Explain the extraction and constraction of water at 4°c.
Answers
Answer:
A common observation seen in the behaviour of the substances is that they expand when heated as the density decreases and vice versa takes place when the material is cooled. This is how substances generally react to heat. Let us now look at how water behaves when heated.
The general tendency of cold water remains unchanged until 4o C. The density of water gradually increases as you cool it. When you reach 4oC, it’s density reaches a maximum. What water does next will astound you. When you cool it further to make some ice, i.e. 0oC, water expands with a further drop in temperature, meaning the density of water decreases when you cool it from 4oC to 0oC. The below graph explains this behaviour.
The effect of this expansion of water is that the coldest water is always present on the surface. Since water at 4oC is the heaviest, this water settles on the bottom of the water body and the lightest ie the coldest layer accumulates on the top layer. So in the winter, the top of the water is always the first to freeze over. Since ice and water both are a bad conductor of heat, this top layer of ice insulates the rest of the water body from the cold of the winter, thereby protecting all the life in the water body. Now you can truly comprehend how essential the anomalous properties of water are for life.

Why does it happen?
A water molecule is made of one oxygen atom combined with two hydrogen atoms as shown below. At normal temperature, the water molecules are held together in a liquid state because of the inter-molecular attraction of the water molecules. In a liquid state, the water molecules are constantly whizzing around in the container and are constantly being rearranged.
Keep in mind that Hydrogen too contributes to the inter-molecular attraction. The attraction between the hydrogen atoms of one water molecules and the oxygen atom of another water molecule is also present. The distance between the H-O bond is more than the O-O attraction in the water. On cooling water, the rate at which the molecules are whizzing around decreases as they lose energy. On further cooling, the water molecules start squeezing together, increasing their density. At 4oC the density reaches its maximum and after this, the water molecules can squeeze no further.
So, the water freezing over into ice is held together not by the O-O attraction but by the H-O attraction. The lattice arrangement of ice prevents the movement of water molecules. But since the H-O is not quite as tight as the O-O bond, it experiences a little expansion once the H-O bond takes over. It is like people packed in a busy subway, more of them can fit into the subway if they tuck their hands inside their pockets than if they all hold hands and stand. This arrangement although is stronger, but it also occupies more space. Water experiences this same effect.