ICSE Revision Notes for Chemical Changes and Reactions Class 9 Chemistry
Chapter Name | Chemical Changes and Reactions |
Topics Covered |
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Related Study |
Chemical Changes and Reactions
- A chemical reaction is the process of breaking the chemical bonds of the reacting substances (reactants) and making new bonds to form new substances (products).
- A chemical bond is the force which holds the atoms of a molecule together, as in a compound.
Conditions Necessary for a Chemical Change
The following conditions are necessary for a Chemical Change.
| Close Physical Contact (Mixing) | A chemical reaction occurs when two substances are mixed in their solid state. Iodine and sulphur react explosively when brought into close contact. |
| Solution | A chemical reaction occurs when two substances are mixed in the solution form Sodium carbonate and tartaric acid vigorously react only in the solution state. |
| Heat |
Some chemical reactions occur only on heating. |
| Light |
Reactions which occur by the action of light are
called photochemical reactions or photolysis. Molecules of the reactants
absorb light energy, get activated and then react rapidly.
Photosynthesis: |
| Electricity |
Chemical reactions such as decomposition of
compounds occur only when electricity is passed through the substance. |
| Pressure |
Some reactions occur only when substances are
subjected to high pressure. |
| Catalyst |
Some chemical reactions need a catalyst to accelerate
or decelerate their rates of reaction. Catalyst themselves do not take part in
the reaction. A catalyst such as Pt or MnO2 initiates a change in
the rate of the reaction without undergoing any change in its chemical
composition. |
| Sound |
Some chemical reactions proceed only by absorption of
sound energy. Sound energy speeds up the reacting molecules, atoms or ions
causing a reaction to occur. |
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1. Evolution of gas |
In a chemical reaction, a gas may be one of the products. Zn + H2SO4 → ZnSO4 + H2 ↑ |
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2. Change of colour |
Some chemical reactions are characterised by a change in the colour of the reactants. Fe + CuSO4 → FeSO4 + Cu |
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3. Formation of precipitate |
Some chemical reactions are characterised by the formation of a precipitate. The precipitate is an insoluble solid substance. AgNO3 + NaCl → AgCl + NaNO3 |
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4. Change of state |
In some reactions, a change of a state is observed. The reaction starts with solid or liquid reactants and ends up with gaseous products and vide versa. |
Characteristics of a Chemical reaction
Energy Changes in Chemical Reactions
- Each substance has a fixed amount of stored energy, which is in the form of potential energy. This energy is called its chemical energy.
- The formation of gas bubbles in a liquid during a reaction is called effervescence.
Exothermic change
A chemical change which takes place with the release of heat energy is called an exothermic change.
C + O2 → CO2 + Heat
Endothermic change
A chemical change which takes place with the absorption of heat energy is called an endothermic change.
C + 2S + Heat → CS2
Photochemical reaction
A chemical reaction which proceeds with the absorption of light energy.
Electrochemical reaction
A chemical reaction which proceeds with the absorption of electric energy.
