Question

if an ice cube melts at 0 degrees Celsius but water also freezes at 0 degrees Celsius, what is the difference between melting and freezing in the terms of (a) the energy involved in the phase change and (b) particles of substances during the phase changes?

Answer 1

A) if energy is being absorbed, then more hydrogen bonds are being broken (melting). If energy is being released, more hydrogen bonds become fixed (freezing). 

b) during melting, hydrogen bonds get broken and movement can occur. During freezing, hydrogen bonds become fixed and water molecules cannot move as much.


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Answer 2

(a) During melting, energy is absorbed by the reaction, while during freezing, energy is released.

Explanation:

Melting occurs when a solid substance (e.g. ice) turns into liquid state (liquid water). In this process, energy is absorbed by the substance, and this energy is used to break the bonds between the molecules of ice. On the contrary, freezing occurs when the liquid substance (water) turns into solid state (ice). In this case, molecules at the beginning have more energy (they move faster), so energy is released and the molecules become slower (in fact, they do not move freely when they are in solid state).

(b) After melting, particles spread out and move faster, while after freezing, they become more stationary and closer to each other

Explanation:

In a solid, particles are bond together and cannot move freely, but only vibrate around their position. In a liquid,  particles are more free to move since they are not bond, so they are generally farther from each other.

Therefore, after melting (from solid to liquid), particles become more spread out (since in the liquid state they are more free to move), while after freezing (from liquid to solid), the particles come closer together since they are bond to each other and not free to move.