Chemical Reaction

Take a look at the different processes taking place around you:

• Plants preparing food by the process of photosynthesis
• Iron structures turning reddish brown with time due to corrosion (rusting)
• Dirt in clothes getting removed on washing with soap and water
• Cake batter rising and getting cooked on baking
• Burning of wood at a campfire

All of these processes that we observe in our day to day lives are a result of Chemical Reactions.

What are Chemical Reactions?

It is a process by which new substances with new properties are formed .

In a chemical reaction one or more substances combine to form new substances.

Example : 

Hydrogen reacts with Oxygen to form Water

Hydrogen + Oxygen     →        Water

        (Reactants)                   (Product)

Note : When Hydrogen and Oxygen react, they form a new substance Water, hence it is a chemical reaction

Is chemical reaction a result of Physical change or chemical change?

Chemical Reaction results in the formation of new substances. 

Thus we can say, it occurs because of chemical changes

Explanation

Based on the above explanation of physical and chemical changes, we can confirm that chemical reactions occur as a result of chemical changes , as;

• New products are formed as a result of chemical reactions and
• The reaction is usually not reversible under the same conditions

Note : If the products can react to produce the reactants, then the reaction will be considered reversible.

Do elements change because of chemical reaction ?

The simple answer is, No

Atom of one element does not change into another element

They only rearrange to form new substances

Example

Magnesium + Oxygen → Magnesium Oxide

During this reaction, Magnesium Oxide is formed which contains only Magnesium and Oxygen element (and not any new element )

Note- Magnesium ribbon burns with a dazzling white flame and changes into white powder. The white powder formed is Magnesium Oxide.

What are reactants and products?

The substances which undergo chemical change in a chemical reaction are called reactants .

The substances produced as a result of chemical reaction are called products

Example

Magnesium + Oxygen → Magnesium Oxide

In this case Magnesium and Oxygen are called Reactants

Magnesium Oxide is a Product

Magnesium + Oxygen → Magnesium Oxide

           (Reactants)                 (Product)

How to find whether chemical change has taken place?

If a chemical change takes place, it usually leads to following:

• Change in state
• Change in color
• Evolution of Gas (Gas being formed)
• Change in Temperature
• Formation of precipitate

Example- Change in color of substances and evolution of gas in a chemical reaction

When we heat green coloured Ferrous Sulphate crystals in a boiling tube,

We observe that the green colour of Ferrous Sulphate changes .

We can also observe smell on burning of sulphur

This is because  on heating,

Ferrous Sulphate breaks down into Ferric Oxide (solid) , Sulphur dioxide (gas) and Sulphur Trioxide(gas)

2FeSO ₄ → Fe2O ₃ ​+ SO ₂ ​+ SO ₃

                                                 (Reactant)           (Products)

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Example- Change in Temperature

Calcium Oxide (Quick Lime) reacts with Water to produce Calcium Hydroxide (Slaked Lime) and Heat

CaO + H ₂ O → Ca(OH) ₂

                                                         (Reactants)      (Product)

Example- Formation of precipitate

When Sodium Sulphate solution is mixed with Barium Chloride solution

It forms Barium Sulphate precipitate and Sodium Chloride

(Insoluble solid is called precipitate)

Note : This Barium Sulphate is formed because of the reaction of positively charged ions of Barium and negatively charged ions of sulphur

Intext Question - Page 6 Q1, 

NCERT Exercise -