Give a reason why -

(a) Freezing of water is a reversible change while burning of a candle an irreversible change.

(b) Change of seasons is a periodic change while change of weather is a non-periodic change.

State which of the following pertain to - chemical changes.

(a) Substance retains its identity.

(b) The change is permanent.

(c) No new substance is formed.

(d) Is easily reversible.

(e) Produces one or more new substances by change in its composition.

(f) The change is temporary.

(g) The composition and properties of the original substance are not altered.

Give four reasons why burning of a magnesium ribbon in air is considered a chemical change.

Compare the energy changes involved during a physical and a chemical change.