CLASS SET, PLEASE DON’T TAKE A COPY!!!!!!!
Answer Key - Freezing Point Depression – “Hail in a Test Tube” lab on page 34 of the packet.
I HAVE PUT A LOT OF THE VOCAB YOU NEED TO KNOW INTO BOLD PRINT
1. What happens to the heat energy of the molecules in the test tube during this lab? (easy)
A: It is transferred to the ice/salt-water bath to warm it up, while the test tube water cools off (loses heat)
2. What happens to the heat energy of the molecules (both ice and water) in the beaker during this lab?
(tricky)
A: Ice: The ice (don’t forget that ice is made of water) molecules gain heat from the liquid water
molecules. This heat allows the ice molecules to bump against each other hard enough to escape
from the crystal structure of the ice (they melt). To melt the ice, it takes 80 calories of heat per gram
of ice. The heat for melting the ice comes from the liquid water molecules, and that is true in an iced
glass of pop in the summer, just like it is in this lab. We made it work for us by adding the salt to
lower the freezing/melting point of the water in the beaker. Since the ice temp was now above the
new freezing point, the ice began to melt (just like the ice on a street melts when you add salt to it).
The heat energy that was still required to melt the ice came from the liquid water (and some heat
came from the room, too, but we are ignoring that for the time being).
B: Water: The liquid water molecules lost heat energy when they melted the ice. They had to give up
the 80 calories required to melt each gram of ice. Because it only takes 1 calorie per gram to cool
liquid water by 1 degree Celsius, every 80 mL of liquid water was cooled by 1 degree Celsius
whenever a gram of ice melted. Since we only had 50 mL of water in the beaker, it was cooled off by
more than 1 degree Celsius for each gram of ice that melted.

ULTIMATELY, IF WE IGNORE THE HEAT IN THE ROOM, THE AMOUNT OF HEAT
ENERGY IN THE ICE AND WATER IN THE BEAKER STAYED THE SAME, IT WAS JUST
TRANSFERRED AROUND AS THE WATER COOLED AND THE ICE MELTED!
3. What does the salt do to the temperature of the salt-water mixture in the beaker? (easy)
A. Since the salt-water combo is at about room temp, and the salt was about at room temp, nothing really
changes here. I meant to write ice/salt-water mixture. In that case, the salt makes the temperature
drop below freezing.
4. HOW does the salt accomplish what you wrote for the previous question? (hard)
A. The salt dissolves in the water, separating into Na+ and Cl-. The Na+ is attracted to the slightly
negative oxygen atom in each water molecule. The Cl- is attracted to the slightly positive hydrogen
atoms in each water molecule. When water is cooled and would normally form ice crystals at 0oC,
the Na+ and Cl- ions that are near the O and H atoms in the water molecule get in the way of the water
molecules and make it more difficult for them to arrange themselves into a crystal lattice. Also, the
heat in the water is used to melt the ice cubes, but it takes 80 cal/gram to melt the ice, while it only
takes 1 gal per gram to raise or lower the temperature of liquid water by 1oC. As a result, for every
gram of ice that is melted using energy from the water, you will see 80mL of the water get 1oC
colder. Since you only had 50mL of water in the beaker, and 15mL of water in the test tube, every
gram of ice that melted dropped the temp of both by at least 1oC. After 6 minutes of the lab, the saltwater/ice in the beaker dropped to such a low temperature that the lack of heat caused the plain water
in the test tube to freeze as 80 cal of heat per gram of test-tube water was absorbed by the saltwater/ice mixture in the beaker. Wow.
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CLASS SET, PLEASE DON’T TAKE A COPY!!!!!!!
Note: Salt also raises the boiling point of water because the Na+ and Cl- ions in the water isolate the
water molecules further away from other water molecules, and their ions’ attraction to the water
molecules keeps the molecules from being able to escape as a gas until they gain a lot more heat
energy. To get enough heat energy, the temperature has to go well above the 100oC temperature at
which water normally boils and releases the water molecules into the air as a gas. (Amazingly, it
takes 540 calories per gram to turn UNSALTED liquid water at 100oC into steam at 100oC. Just like
the freezing/melting example, the temperature doesn’t change as the phase changes from liquid to
gas).
5. What does the heat energy of the water in the beaker do to the ice? (easy)
A. The heat energy is used to melt some or all of the ice
6. How does the polar nature of the water molecules impact what happens during the lab? (think about the
salt-water mixture in the beaker and the drawings on the white board, plus the video)
A. Without the water molecules having a polar nature, the salt wouldn’t have dissolved and the Na+ and
Cl- ions wouldn’t have been available to lower the freezing/melting point of the water.
7. Knowing that ice takes 80 cal/gram to melt, and liquid water only needs to lose 1 cal/gram to drop its
temp by 1 degree C, how can you explain the drop in temperature of the ice-water mixture in the beaker?
Please assume that the salt already did its job of lowering the freezing point of the liquid water in the
beaker when you craft this answer. (tricky)
A. Like the answer to question #4. The liquid water gave heat energy to the ice so the ice could melt
into liquid water. But, since it takes so much more heat energy to melt ice than it does to cool liquid
water by 1 degree Celsius, the liquid water lost a lot of heat energy and the temperature started to
drop. For every gram of ice melted, 80 calories were taken from liquid water, which only needs 1
calorie per gram to drop its temperature by 1 degree Celsius. Since there were only 50mL of water in
the beaker, and 15mL of water in the test tube, the 80 cal/gram needed to melt the ice was enough to
cool all 65 grams of the water by a little over 1oC for every gram of ice that melted. After 20-30
grams melted, the salt water mixture in the beaker was well below zero and the water in the test tube
was a liquid at 0oC. If things were perfect, each gram of ice that melted at that point would absorb
enough energy to freeze a gram of water in the test tube. As the melting/freezing occurred, some heat
would be lost to the salt-water mixture and some to the beaker, test tube, and room itself.
8. If you ignore the room, what happens to the heat energy in the test tube-beaker combination during this
lab? (easy)
A. Ignoring the room, there is as much heat in the test tube-beaker combo at the end as there was at the
start. The molecules in the water may lose heat energy to the ice, but none of that energy is lost in
any part of this lab, so the total heat energy will stay the same. (remember, the heat energy is based
on the motion of the atoms and molecules in the lab, and it is really a measure of how fast they are
moving around – their kinetic energy.)
9. What happens to the heat energy in the test tube-beaker-classroom combination during this lab, if we
assume that they all started out in the room? (easy)
A. It stays the same as it was at the start of the lab. It’s not spread out in the same way it was at the start
of the lab, but it is the same amount of energy.
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CLASS SET, PLEASE DON’T TAKE A COPY!!!!!!!
10. Draw a few water molecules (space filling models) that show how they are arranged in a crystal in the
ice.
A. Draw something that shows a regular, repetitive arrangement of the water molecules. The slightly
negative oxygens should be near the slightly positive hydrogens from other water molecules as they
arrange into a crystal lattice (that’s a vocab term and it describes the regular, repetitive pattern seen
in the example to the right).
11. Draw a few liquid water and solid salt molecules (in a crystal) to show how they look before the salt
dissolves, but after it’s dropped into the water at the start of the lab.
A. This one should look like the example in the video we watched in class. (https://goo.gl/hz3RZT )
12. Draw the molecules in the salt-water mixture after the salt dissolves in the liquid water.
A. This should also look like the example near the end of the video used in the prior question.
(https://goo.gl/hz3RZT ) The general idea is that the Na+ ions should be associated with the slightly
negative end of the water molecules (oxygen’s end) and the Cl- ions should be associated with the
slightly positive ends of the water molecules (hydrogen’s end). All of the molecules and ions will be
evenly spread out if the mixture is a true solution (another vocab word).
13. The salt-water mixture is a special kind of homogeneous mixture. What do we call a mixture where one
item is dissolved in another item?
A. A solution.
14. What is the solute in the salt-water mixture?
A. The salt
15. What is the solvent in the salt-water mixture?
A. The water is the solvent. Water is known as the universal solvent (more vocab), since it will
dissolve or mix with so many things.
16. What does homogeneous mean?
A. Homo means “the same”, so homogeneous (vocab) means that the matter appears to be the same
throughout. Homogenized milk has been mixed so well that the fat doesn’t rise to the top, so it is
homogeneous.
17. What does heterogeneous mean?
A. Heterogeneous means mixed, or different. Heterogeneous (vocab) means that it is not the same
throughout.
18. Look up the classification of matter and make a chart that properly arranges the following terms:
Mixture, Matter, Pure Substance, Homogeneous Mixture, Heterogeneous Mixture, Solution,
Compound, Element.
A. You should have this one done. I’m out of space, and it’s readily available online.
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