Pre-Freeze/Melt Lab
(BHT-TOP)
Purpose:
If you live in a part of the country where it
snows, you know a larger pile of snow takes a
longer amount of time to melt compared to a
smaller pile of snow: this is due to more or less
substance being present; however, their melting
points are both the same = 0 degrees Celsius.
You also know that the melting point of ice
(water) is different than that of candle wax. This
phenomenon is due to an entirely different
reason: every substance has its own
melting/freezing point, called a characteristic
property.
• Key Question:
Can you use freezing point to determine if
the temperatures at which a sample of two
different substances melt/freeze is really a
characteristic property?
Background:
define:
-pure substance
-phase change
-melting point
Drawing:
draw the lab set up: drawing on the board
Safety:
always wear goggles!
Materials:
Ring stand
Ring stand clamp x2
Test tube with chemical BHT or TOP and a thermometer
Thermometer in a rubber stopper
400 ml beaker full of hot water
Teacher:
-two pure substances, BHT and TOP, each in their
own test tube with thermometer and are in the liquid
state (taken from two water baths),
-2 two liter flasks of boiling water
Procedure:
1. Create a data chart with: time, temperature of water,
temperature of BHT and temperature of TOP on the
header bar. The number of rows will be very long due
to the amount of time involved to cool the
substances.
2. At lab stations, pairs of students set up one ring
stand w/ two ring stand clamps attached.
3. Bring your 400 ml beaker to teacher to retrieve: 1)
hot water and 2) a test tube w/ its own liquid chemical
and thermometer (B = BHT or A = TOP: one sample
of each will be used at EACH lab station).
4. Place the test tube w/ liquid in the hot water beaker
and return to your lab station. Immediately place the
test tube in the clamp keeping test tube submerged in
hot water: DO NOT ALLOW IT TO TOUCH THE
BEAKER. Stir constantly w/ thermometer.
5. Place the thermometer already in the rubber stopper
in second clamp and submerge it in the beaker of water:
NOT TOUCHING the beaker or test tube.
6. Immediately measure and record in a data chart
temperatures of both the water bath and the assigned
chemical every 0.5 minutes. You will need to designate
which person will be reading both the water temperature
AND the chemical temperature and which will read the
time and record the two temperatures.
7. Continue recording both temperatures until two
minutes after the chemical is “ROCK SOLID”.
8. Return your chemical to the teacher’s water bath: be
absolutely certain it goes in the correct chemical’s
water bath: B = BHT, A = TOP
9. Graph all three substances (water, BHT, TOP) with
temperature depending on time, following the graphing
rules (share your lab station’s BHT or TOP data with the
other pair of students at your lab station).
10. Locate and label on the graph the freezing point of
the two chemicals (what will the graph look like at this
point?)
Questions:
1. Describe what the direction the graph takes when the
chemical is in only the liquid state.
2. Compare your classmates’ BHT and TOP graphs,
what does the freezing point look like?
3. Using your data and classmates’ data, what is the
freezing point for:
-BHT:
-TOP:
-water:
4. Does the freezing point of a substance depend on the
amount of substance, explain?
5. Based on the cooling curves of BHT and TOP,
explain using the data (numbers) why the freezing
temperature of a substance is a characteristic property.
6. The freezing point of your chemical should have a
slight elevated rise called the “heat of crystallization”,
what causes this rise?