Irreversible Changes

Chemical Changes

Irreversible Changes

What are Chemical Changes ?The change is said to be chemical if new substances with different proprieties are formed.

Chemical changes are permanent and cant be reversed to give back the substances in their previous forms.

Burning of paper is an example of chemical change because after burning the ash left and it cant be reversed.

ContactA chemical reaction between two substances takes place when they are in close contact with each other.

For example a sodium comes in contact with water, a reaction takes place to give hydrogen and sodium dioxide.

How ever sodium and water does not react with each other if they are not in contact.

SolutionIn some cases chemical reactions occur only when solutions of the reactants mixed together.

For example Baking soda powder and tartaric acid crystals do not react when they are mixed.

But, a vigorous reaction occurs when tartaric acid crystals are added to a solution of baking soda.

heatHeating is necessary for some chemical reactions.

For example iron and sulphur, when mixed, does not react unless heated.

The study of chemical changes occurring due to heat is called thermo chemistry.

LIGHTA number of chemical reactions takes place in presence of light.

Photosynthesis is a chemical reactions and it cannot takes place at night.

A photographic plate is affected, when exposed to light. The study of such kind of reactions is called photochemistry.

electricityThe passage of electric current can bring about a chemical change, which may not occur otherwise.

Water decomposes into hydrogen and oxygen when electric current pass through it ( Figure 6.5 ).

Chemical changes due to electricity are studied under a separate branch called electrochemistry.

PressureSometimes a change in pressure brings about a chemical change.

For example, a mixture of potassium chlorate and sulphur explode when struck with a hammer.

catalystSubstances which support chemical reactions without undergoing any change themselves are called Catalyst's.

Manganese dioxide increases the rate of decomposition of potassium chlorate when it is heated strongly to get oxygen.

When some Manganese dioxide is mixed with potassium chlorate, oxygen is given off rapidly, without strong heating.

The Catalyst, Manganese dioxide , is recovered unchanged as it does not take part in the reaction.