8th Grade Science – Professor Shai

Freezing-Melting Point Experiment

Section 3.2 – Page 41

 

Introduction

 

We studied density in detail as a characteristic property of matter.  Another such identifying property is the freezing-melting point of a substance.  The freezing-melting point of a substance is the temperature at which it changes from a liquid to a solid (freezing) or vice versa from a solid to a liquid (melting).  It is neat that these two temperatures are usually the same!  Moreover, the change in state from solid to liquid requires heat.  The heat does not change the temperature the substance.  Instead, the heat is used exclusively to melt the substance that for the most part remains at a constant temperature throughout the melting process.  Once all the substance is melted, the heat again raises the temperature of the substance until it starts to boil, but that’s our next lab.

 

The freezing-melting point of water is well-known to you;  it is 32 degrees F and zero C. 

 

In this experiment, you will measure the melting-freezing point of two unknown substances.  (If you really care, the substances are called butylated hydroxy toluene (BHT), and tert-octylphenol (TOP)).  The two substances have different freezing-melting points, both well above room temperature.  So at room temperature, both substances are solids.

 

Procedure

 

I will give you some random amount between 7 and 13 grams of one of the substances.  Recall that characteristic properties are independent of the amount of the substance being measured.  You will measure the freezing-melting point of your substance and produce a graph of temperature versus time.


It easier to notice when something completely freezes than when it completely melts.  Therefore, we look for freezing rather than melting.  In order to do this, we heat up the substance until it is a liquid, and we plot the graph as it cools down and turns into a solid.  We heat the substance using a double-boiler.  That is, we put the substance in a test tube inside a beaker with water and we heat the water.  This technique is used so that we heat the substance gently and evenly and do not burn anything.  It is a common technique used in cooking delicate items like chocolate sauce, yum!

 

  1. Put the substance in a test tube.
  2. Place a thermometer in a slit stopper in the test tube, and let it rest on the bottom.
  3. Place the test tube in a 100 ml beaker filled with water, so that the water level is higher than the top of the substance in the test tube.
  4. Place another thermometer into the beaker.
  5. Ask me to light your burner.
  6. Place the beaker on a heat-spreading screen and heat the water until it boils.
  7. Stir the substance when it starts to turn liquid until it is completely liquid.
  8. When the substance is completely liquid, ask me to come turn off your burner.
  9. Every 30 seconds, stir the substance read the temperature of the water and the substance. 
  10. Graph two points for each reading, one for the water, and one for the substance.
  11. Wait and keep measuring every 30 seconds until the substance solidifies.
  12. Keep measuring and graphing until the solid cools at least 10 degrees below its freezing point.


Worksheet

Time(minutes)      Temperature Water         Temperature Substance

0
.5
1
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
7.0
7.5
8.0
8.5
9.0
9.5
10.0
...  continue as needed.  Draw a graph on the graph paper provided.



Write down the freezing-melting point temperature of your substance as indicated by your graph to the nearest tenth of a degree Celsius.  Hand in your graph to me.



Do problems 2-5 (pages 43-45), and 28-30, 32 (pages 60-62).  Ask me for help if you are not sure how to do them.