Question

which of the following are physical changes that may have occurred in this image?
the fallen leaves started to decompose.
water reacted with calcium carbonate to form calcium oxide to make the concrete on the bridge.
the leaves changed color from green to red, orange, and yellow.
none of these are physical changes.

Explanation:

A physical change is a change in which no new substances are formed. The leaves changing color from green to red, orange, and yellow is a physical change. It is due to the breakdown of chlorophyll (which gives leaves their green color) and the revealing of other pigments like carotenoids and anthocyanins. Decomposition of leaves is a chemical change (involves the breakdown of complex organic matter into simpler substances by microorganisms). The reaction of water with calcium oxide to form calcium hydroxide (and then further reaction to form calcium carbonate in concrete) is a chemical change ($CaO + H_2O=Ca(OH)_2$ and $Ca(OH)_2+CO_2 = CaCO_3+H_2O$).

Answer:

The leaves changed color from green to red, orange, and yellow.