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Description:
Who doesnt like ice cream, eating it is fun
too, but making it yourself is so much better.
Resident will be able to make their own ice
cream and even choose their own flavor(s). Through
the use of ingredients such as milk, sugar and
cream, residents will make their own ice cream
while learning the fundamental concepts of freezing
point and energy absorption.
Instructions:
Materials
= cup milk
= cup whipping cream (heavy cream)
< cup sugar
< teaspoon vanilla or vanilla flavoring
= to > cup table salt or rock salt
2 cups ice
1-quart ZiplocTM bag
1-gallon ZiplocTM bag
Procedure
1. Add < cup sugar, = cup milk, = cup whipping
cream, and < teaspoon
vanilla to the quart ziplocTM bag. Seal the
bag securely.
2. Put 2 cups of ice into the gallon ziplocTM
bag.
3. Add = to > cup salt to the bag of ice.
4. Place the sealed quart bag inside the gallon
bag of ice and salt.
5. Seal the gallon bag securely.
6. Gently rock the gallon bag from side to side.
Continue to rock the
bag for 10-15 minutes or until the contents
of the quart bag have solidified into ice cream.
Explanation
Ice has to absorb energy in order to melt, changing
the phase of water from a solid to a liquid.
When you use ice to cool the ingredients for
ice cream, the energy is absorbed from the ingredients
and from the outside environment (like your
hands, if you are holding the baggie of ice!).
When you add salt to the ice, it lowers the
freezing point of the ice, so even more energy
has to be absorbed from the environment in order
for the ice to melt. This makes the ice colder
than it was before, which is how your ice cream
freezes.
Ideally, you would make your ice cream using
'ice cream salt', which is just salt sold as
large crystals instead of the small crystals
you see in table salt. The larger crystals take
more time to dissolve in the water around the
ice, which allows for even cooling of the ice
cream.
You
could use other types of salt instead of sodium
chloride, but you couldn't substitute sugar
for the salt because (a) sugar doesn't dissolve
well in cold water and (b) sugar doesn't dissolve
into multiple particles, like an ionic material
such as salt. Compounds that break into two
pieces upon dissolving, like NaCl breaks into
Na+ and Cl-, are better at lowering the freezing
point than substances that don't separate into
particles because the added particles disrupt
the ability of the water to form crystalline
ice. The more particles there are, the greater
the disruption and the greater the impact on
particle-dependent properties (colligative
properties) like freezing point depresssion,
boiling point elevation, and osmotic pressure.
The salt causes the ice to absorb more energy
from the environment (becoming colder), so although
it lowers the point at which water will re-freeze
into ice, you can't add salt to very cold ice
and expect it to freeze your ice cream or de-ice
a snowy sidewalk (water has to be present!).
This is why NaCl isn't used to de-ice sidewalks
in areas that are very cold.
How
to Make Ice Cream doc
Other Considerations: it can get messy
and wet, so i usually have this activity outside
in our patio or courtyard.
Vay
San Jose State University
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