MadSci Network: Chemistry

Re: Making ice without machinery

Date: Thu Jun 10 19:11:54 1999
Posted By: John Christie, Faculty, School of Chemistry, La Trobe University, Bundoora, Victoria, Australia
Area of science: Chemistry
ID: 928339541.Ch

These chemical recipes for making ice are fine for demonstration purposes 
with small quantities, but rather inefficient and expensive when scaled up 
to try to use them for any practical purposes.

The Glauber's salt recipe that you have come across does specifically 
require that Glauber's salt be used. No other salt will do. The chemical 
process is

Glauber's salt + sulfuric acid solution --> sodium bisulfate + water
Na2SO4.10H2O (solid) + H2SO4 (aq) --> 2 NaHSO4 (solid) + 10 H2O (liq)

This reaction will take in about 105 kJ of heat from the surrounding 
environment per mole of reactants. Scaling this up, we arrive at
330 kJ of heat per kg of Glauber's salt and 300 g of sulfuric acid. For the 
sulfuric acid to be truly "aqueous", it would need to be in at least 1 
litre of water. Any less water, and heat would actually be given out when 
any residual sulfuric acid reacted with water released by the reaction. But 
of course too much water would lead to sodium bisulfate being formed in 
solution rather than as a solid.

If you are operating in Puerto Rico, I would suppose that the water you can 
get hold of during a power failure would be at about 25 deg C, so the 1 
liter (plus) of cooling solution would itself need to be chilled from that 
temperature to about -5 deg C in order to do any freezing. That would 
require at least 1 kg * 4.184 kJ/kg/deg C * 30 deg C = 125 kJ of heat to be 
removed, leaving up to 200 kJ maximum of potential heat removal to do your 
freezing. By the time we have allowed for heat losses in the apparatus, and 
the fact that it might be necessary to use slightly more water in 
proportion, a realistic figure might be 120 kJ.

Another recipe that I am personally more familiar with, and that may well 
work better for your purposes is simply to take the coolest available 
water, and try to dissolve up some ammonium nitrate in it. Ammonium nitrate 
is a commonly available nitrogen fertilizer. It is very soluble. 1 kg of 
ammonium nitrate will dissolve in 1 litre of water. It will remove about 
28 kJ of heat from the surroundings per mole of ammonium nitrate, which 
works out at 350 kJ per kg of ammonium nitrate -- very similar to the 
Glauber's salt system. Obviously you would need to get hold of your 
ammonium nitrate in a powdered form of some sort, rather than in a slow 
dissolving, slow release form.

How much freezing will 120 kJ of heat removal do? If we take 1 gram of 
water at 25 deg C, we need to remove 4.184 * 25 = 104.6 joule to chill it 
to water at 0 deg C, and then 334 joule to freeze that water -- 449 joule 
per gram = 0.449 kJ per gram. 120 kJ is therefore enough heat removal to 
freeze 120/0.449 = 267 gram of water.

1 imperial gallon of water is about 4520 gram; 1 american gallon is 
therefore about 3620 gram. Puerto Rico sounds more American than imperial, 
so I'll guess that you mean the latter ;-)

That would mean a scale-up by a factor of about 14: You need either

14 kg of Glauber's salt in 14-20 litres of previously made up dilute 
sulfuric acid (20 to 25% by volume of concentrated sulfuric in water, or 
about one part of water to 2 parts of battery acid. NB extreme care 
required -- a lot of heat is given off when sulfuric acid and water are 
mixed. Always add acid to water, slowly.)


14 kg of ammonium nitrate fine crystals in 14 litres of water.

A plastic jug is a bad way to go -- the plastic is an insulator, and the 
jug shape is bad for heat removal from the centre. You need at least one 
small dimension for rapid heat exchange. A number of smaller containers or 
a book-shaped thin rectangular container would be better.


The second part of your recipe about dry ice seems to me to be nonsense. 
Yes, it is possible to remove heat with a reaction that produces a gas, 
like the vinegar/carb soda reaction. But to make dry ice you need to 
achieve temperatures of -80 deg C, and there is no way you are going to do 
that in or near an aqueous environment (and especially not in Puerto 

Current Queue | Current Queue for Chemistry | Chemistry archives

Try the links in the MadSci Library for more information on Chemistry.

MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci

MadSci Network,
© 1995-1999. All rights reserved.