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Note on Chemical Reaction

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Physical change

Physical change is a temporary or reversible change in which no new substances are formed. Examples:

  • Lightening of an electric bulb.
  • Magnetizing an iron nail.
  • Preparing a solution.
  • Changing of ice into the water and water vapour in heating.
  • Making of different objects from the soil.

Characteristics of physical changes

  • Physical changes are temporary a change.
  • They are reversible change.
  • In these changes, no new substances are formed.
  • In these changes, only physical properties are changed but chemical compositions are not changed.

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Chemical change

A chemical change is a permanent and usually irreversible change in which new substances with different properties are formed.

Characteristics of chemical change

  • Chemical changes are permanent changes.
  • They are usually irreversible changes.
  • In this change, new substances are formed.
  • Rusting of iron.
  • Burning magnesium ribbon in air to form magnesium oxide.

Differences between physical and chemical changes

Physical change Chemical change
No new substances are formed. New substances are formed.
It is a temporary change in which chemical composition of the substance remains the same. It is a permanent change in which chemical composition of the substance changed.
Change in energy is not usually seen in it. Change in energy is visible.
Change in mass is not seen. E.g., dissolving the salt in water Change in mass is seen in it, e.g., rusting of iron.
Only the physical properties like colour, taste or state are changed. Chemical, as well as physical properties, are changed.
It is usually reversible. It is usually irreversible.

Chemical reaction

The process by which a chemical change takes place is called as a chemical reaction. Those substances that take part in a chemical reaction are called reactants. Those substances which are formed by a chemical reaction are called products.

Word equation

The chemical reaction which is expressed in terms of full names of reactant and product molecules is called word equation. For example, Hydrogen reacts with oxygen to produce water.

Chemical Reaction Hydrogen + oxygen→Water
Chemical Reaction Hydrogen + oxygen→Water

Hydrogen + oxygen→Water

(Reactants) (Products)

Chemical equations

The chemical reaction which is expressed by using themolecular formula of reactants and products is called chemical equation. The chemical equation is also called formula equation.

2 H2 + O2→2 H2O

  • Physical change is a temporary or reversible change in which no new substances are formed.
  • A chemical change is a permanent and usually irreversible change in which new substances with different properties are formed.
  • Chemical changes are permanent changes.
  • Physical changes are temporary a change.
  • The chemical reaction which is expressed by using molecular formulae of reactants and products is called chemical equation
  • The chemical reaction which is expressed in terms of full names of reactant and product molecules is called word equation. 
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Very Short Questions

A physical change is a temporary change in which only the physical state or appearance of the substance changes and no new substances are formed.
For example: Melting of ice, evaporation of water etc.

A chemical change is a permanent change in which new substances with different properties are formed.
For example: Electrolysis of water, burning of kerosene etc.

The differences between physical and chemical change are as follows:

Physical Change Chemical Change
In a physical change only physical properties are changed. In a chemical change, the chemical composition and chemical properties are changed.
No new substances are formed New substances are formed.
It is generally a temporary change. It is a permanent change.

The main factors that bring out a chemical change are:

  • Heat, electricity and light
  • Simple contact
  • Contact by solution
  • Pressure
  • Catalyst

Heat supplied gives energy to the molecules of the reactants and makes them collide to each other to bring about chemical reaction. Examples are:
CaCO3 Heat → CaO + CO2
2KClO3 Heat → 2KCl +3O2

Electricity brings both synthesis and decomposition reaction. For example:
2H2O Electrolysis→ 2H2 + O2
2H2 + O2 Electrolysis→ 2H2O

Some of the chemical reaction takes place in the present of light. For example:
2AgBr Sunlight→ 2Ag + Br2
CH4 + Cl2 Uv-rays → CH3Cl + HCl

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  • Which is a chemical change?

    breaking glass
    Boiling tea in water
    making water sweet by adding sugar
    Color change in a peeled banana
  • Which statement is false about physical change?

    In physical change properties like color, odor, taste, physical states are changed
    NaOH solution and HCl solution react to give NaCl solution. It is possible to get back the starting compounds by heating the salt solution to 1000oC
    There is new substance formed during chemical change
    It is impossible to recover tea once it's color goes into boiling water
  • Which statement is false about chemical change?

    Chemical changes are permanent changes
    Chemical change produces new substances
    Chemical change is irreversible
    Adding salt to snow makes snow melt into water faster. This is an example of chemical change
  • Which one is not true?

    Chemical changes are permanent changes
    new substances are formed after chemical chaness
    Chemical changes are usually irreversible changes.
    Chemical changes are reversible change
  • Lightening of electric bulb is the example of___

    Electric changes
    Physical change
    all of the above
    Chemical changes
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Calcium

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Chemical

Name a chemical found in had water.


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Calcium

If 20 gm of salt is dissolved in 25 gm of water at 60°c .calculate rhe solubility of salt.


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Calcium

Draw the molecular weight of magnesium phospha


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