Hi Gang! Does cold water boil faster than hot water?
The answer is no. But why might a lot of people people think
it does? In this short science video I'll show some
rigorous science experiments I did, that give some clues into all this.
The usual way this experiment is done is to time how long a pot of cold water takes
to boil, and then how long a pot of hot wire takes
to boil. But I wanted to go deeper.
I wanted to see how fast the temperature changes when the water is cold,
how fast it changes when it's warm and how fast it changes when it's hot.
From known physics I already suspected there would be a difference,
so I set up an experiment to test it.
I started with the usual pot and the usual cooking thermometer.
The problem there is that most of the cooking thermometer
isn't even in the water. When I did this I found the water was actually
boiling while the thermometer was saying it was only
around 70 degrees celsius or 160 fahrenheit.
The boiling temperature of water is 100 celsius or 212 fahrenheit.
The part of the thermometer in the air was making the thermometer cooler than the
water and not telling me what I needed to know.
So I got one of these. It's an automobile coolant temperature sensor.
But underneath the protective metal case, it's a thermistor.
Of course, I made some additions to make it useful for my purposes.
I also made a holder for the thermistor that I could use to sit it in the bottom of
my pot. It's made of clothes hanger wire
and is adjustable as needed by bending the wire.
Notice that when it's in the pot with water, the majority of the sensor is actually in
the water, which is what I was after.
One way to use it is to connect it to a meter that's set to measure resistance.
As the temperature here increases, the resistance on the meter decreases.
If you know what resistance equates to what temperature
then you can monitor the temperature that way
by taking resistance measurements. In another video I show how to figure out
the resistance temperature relationship, since I couldn't find that information anywhere
for this one. More about that at the end.
Now for the experiment.
First I wanted to test starting with cold water.
I put 250 millilitres or 1 cup of water in the freezer
with my thermistor in it such that the bottom part was in the water.
When the resistance was above 528 ohms, which according to my table is 6 celsius or
43 fahrenheit, I took it out.
I put the thermistor in the bottom of the pot
and connected the meter to the thermistor. With the stove element nice and hot
I poured the water into the pot. I put the pot on the stove element,
started the timer and started recording resistance values from
the meter every 10 seconds.
Once I reached 31 ohms on the thermistor, which is around boiling,
I stopped recording.
Next I repeated the experiment using the hottest water
I could get from the hot water tap.
Here are the results of the cold water test, and here are the results of the hot water
test. And overlapping them we get this.
Horizontally across the bottom we have time, and vertically on the sides is temperature.
So the cold water started at 15 celsius or 59 fahrenheit
and it ended at 96 celsius or 205 fahrenheit, the hottest I measured before stopping.
The cold water took 3 minutes to boil. The hot water took 1 minute and 50 seconds
to boil. So the answer to the first question is
that hot water does boil sooner than cold water,
or faster as it's often stated.
But what about the second question? Why is there this idea that cold water might
boil faster?
Look carefully at the chart. Notice that the data forms curves, not straight
lines. The heating is faster during this part
where the water is still cool. Then the heating slows down when the water
is warm. And then it really slows down when the water
is hot. This is actually known science.
The greater the temperature difference you have between the hot side
and the cold side, the faster the heat is transferred.
And the closer the two temperatures are the slower the heat is transferred.
And that's likely where the misconception comes from.
When the water's cold, the heat is transferred faster.
But only when it's cold. Once it reaches the same temperature
as the water that was started at a hotter temperature,
the heat is transferred at the same rate as for that hot water.
From then on they follow the same curve. The rate of heating for the cold water has
caught up to the rate of heating for the hot water
and from then on, they take the same amount of time.
Well thanks for watching!
See my youtube channel, rimstarorg for more fun science videos like this.
That includes the one I mentioned talking about how I adapted
the automobile coolant temperature sensor, or thermister,
and made the necessary resistance to temperature measurements table.
One on how to make a steam vacuum and how it works.
And one explaining what fire is.
And don't forget to subscribe if you like these videos,
or give a thumbs up or leave a question or comment below.
See you soon!