Rancidity is a condition in which the food items develop bad odour due to chemical reactions by microbes.

Chemical reactions carried out by heat are called thermolysis.

Chemical reactions carried out by electricity are called electrolysis

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.

Shiny metal surfaces and other articles lose their shining appearance due to the chemical reaction on the surface. This is called tarnishing.

Any unwanted change in physical, chemical and biological properties of the environment is termed as pollution.

When an iron nail is placed in copper sulphate solution, the blue colour of the copper sulphate solution slowly changes into green due to the formation of ferrous sulphate as a result of chemical reaction between iron and copper sulphate solution.

Chemical substance used to alter the speed of the reaction is called a catalyst and the process is called catalysis.

i) A chemical change is a permanent, irreversible change and produces a new substance.
ii) chemical changes are otherwise called as chemical reactions.