, and this burns (at about 2,000 degC at it's hottest point) heating the air around the flame, that drives convection upwards, and radiation also comes from that hot flame. Both impinge on the bottom of the pan, which because it is colder (at 25degC to start) absorbs some of the heat released by the burning gas. The metal of the pan warms, and conduction carried heat to the water within. Some heat is lost from the outside surface of the pan, as air currents convect it away and into the bulk of the 25 degC air in the room. The hotter the pan gets, the more heat convects away, but there is not much radiation because the temeprature of the pan itself is reasonably low (lets say 150 degC on the outside)
As the water in the pan warms, then it too starts to loose heat to the environment. Some water also evapourates off even before the bulk water volume starts to boil off. The hotter the water, the higher the deltaT to the room, the more heat is lost.
So at first, the rate of rise of the water is slow, because the pan must heat first
Then the rate of rise of the water is fast because the biggest thermal graident exists between the flame and the water, and the water is still close to room temp so little heat is lost to the rom
Then the rate of rise slows as the water gets hotter and hotter and the thermal gradient to the room grows, and of course the deltaT to the flame falls.
Ultimately the shape of the temperature increase asymptotically approaches that of the pan itself. However, at some point, the water starts to boil, and here, the latent heat of vapourisation takes the heat flux, and uses that to boil the water to steam, and the temperature of the water is stuck at 100 degC (assuming 1 bar abs pressure and pure water)
ruggedscotty said:
just wait till he seals the pan up and lets the pressure build.... water gets hotter before it boils.... physics can be fun
What could possible go wrong? 
At risk of whooshing you might want to be very careful trying that at home.
Might one notice a difference just by boiling in a pan with and without a lid?
Both rate of rise (convection/evaporation to environment reduced) and boiling point (higher pressure).
I would stab at yes and no respectively.
You would want to repeat the tests several times. Repeatability and selectivity would be covered.
Is there sufficient consistency that a conclusion can be drawn.
Have they covered sources of error yet?
That type of question does my head in - its quite a complex subject, but you can't really tell what level of detail they want in the answer, and the initial statement (water takes longer to heat the hotter it gets) is only correct for certain circumstance. In my camping cooker in the winter the converse is true because I put the gas cylinder on top of the pan to preheat the gas.
If someone asked me if water takes longer to heat when it gets hotter without context, I think my answer would be "it depends..."
If someone asked me if water takes longer to heat when it gets hotter without context, I think my answer would be "it depends..."
Edited by Mave on Wednesday 13th January 14:11
Water is weird stuff.
The concept of supercooled water is foreign to most, but basically you can get water in liquid state down to well below zero C (in fact, down as low as -48C) -- the key thing being that in order to freeze it needs to crystallise, and in order to do that it needs something to nucleate around.
Effectively if your water is clean enough and the container is smooth enough, and it's not moving, it won't freeze.
Many examples on youtube if you just search for "supercool water"
The concept of supercooled water is foreign to most, but basically you can get water in liquid state down to well below zero C (in fact, down as low as -48C) -- the key thing being that in order to freeze it needs to crystallise, and in order to do that it needs something to nucleate around.
Effectively if your water is clean enough and the container is smooth enough, and it's not moving, it won't freeze.
Many examples on youtube if you just search for "supercool water"
ch37 said:
Key Stage 3 Science criteria says kids should know it takes longer to heat water, the hotter the water gets.
Does it?!
Everything I can find online suggests that temperature increase is uniform after 0 and up to 100. All the graphs I've seen are a nice straight (diagonal) line between those values before vapour comes into play.
The experiment (which we don't have the kit to do) is for between 20 and 80, but the example values given show a significant slowing down of temperature increase as you go above 50 degrees.
Not often KS3 Science gets me but I've drawn a complete blank on this one.
IMO this is the sort of really sDoes it?!
Everything I can find online suggests that temperature increase is uniform after 0 and up to 100. All the graphs I've seen are a nice straight (diagonal) line between those values before vapour comes into play.
The experiment (which we don't have the kit to do) is for between 20 and 80, but the example values given show a significant slowing down of temperature increase as you go above 50 degrees.
Not often KS3 Science gets me but I've drawn a complete blank on this one.
t science teaching that has destroyed a generations ability to do science properly. A load of kids will take away from this that it takes more energy to raise a kilo of water from 70 - 71 C vs 20 - 21C.In reality, the specific heat capacity of water is constant - 4184J/Kg/K.
So an experiment where a kilo of water is heated in isolation (insulated etc) would yield a straight line. Boling a pan on a stove would lead to a curve because of all the reasons listed above.
When I did this at about KS3, the experiment was to heat a block of aluminium with an electrical heater buried in it. The block was wrapped in insulation. Lo, the amount of energy input was linearly proportional to the temperature raise - due to the specific heat capacity being constant. After getting that across, there was a discussion about the quality of the insulation and heat loss. Then at some later stage we covered latent heat of condensation/vapourisation etc
ch37 said:
Key Stage 3 Science criteria says kids should know it takes longer to heat water, the hotter the water gets.
Does it?!
I'm guessing that this is supposed to be about heat loss to the surroundings and by evaporation, rather than specific heat capacity.Does it?!
The point being that a pan of water on a stove will lose heat to the surroundings as well as gaining heat from the stove. The hotter the water gets the more heat is lost, and therefore the rate of heat rise slows down. A major part of the heat loss is through evaporation of water from the surface. When the water reaches boiling point, the capacity for heat to be removed increases dramatically to exactly balance the heat entering from the burner - as a result, once boiling point is reached, the temperature stops rising.
Specific heat capacity can be reasonably assumed to be constant - i.e. it takes the same amount of energy gain to raise the temperature of 1 kg of water by 1 C.
(A pedantic point is that specific heat capacity is not constant and depends on temperature, but unless you are working with very high pressure systems, like industrial steam turbines, the change is so small it can be ignored - interestingly, the change to specific heat becomes extreme at temperatures around 360 C, which is turns out to be a highly convenient property for high efficiency steam engines).
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