Lesson 3.4 - Melting Points

In Lesson 3.3, you learned the scientific definitions of pure substances and mixtures. You also learned how the properties of antifreeze and water change when they are mixed. Read page 17 to begin the study of using salt on roads and sidewalks to keep them free of ice.

You are likely familiar with the sanding trucks on the roads when there is a snow or sleet storm. Do you know what they are spreading on the road? It is not just sand. It is a mixture of sand and salt. Why is there salt in this mixture? Have you ever had to put salt on an icy sidewalk? What kind of salt is used on icy sidewalks and roads? All these questions are answered in this lesson.

Investigation: Melting Point of a Salt-Water Mixture

Read the entire investigation on pages 18 and 19 in your textbook.

If you have access to a supervised laboratory, perform the investigation as outlined in the text.

If you do not have access to a supervised laboratory, you will not perform the physical part of the investigation. Instead, you will use the sample data collected by a student to answer the analysis questions.

Problem: How does the quantity of salt in a salt-water mixture affect the melting and boiling points of the solution?

Apparatus, Materials and Procedure: As specified on page 18.

Observations:

Beaker

Melting Point

Boiling Point

A

-2°C

100°C

B

-4°C

102°C

C

-5°C

103°C

D

-6°C

105°C

Analysis:

  1. Graph the results. Use the x axis for the quantity of salt added and the y axis for the temperature.

  2. Predict the freezing point and boiling point of a salt-water mixture if 80 g of salt were added to 200 mL of water.

Then, return here to continue this lesson.


The investigation shows clearly that the melting point and boiling point of a salt-water mixture is different from the melting point and boiling point of pure water.

If you have some sidewalk de-icing salt, you might try this little investigation.

  • If it is winter, put a teaspoon of ordinary table salt on some ice outside and beside it (but not too close) put a teaspoon of sidewalk de-icing salt.
  • If it is summer, you will have to put an ice cube into each of two plastic containers and put the salt on one ice cube and sidewalk de-icing salt on the other. Then, put the containers into the freezer of your fridge or into the deepfreeze for about 5 minutes. Be careful not to spill the contents.

Ordinary table salt is called sodium chloride. Sidewalk de-icing salt is usually calcium chloride. You should notice that the sodium chloride does not do much, but the calcium chloride melts a hole into the ice quickly. The reason for this is that sodium chloride requires heat energy to dissolve in water while calcium chloride gives off heat energy when it dissolves in water. Therefore, the calcium chloride heats up the ice water as it is dissolving and melts a hole in the ice. The sodium chloride just makes the ice colder as it dissolves.

Calcium chloride is very expensive compared to sodium chloride. That is why the sand-salt mixture that is put on icy roads may have both kinds of salt in it. The more expensive salt will melt the ice into water and the less expensive salt dissolves into the melt water and lowers the freezing point so that ice does not form again. The sand is used in the mixture to provide instant traction on icy roads.


Now, do assignment 1C. Please page forward to the computer-scored assignment.











Answers to Questions:

Analysis 1. Graph the results. Use the x axis for the quantity of salt added and the y axis for the temperature.

Your graph should be similar to the following. Be sure to label the axis and give the graph a title.

Analysis 2. Predict the freezing point and boiling point of a salt-water mixture if 80 g of salt were added to 200 mL of water.

If you extend the graphed lines, you should predict the melting point to be about -7°C and the boiling point to be about 106°C.


Click to return to where you left off in this lesson.