Question

Which of the following statements are true:
A) Rubber contracts on heating
B) Water expands on freezing
C) Water contracts on heating from ${0^ \circ }C$ to ${4^ \circ }C$
D) Water expands on heating from ${4^ \circ }C$ to ${40^ \circ }C$

Answer 1

Entropy increases as the randomness increases and heating leads to decrease in intermolecular distance. Entropy of a system depends on the degree of randomness of the molecules in the system.

Complete answer:
As we know rubber bands are made from loosely packed chains of atoms and entropy is a way of measuring the amount of disorder in a system. If the long chain of atoms are lined up in a way, we say that entropy is low and if tangled up in a mess, then, entropy is high.
When we stretch a rubber band, the messy tangles line up a bit which decreases entropy and heat goes out of the system and when entropy increases, the system takes in heat from around it.
When we heat this rubber band, the band contracts itself which leads to decrease in intermolecular distance between the atoms and pulls itself together which increases the entropy of the system. The chains become mobile(moving) and slide past each other which decreases the intermolecular distance between atoms. Hence: rubber contracts on heating.
Rubber is a polymer chain which breaks on heating leading to decrease in intermolecular distance. Hence, it contracts.
Water at ordinary temperatures contracts and increases in density as it is cooled. But at about ${4^\circ}C$, it reaches a maximum density and then decreases in density as it approaches the freezing point. The water further expands on freezing which causes the ice to float on water.
Water molecules on freezing tend to form a hexagonal structure. Due to the hydrogen bonding, each hydrogen atom tends to line up with two oxygen atoms one on each side of the hydrogen atom. The tendency of the hydrogen bonding increases with the decrease in temperature. This alignment of the hydrogen atoms due to hydrogen bonding explains the expansion while freezing.
At ${4^\circ}C$, the density of water is maximum. When it is cooled below it, the water molecules start forming hydrogen bonds with each other. After a certain temperature, water forms a hexametric or cage like structure. Upon freezing, this causes the density to decrease as now water exists as a cage like structure.
Since the density decreases, volume increases, so, water expands on freezing.
Now when water is heated from ${0^ \circ }C$ to ${4^ \circ }C$ its volume decreases and density increases up to ${4^\circ}C$. Because at this range, some of H-bonds are broken because of heat, as a result, water molecules come closer to one another.
As density is mass/volume, the mass remains constant but volume is decreasing which results in increase of density so the water contracts from 0 to 4.
Now we heat the water from ${4^ \circ }C$ to ${40^ \circ }C$, the density is maximum at ${4^\circ}C$. At ${4^\circ}C$, all the bonds are broken, the clusters start forming. The molecules are coming closer due to the decrease in their speeds and their motion is coming to an extinction. However the formation of clusters results in the molecules staying away from each other. On further heating, the volume of water increases normally and density will be decreasing.
So, the water expands on heating from ${4^ \circ }C$ to ${40^ \circ }C$.

Note: Hydrogen bond is the weak bond developed between N/O/F and H atoms because of their charge difference. The entropy of the system decreases when water freezes. This is due to the restraining of the motion of the water molecules. But for rubber the entropy increases on expansion.