A change in which one or more than one new substances are formed is known as a chemical change. 

Sheet erosion is the detachment of soil particles by raindrop impact and their removal downslope by water flowing overland as a sheet instead of in a definite channel.

All of the following changes are irreversible because we can not convert the product into reactant again.
1) Milk to paneer.
2) Cow dung to biogas.
3) Grain to its flour.

Burnt sparklers and burnt books pages are examples of irreversible changes, as they are permanent changes which cannot be undone. Once these materials are burnt in to ashes, the original materials cannot be recovered back.

Irreversible changes $-$ because when they are burnt formation of gas and ashes takes place i.e. chemical reaction is taking place and new substance are formed which can't be reversed back.

The following changes are irreversible.We cant covert product into reactant in these processes

The following represents a reversible change. A metal rod on heating expands (its length increases). A metal rod on cooling contracts (its length decreases).

When solid is placed in water, the density of solid decides if it will float or sink in water. 

Dissolving is an example of a reversible change. When salt is mixed with water it disappears because it dissolves in the water to make salty water. But we can get the salt can back again by boiling off the water. That leaves the salt behind.

The railway tracks are made up of metals and hence they have a property of expansion when heated and contraction when cooled.

Either reversible or irreversible change.

Gases have a greater tendency to expand because it has no or negligible intermolecular force of attraction between the particles. 

The brown colour of the apple after cutting is due to the chemical change of the enzymes.
When an apple is cut, oxygen is introduced into the injured plant tissue. When oxygen is present in the cells, polyphenol oxidase enzymes(PPO) in the chloroplasts rapidly oxidize phenolic compounds naturally present in the apple tissues to o-quinones, colourless precursors to brown coloured secondary products. o- quinones then produce the brown colour by reacting to form compounds with amino acids or proteins they self-assemble to make polymers.

When heat flows through the ice, the ice will melt. 

All of the following is/are different ways to bring a change.
Expansion, Contraction, Evaporation,Melting.

D, Temperature and close contact plays important roles in chemical reactions. These factors affects the activation energy of the reaction.

Both expansion and contraction is used during the change.
The iron blade has a ring in which the wooden handle is fixed. The ring is slightly smaller in size than the wooden handle.The ring is heated and it becomes slightly larger in size (expands). The handle  fits into the ring. When the ring cools it contracts and tightly fits on the handle.

The iron blade has a ring in which the wooden handle is fixed. The ring is slightly smaller in size than the wooden handle.
The ring is heated and it becomes slightly larger in size (expands). The handle  fits into the ring. When the ring cools it contracts and tightly fits on the handle.