what is meant by anomalous expansion of water ?
Water shows unusual expansion when heated from 0 oC to 4 oC. Instead of expanding with rise in temperature, water contracts when its temperature increases from 0 oC to 4 oC.
Here is an example:
Take a cube of ice at -3 oC and heat it. The cube will expand till ice starts melting. After complete melting of ice, it contracts till the temperature reach 4 oC and then starts expanding. Thus, water has its maximum density and minimum volume at 4 oC.
This phenomenon is called anomalous expansion of water.