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Optional problems. Can you devise an experiment to show whether the practice of some pupils of putting pencils in their mouths is wise? Why is the air one of the leading means of distribution of bacteria ? What are some of the leading methods used in sterilizing fruit for canning and for preventing growth of bacteria in the canned fruit ?

CHANGES IN VOLUME WHEN WATER FREEZES (X-1)

The problem. We have seen that air, water, and mercury expand when heated and contract when cooled. This is true of most substances. Thus, if melted lard, tallow, wax, or paraffin is poured into a vessel and allowed to cool, it will be found that when it has solidified there is a depression of the surface and possibly there are cracks in the center, indicating that the material has contracted noticeably even after it began to harden. On the other hand, it is commonly noted that water when freezing may break the containing vessel, or the surface of the mass of ice inay be bulged up, thus apparently showing expansion. What are the facts regarding the behavior of water when it freezes ?

Fig. 20 What to use. Thermometer, glass tube, two-hole rubber stopper, beaker or drinking glass, ice, salt, sheet of centimeter cross-section paper, and a small metal can, glass vial, or test tube. A small baking-powder or paprika can is satisfactory.

What to do. 1. Assemble the apparatus as shown in figure 20. Fill the metal or glass vessel with water which has a temperature of about 40° C., and close it with a two-hole rubber stopper containing a thermometer and a glass tube. In handling thermometers in stoppers care is needed to avoid breaking the thermometer. Thermometers and glass tubes should be grasped near the stopper, turned slightly, and thrust into the stopper. Push the stopper into the can or vial until the glass tube is about one fourth full of water. Fasten a paper centimeter scale back of the tube.

2. Set the metal vessel in a beaker or cup and surround it with a freezing mixture made by adding ice or snow to a strong salt solution.

3. Mark the water level on the scale. Call all readings below this minus (-) and all above it plus (+). Record the readings every minute for from thirty to sixty minutes as follows:

TIME

TEMPERATURE

CHANGE IN VOLUME

Questions. Will the experiment be a fair test if there is air between the stopper and water level? What temperature is produced by the freezing mixture ? Describe the changes in volume. At what temperature does the water have the least volume ? greatest volume ? At what temperature is a cubic inch of water the heaviest ? Why is the freezing temperature of water used in making thermometer scales ?

Suggestions for report. Make a graph showing the change in volume with the change in time. On the same page show the change in temperature with the change in time. On the blank portion of the cross-section paper draw a cross-section diagram of the apparatus used.

Reference work. Read sections 131, 134, 144, and 145. Explain the following: Why does ice float ? Why do lakes and ponds not freeze solid? Why do water pipes burst in cold weather ? Optional problems. Try the above experiment with other liquids.

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