Question

b. choose the word or phrase that best completes each sentence. write its letter on the line. 1. if a substance has neither definite volume nor shape, it is a (a) solid (b) liquid (c) gas. 2. the temperature at which there is a phase change from liquid to solid is the (a) condensation point (b) boiling point (c) freezing point. 3. a phase change from liquid to gas that occurs at any temperature is (a) evaporation (b) boiling (c) sublimation. 4. the temperature at which there is a phase change from liquid to gas inside a liquid is the (a) melting point (b) freezing point (c) boiling point. 5. when a substance changes phase before melting, it (a) absorbs heat energy (b) gives off heat energy (c) produces heat energy. challenge a. find and list the boiling points and freezing points of 5 different substances. use references to help you. b. is there any difference between the freezing point and the melting point of a substance? explain. note for page 129 the term “substance” as used here has the same meaning as element or compound - a material with a definite chemical makeup. these concepts, however, are not developed until units 9 and 10. students, at this stage, should understand that most materials in everyday use are not single substances, but are mixtures. they cannot be identified by such properties as density and melting point. another point to bear in mind is that only crystalline substances have definite melting points. most organic substances, such as plastics and butter, may gradually soften, rather than melt at a particular temperature. many materials also burn, char, or decompose before melting.

Explanation:

1. Gases have neither definite volume nor shape.
2. The temperature of phase - change from liquid to solid is the freezing point.
3. Evaporation is the phase change from liquid to gas at any temperature.
4. The boiling point is the temperature of phase - change from liquid to gas inside a liquid.
5. When a substance changes phase from solid to liquid (melting), it absorbs heat energy. For the challenge part A, one can refer to chemistry textbooks or online resources to find boiling and freezing points of substances like water, ethanol, mercury, etc. For part B, the freezing point and melting point of a pure substance are the same temperature; it's just the direction of the phase - change (solid to liquid for melting, liquid to solid for freezing).

Answer:

1. (c) gas
2. (c) freezing point
3. (a) evaporation
4. (c) boiling point
5. (a) absorbs heat energy

Challenge A: Answers will vary based on the substances chosen and references used.
Challenge B: For a pure substance, there is no difference in temperature between the freezing point and the melting point.