Since water expands on freezing, pressure would on compressing ice at 0°C., tend to turn it into water. Pressure does lower the melting point of ice, so that a little ice may melt when it is subjected to pressure. On removing the pressure the water freezes. This may be shown by placing a loop of fine piano wire (see Fig. 152) over a piece of ice supported so that a weight may be hung upon the wire. The wire will be found to gradually cut through the ice, the melted ice refreezing above the wire. Important Topics 1. Specific heat. 2. Heat of fusion of ice. 3. Crystalline substances have fixed melting points. 4. Expansion on freezing, importance. Exercises 1. What are two advantages in the high heat of fusion of ice? 2. What are two advantages in the expansion of water while freezing? 3. How much heat will be required to melt 1000 g. of ice and warm the water to 20°C.? 4. How many grams of ice at 0°C. can be melted by 400 g. of water at 55°C.? 5. What are two advantages of the high specific heat of water? Two disadvantages? 6. If the specific heat of iron is 0.1125, how much ice at 0°C. can be melted by a 200-g. ball of iron heated to 300°C? 7. What is the temperature of a hot ball of iron weighing 80 g., if when placed on a piece of ice at 0°C. it melts 90 g. of ice? 8. If 500 g. of copper at 400°C. are placed into 3000 g. of water at 10°C. what will be the resulting temperature? 9. What weight of water at 90°C. will just melt 10 kg. of ice at 0°C.? 10. If the smooth dry surface of two pieces of ice are pressed together for a short time the two pieces will be frozen into one piece. Explain. 11. Tubs of hot water are sometimes placed in vegetable cellars to prevent the vegetables from freezing. Explain. 12. How many B.t.u. are given out when 2 lbs. of water freeze? (2) HEAT AND CHANGE OF STATE [Illustration: FIG. 153.--The black cube in the upper corner represents one cubic inch of water. The entire cube represents the space occupied by the cubic inch of water in the form of steam. The reduced spaces at the bottom and sides show how much short the cube is of being one cubic foot. (American Radiator Co.)]