Tag: introduction to physical and chemical changes

Melting of wax is a physical change. In a burning candle there are both physical and chemical changes. The burning of the solid wax to form liquid wax and the evaporation of liquid wax to form wax vapour are physical changes. The burning of wax vapour is a physical change.

All of the following represents a physical change.
Stretched rubber band.
Melting of ice cream.
In winter, hair oil freezes.

When a metal rod is heated, it expands. During expansion, the length of metal rod increases.

All of the following represents a chemical change.

No new substances are formed in all these three changes. So these are physical change. 

Zinc oxide is a white powder, when heated strongly (above $800^0 C$) it turns yellow, but again becomes white again when cold. The hot zinc oxide absorbs some of the blue wavelengths of light making the reflected light more yellow. The loss of oxygen makes it a reversible chemical change.

Iron is magnetic at room temperature, but at 770 degrees Celsius iron loses its magnetism. However this change is temporary, hence it is an example of physical change.

and the change in color is a physical change because it is easily reversible without affecting the chemical nature of it.

In physical change properties of molecules remain the same even after the change. Physical properties like colour, density, volume, mass, boiling point, melting point only have an effect on their physical nature, the chemical nature of the substance remains the same. Physical properties are used to observe and describe matter.