Pre-Lab 10B: Determining Freezing/Melting Point

Purpose

• A cooling/heating curve is a plot of temperature vs. time/energy graph.

• It illustrates the effect on a substance as the temperature decreases/increases through a phase change.

• In this experiment, you will study the effects of cooling and heating cetyl alcohol through a phase change.

• The data will be recorded using a data collector and then plotted on graphs.

• Based on your observations, measurements and graphs, you will be able to determine the freezing/melting point of cetyl alcohol.

Key Question

• How do you determine the freezing/melting point of cetyl alcohol?

Background

• Define:

Phase change

Freezing point

Melting point

Safety:

wear goggles with heating and chemical use

Drawing

• Draw this:

note probe is in

the test tube

IMPORTANT PROBE DATA COLLECTOR USE:• probe must be in chemical before you press “Go”• when “Go” is pressed, data collection begins, even

though you can not see it• press “graph” to view graphing data• there is no pause button• mark last data point you want by pressing screen at

that point on the data line. This will put a line on the graph marking the end of your data, BUT the temp will keep going: do not use this extra data

• DO NOT REMOVE TEMP PROBE FROM SOLUTION UNTIL THE GRAPH HAS BEEN DRAWN

ProcedureRead directions very slowly and accurately

there are two parts to this experiment:

part one, melting point: in a 250ml hot water bath, measure/graph the temperature of the test tube with solid turning to liquid

You will get the 250ml hot water from me at the side of the class, carefully hold the lip of the beaker and return to your lab station

part two, freezing point: remove the test tube with melted solid from the water bath and measure/graph the temperature the liquid turns to solid

Section 2:The first part of the lab: you do not need a 250ml beaker for cold water, just put

the temperature probe under cold running water before use

Step 4 has a typo-error:Measure the temperature of the hot water to make sure it is within the range in step “1”; this number should be step 2, please change this in our lab books

• Place the temperature probe into the test tube and put the test tube containing the solid into the hot water bath and press START on the Data Collector.

• View the experiment in graph mode.

Section 3:

• Describe and carefully sketch the shape of the graph from Part 2.

• remember this graph is a sketch: do not use graph paper

• use straight edge for x and y axis

• write title

• label x and y axis: variable and unit

• record numbers on each axis

• Include data line

Section 4:

The second part of the lab:

• Observe the cetyl alcohol in the test tube to make sure it is still all liquid.

• If some solid has formed, pour out the water in the warm beaker and add more hot water to the beaker.

• You must start the experiment w/ all the chemical in the test tube in the liquid state

• Remove the test tube from the warm beaker, stir (?) and start a new experiment on the data collector. DO NOT HOLD THE T.T.?

Section 5:

• Describe and carefully sketch the graph from Part 4.

• Wash your temperature probe so that no chemical remains on it , why?

Post Lab 10B: Determining freezing/melting point

Key Question:

How do you determine the freezing/melting point of cetyl alcohol?

Background

• Define:

Phase change

Freezing point

Melting point

Purpose

• A cooling/heating curve is a plot of temperature vs. time/energy graph.

• It illustrates the effect on a substance as the temperature decreases/increases through a phase change of cetyl alcohol

• you will be able to determine the freezing/melting point of cetyl alcohol.

Questions:

1) what is the frz/mlt point temperature range for a pure substance?

2) If the ceytl alcohol becomes contaminated, how would this effect the fz/mlt pt?

3a) What is the effect of sample size on the melting/freezing point?

3b) Describe and carefully sketch the shape of the graph from heating the sample.

5a) Describe and carefully sketch the graph from cooling the substance.

5b) Referring to both of your graphs, determine the melting and freezing point of cetyl alcohol. Are they the same? Should they be?

5c) Based on the shapes of your curves, which data do you think is more reliable - the heating or cooling data? Why do you think this is so?

5d) What is happening to the molecules of cetyl alcohol during the diagonal portions of the heating curve? What about the plateau?

5e) Your graph from Part 2 shows that during a change from solid to liquid, the temperature stays the same. Explain why the temperature does not increase, even though energy is being added.