Science lecture notes

1. Introduction to
Colligative (collective) Properties
Goal 1: Get in the way
Goal 2: Create Disorder
or
When quantity not quality is the
only thing that matters.

2. An ice storm is coming. Your car’s radiator has no
antifreeze in it; all stores are closed, and you’ve
got to use whatever you have around the house to
save your car’s engine.
Also remember the motto for
Colligative Properties
Getting in the way is Good,
Creating Disorder is Better
It would be ideal to have antifreeze
(ethylene glycol) but remember it’s
quantity not quality that counts here.

3. The next morning you wonder if what you did
saved you thousands of dollars on a new engine.
Rule one: What you add has to dissolve in water.
Rule two: If you add a solid that dissolves in
water, it doesn’t matter what it is, just the amount.
Rule three: Just like antifreeze, your goal is to
replace about ½ of the water with a solid or liquid
that is miscible with water.
Examples 1: Sugar, salt, baking soda, shampoo,
laundry detergent, pancake syrup.
Examples 2: Rubbing alcohol, brake fluid

4. Again, it seems odd that the chemical nature of these
different compounds are not a factor in regards to how
much they depress the freezing point.
Examples 1: Sugar, salt, baking soda, shampoo,
laundry detergent, pancake syrup.
It reminds us of the ideal gas law: PV=nRT
The properties are not affected by what kind of
gas we are talking about (at least in situations
where they are behaving as an ideal gas).

5. How does it work?
1)Get in the way.
2)Create Disorder
Reason 1: As the water tries to freeze,
the other molecules get in the way.
Reason 2: By adding these other substances,
you’ve added disorder to the mixture. Nature
tends to favor disorder (entropy). When water
tries to freeze, it has to get organized, which will
take more energy.
Freezing point
depression

6. This frog is frozen, but the water in it did not turn
into ice crystals, which would have ruptured the
cells in its body. Why didn’t ice crystals form?
Glucose and glycerol in its blood and cells
prevent water from freezing.

7. Dissolved sugars also play a role in helping
plants to be frost tolerant.

8. This graphic helps us see the randomness that mixing causes. As a mixture, the
liquid that would normally freeze at a higher temperature is prevented from freezing
by the other liquid or solid. Again the cause is the solute gets in the way and the
freezing as to reverse the randomness (entropy) which takes more energy.

9. Antifreeze is often called
“coolant” because it not
only can lower the freezing
point of water, it can
elevate the boiling point of
water.
So, if you don’t have any
antifreeze (coolant), what
could you use instead?
Yes, the same items that
you picked to keep it from
freezing.

10. Notice that ice cream melts differently
than ice. Ice stays hard until it melts.
Ice cream gradually get softer and
softer.

11. Ice is a pure substance but ice
cream is a mixture. In other
words, there are other
chemicals that get in the way
of water freezing.
So you have get
colder than 0°C
to get it to
freeze.
About 30% of the water in
ice cream never freezes
because of the high level
of dissolved solids like
sugar, fats, and proteins.

12. Chocolate
behaves is a
similar fashion.
As a mixture, it
gradually softens
before it melts.

13. Most anything
will reduce its
melting point if it
is mixed with
other
substances.

14. Pure molten silica (SiO2)
freezes into quartz at
around 2000°C, but if
mixed with CaO and
Na2CO3, it freezes at
about 1000°C. Other
additives can bring it
down to 500°C. Actually,
at room temperature it is
still not completely frozen.
Glass is classified as a
supercooled liquid. Over
many millenia, this
drinking glass will slowly
“melt” into a pool of glass.

15. Eggs are mostly
water, but
dissolved
proteins keep
them from
freezing at 0°C.
Chefs take
advantage of this
in frozen
desserts.

16. How do you make red blood cells go from this shape to this?
Get in the way
And quantity not quality is all that counts.

17. Cell Membrane
Inside cell
Outside cell
Inside and outside water passes
through the permeable
membrane at equal rates.

18. Cell Membrane
Inside cell
Outside cell (hypertonic)
Na
Cl
Cl
Cl
Cl
Na
Na
Na
Na
Na
Eventually an equal number will
pass back and forth, but only after
the outside has a greater share of
the water molecules.
Anything soluble will do the same
thing (quantity not quality) For
example, any used for freezing
point depression would cause
this.
The extra solute (e.g., NaCl) gets in
the way of the outside water passing
through the membrane. Note:
Solutes cannot pass through membrane.

19. Vapor pressure is reduced by the addition of a non-
volatile solute for the same reasons as freezing point
depression and boiling point elevation.