write shortnote on anomalous behaviour of water
Answers
Answer:
Anomalous Expansion of Water
Explanation:
A common trend seen in substances through out, is the fact that all substances expand when they are heated and their density decreases and vice versa and when you cool something, it contracts and increases in density. This is how substances generally react to heat. Let’s now look at what water does.
If you cool water, it follows the general trend till 4o C. Densityof 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, ie 0oC, water expands with further drop in temperature meaning the density of water decreases when you cool it from 4oC to 0oC. This behaviour is charted in the graph below.
The effect of this expansion of water is that the coolest 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 coolest 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.
It happens due to:
A water molecule is made of one oxygen atom combined with two hydrogen atoms as shown below. At a normal temperature, the water molecules are held together in a liquid state because of the intermolecular 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 its density. At 4oC the density reaches its maximum and after this, the water molecules can squeeze no further.
So now, 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 isn’t quite as tight as the O-O bond, it experiences a little expansion once the H-O bond takes over. It’s 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.
P.S. : You don't need to write Why it happen if its a 3 marks question. If 5 marks are there then only you need to write that part
