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Adam Munich

Sat Oct 16 2010, 06:03AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

I heard that hot water freezes faster than cold water today. I looked it up and apparently it's true.

The effect is named for the Tanzanian high-school student Erasto Mpemba. Mpemba first encountered the phenomenon in 1963 in Form 3 of Magamba Secondary School, Tanzania when freezing hot ice cream mix in cookery classes and noticing that they froze before cold mixes. After passing his O-level examinations, he became a student at Mkwawa Secondary (formerly High) School, Iringa, Tanzania. The headmaster invited Dr. Denis G. Osborne from the University College in Dar Es Salaam to give a lecture on physics. After the lecture, Erasto Mpemba asked him the question "If you take two similar containers with equal volumes of water, one at 35 Â°C (95 Â°F) and the other at 100 Â°C (212 Â°F), and put them into a freezer, the one that started at 100 Â°C (212 Â°F) freezes first. Why?" only to be ridiculed by his classmates and teacher. After initial consternation, Dr. Osborne experimented on the issue back at his workplace and confirmed Erasto's finding. They published the results together in 1969.

I'm going to test this myself. If it works well then it maybe you need to heat your drinks first if you want to get them cold.

radiotech

Sat Oct 16 2010, 07:04AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

If you try this run curves with thermocouple that came with your new
multimeter to see the rate of temp drop at 212 wnen you cut off the power to tne heater and at the point where it freezes. Then see if you can make a phase diagram. Your science teacher could help you. What gizmo could you invent to signal freezing?

Sulaiman

Sat Oct 16 2010, 08:05AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

If you do this experiment,
add a third glass of water that has been boiled then left to cool to 35 Celcius.

Adam Munich

Sun Oct 17 2010, 05:45AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Crap I forgot about them and now they're both frozen...

I'm going to retry this with you're suggestions. Here's the setup.

Pretty much the thermocouple in 1 cup of water. The meter + the windows 95 laptop will log the data, and when the water reaches -1*C it's frozen. First with room temp water, then with boiling, then with the boiled + cooled.

klugesmith

Sun Oct 17 2010, 06:08AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

The "hot water freezes faster" truth or myth has been discussed for more than 50 years.

I think its origin was a truth easy to verify in the days before self-defrosting freezers.
When the floor of a freezer compartment has a thick layer of frost,
and you put in metal ice-cube-making trays, the one filled with hot water will
melt its way down and make a better thermal contact than the one started with cold water.

I bet a simple Google search would be most fruitful in this investigation.

-Rich

Bored Chemist

Sun Oct 17 2010, 11:17AM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

All fascinating stuff; but is there any chemistry involved?

klugesmith

Sun Oct 17 2010, 05:23PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

Bored Chemist wrote ...

All fascinating stuff; but is there any chemistry involved?

No chemical reactions, but no electricity either. The tools (fluid measuring cups, thermometers) are at home in chem labs!

Grenadier: If you do your experiments sequentially, there's an important uncontrolled variable: the cycling of the refrigerant compressor and the self-defrost heater. They could be affected by opening the door, not to mention putting hot things in. How 'bout using your thermocouple logger to record the air temp in freezer for 24 hours while the door stays shut? Maybe you'll catch a defrost cycle.

Adam Munich

Sun Oct 17 2010, 06:15PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Damn! I forgot about the defrost cycle. Now I have to do it again in my 70's freezer in the garage. Grr...

Edit: All right new setup. This time I'm using a teacup because the plastic one shattered. I'm using a different computer also because I realized I won't be able to get the data off of the other one. This computer has seen better days but it still works. Temp will be logged at 30 second intervals. I also turned off the defrost cycle on the old freezer.

radiotech

Sun Oct 17 2010, 06:53PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Of chemical interest is the effect of adding things like Glauber's
salts and see what effects aree noted. Or you cound invent a machine like this :

Adam Munich

Sun Oct 17 2010, 06:58PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

radiotech wrote ...

Of chemical interest is the effect of adding things like Glauber's
salts and see what effects aree noted. Or you cound invent a machine like this :

Like what?

I don't have any sodium sulfate, but I have sodium bisulfate, epsom salt, salt, ammonium nitrate, and a few other soluble things.

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