The Presentation provides knowledge about Chemical Changes and its requirements.

2. What are Chemical Changes ?
 The change is said to be chemical if new substances with different proprieties are
formed.
 Chemical changes are permanent and can’t 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 can’t be reversed.

3. Contact
 A 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

4. Solution
 In some cases chemical reactions occur only when solutions of
the reactants mixedtogether.
 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.

5.  Heating 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

6. LIGHT
 A 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.

7. electricity
 The 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.

8. Pressure
Sometimes a change in pressure
brings about a chemical change.
 For example, a mixture of
potassium chlorate and sulphur
explode when struck with a
hammer.

9. catalyst
 Substances which support chemical reactions without undergoing any change
themselves are calledCatalyst's.
 Manganese dioxide increases the rate of decomposition of potassium chlorate when
it is heatedstrongly 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.