Tag: chemistry

Changes are of two kinds: Physical and chemical changes.
A physical change in a substance does not change what the substance is.
Where as in a chemical change, where there is a chemical reaction, anew substance is formed and energy is either given off or absorbed.

Physical changes require less energy than chemical energies, as they does not involve in making and breaking of bonds as in the case of chemical bonds, which require a lot of energy.

Dissolving sugar in water is a physical change because sugar molecules are dispersed within the water, but the individual sugar molecules are unchanged.

Since the law of conservation of mass is valid under all circumstances, hence, mass always remains the same, whether a substance undergoes physical change or chemical change. 

A plant collecting sunlight and turning it into food is a photosynthesis reaction which is an example of a photochemical reaction. This is a chemical change. 

A change in temperature is characteristic of a chemical change. When the reactants are mixed, the temperature change caused by the reaction is an indicator of a chemical change. 

Solid ammonium chloride vapourizes to form gaseous ammonium chloride. This is a physical change. Ammonium chloride is partially dissociated. This a chemical change. The products ammonia and chlorine recombine to form ammonium chloride. This is also a chemical change.

On burning candle, wax melts which is a physical change. Also, the wax near the flame burns and gives $CO _2$, soot, light and heat which is a chemical change. Hence, burning of candle is a physical as well as chemical change.

Physical change does not involve formation of new products. As cutting of trees do not produce anything it is said to be a physical change.