Question

though you add heat, the temperature of boiling water remains constant because...
it takes energy to break hydrogen bonds.
it takes energy to break covalent bonds.
water has a constant boiling temperature.
it takes energy to circulate water.
none of the above. the temperature rises during boiling.
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part i
which statement helps to explain why ice is less dense than liquid water?
(a) water molecules make hydrogen bonds at definite angles.
(b) cold molecules move less than warm molecules.
(c) hydrogen bonds lengthen in the cold.

Explanation:

1. For the first question, when boiling water, the added heat energy is used to break the hydrogen - bonds between water molecules rather than increasing the temperature until all water is vaporized. Covalent bonds are not broken during boiling. Water has a constant boiling temperature because of the energy consumption in breaking hydrogen - bonds, not the other way around. Water circulation energy is not related to the constant boiling temperature. And the temperature of boiling water at a constant pressure remains constant.
2. For the second question, water molecules in ice form hydrogen - bonds at definite angles, creating an open - lattice structure which makes ice less dense than liquid water. The movement of cold and warm molecules and the change in hydrogen - bond length in the cold are not the main reasons for ice's lower density compared to liquid water.

Answer:

1. It takes energy to break hydrogen bonds.
2. (a) Water molecules make hydrogen bonds at definite angles.