Photosynthesis is an irreversible change. In photosynthesis carbondioxide and water react in the presence of sunlight to produce a new substance called starch. Hence photosynthesis is a chemical reaction

Chemical substance used to alter the speed of a reaction is called catalyst, It does not undergo any change during that reaction.

It is an endothermic reaction. when calcium carbonate is heated above 900^oc it undergoes thermolysis and produces calcium oxide and oxygen. Heat is absorbed in this reaction, so it is an endothermic reaction.

(i) Spoilage of food and vegetables:
Food spoilage may be defined as any change that causes food unfit for human consumption
The chemical reactions catalyzed by the enzymes result in the degradation of food quality such as development of bad tastes and odour, detorioration and loss of nutrients.
Eg: Rotten eggs develop a bad smell due to the formation of hydrogen sulphide gas.
Decaying of vegetables and fruits due to microbes.
(ii) Rancidity of fishes and meat:
Fishes and meat contains high levels of polyunsaturated fatty acids.
These fatty acids undergo oxidation when exposed to air or light.
It results in bad odour. This process is called rancidity.
(iii) Apples and fruits turn brown when cut:
It is due to chemical reaction with oxygen in air.
The cells of apples contain an enzyme called tyrosinase that when in contact with oxygen catalyses a biochemical reaction of plant's phenolic compounds to brown pigments known as melanins.

No, all the fruits and vegetables do not undergo browning. The cells of certain fruits like apples contain an enzyme called tyrosinase when in contact with oxygen catalyses a biochemical reaction of plant's phenolic compounds to brown pigments. So, the fruits and vegetables which contain tyrosinase undergo browning.

No, rusting is not good for iron materials. Rust is formed when an iron surface comes into contact with water and oxygen. It results in the degradation of iron. Thus the metal will be weakened and become brittle and flaky.

Acid rain is caused by the chemical reaction when compounds like sulphur dioxide and nitrogen oxides are released into the air. The exhaust from cars and truck and the smoke released from industries emit enormous nitrogen oxides and sulphur dioxide into atmosphere. It forms the principal cause for acid rain.