Physical and Chemical Changes in Chemistry

Consider the following experiments in order to distinguish between chemical and physical changes.

Experiment 1:

Procedure:      A piece of magnesium ribbon was held by tongs in a Bunsen flame.

Observations: It burnt with an intense brilliant white flame leaving white ashes.

Experiment 2:

Procedure:      Some powdered sulfur was placed in a test-tube. This was heated in a

           Bunsen flame, rotating continuously.

Observations: The sulfur melted to an amber coloured liquid which on cooling returned

to the original yellow solid.

Experiment 3:

Procedure:      A piece of sodium, the size of a small pea, was dropped on the surface of    

some distilled water in a trough.

Observations: The sodium ‘melted’ into a ball and darted over the surface of the water

producing a hissing sound and finally disappeared with a small flash and an explosion. The resultant clear liquid was heated in a crucible until no more steam comes out. A white solid was left after cooling. When distilled water was added, this white solid dissolved but not vigorously as in the case of sodium. A new substance had been formed – caustic soda.

Experiment 4:

Procedure:      Some sodium chloride crystals, were placed in a beaker half filled with

           distilled water. The contents were stirred. A little liquid was then taken

           from the beaker and placed in a watch glass. The watch glass was then

placed on a beaker half filled with tap water and heat the contents.

Observations: When the sodium chloride crystals were stirred, they dissolved. When the

            water in the watch glass evaporated, crystals of sodium chloride were left