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?