This section deals with Physical and chemical changes Class 7 Science Notes. At the end, you will also get to solve some important MCQs on the topic.
CHEMICAL CHANGES AND PHYSICAL CHANGES
What is a physical change?
A physical change is a change that is accompanied by a change in the appearance, smell, or simple display of a sample of matter without a change in its composition.
Physical change is a temporary change. Examples of physical change are freezing of water, melting of wax, boiling of water, etc. The composition remains the same despite its molecules being rearranged.
In a physical change, there is a change only in texture, color, shape, chnage of state and temperature of a given substance.
Some of the processes that cause physical changes include cutting, bending, dissolving, freezing, boiling, melting, liquefaction, smoke and vaporization etc.
Examples of physical changes:
Changing ice into water, Vaporizing liquid nitrogen, Mixing water and oil, bending an iron rod etc.
What is a chemical change?
A chemical change is a change that is accompanied by a change in the composition of the substances in question.
A chemical change occurs when the substance’s composition is changed. When bonds are broken and new ones are formed a chemical change occurs.
A chemical change is a permanent change.
Chemical changes involve the production of energy, and changing both physical and chemical properties of the substance including its composition.
The indicators of chemical change are Change in temperature, change in color, change in odour, formation of bubbles and formation of a precipitate.
Examples of chemical changes:
Rusting of iron: When iron exposed in to air, iron surface turns brown due to the formation of hydrated ferric oxide (Fe2O3.xH2O) which is also called rust.
Blckening of Silver: Silver surface turns black due to the formation of silver sulphide (Ag2S)
Ripening of fruits, changing milk into curd are also the examples of chemical changes.
Classification of chemical changes
The chemical changes/chemical reactions can be classified into following types:
Combination chemical reaction:
A combination reaction is a reaction in which two reactants combine together to form a totally new product.
Decomposition chemical reaction:
The chemical reaction in which one reactant breaks down into two or more products, is known as decomposition reaction.
It can be represented as AB → A + B
Chemical decomposition reaction is considered as opposite to the chemical synthesis.
The stability of a chemical compound is eventually limited when exposed to extreme environmental conditions such as heat, radiation, humidity, or the acidity of a solvent.
The examples of the decomposition reaction are the electrolysis of water, decomposition of Carbonic acid and breakdown of Hydrogen Peroxide.
During electrolysis reaction of water, water breaks down to form Hydrogen gas and Oxygen gas.It is shown below:
2 H2O(l) → 2 H2(g) + O2(g)
Single displacement chemical reactions
The chemical reaction in which one element is substituted for another element in a compound. In such a reaction, a new element and a new compound come out as products.
It can be represented as AB + C → CB + A
An example of Single displacement chemical reaction is the reaction between Silver nitrate and Zinc. When Zinc reacts with Silver nitrate, it displaces silver from the compound and the products are Zinc nitrate and Silver, as shown below:
AgNO3 + Zn → ZnNO3 + Ag
In such reactions, a more reactive element displaces a less reactive element from its compound.
Double displacement chemical reaction
A double displacement is a reaction in which the positive and negative ions of two ionic compounds exchange places to form two new compounds. It can be represented as:
AB + CD → AD + BC
An example of double displacement reaction is the reaction between Silver Nitrate and Sodium Chloride, as shown below:
AgNO3 + NaCl → AgCl + NaNO3
A double displacement reaction is also known as a salt metathesis reaction.
Precipitation chemical reaction
The reaction which is accompanied by the formation of an insoluble salt when two solutions containing soluble salts are combined, is known as a precipitation reaction.
An example of precipitation reaction is the reaction between Silver nitrate and Potassium Chloride, in which Silver chloride is formed as a precipitate. The reaction is shown below:
AgNO3 (aqueous) + KCl(aqueous) → AgCl(precipitate) + KNO3 (aqueous)
Redox chemical reaction
Redox means reduction and oxidation, and hence the redox reactions are oxidation-reduction reactions where reactants undergo changes in oxidation states.
An example of redox reaction is the reaction between Hydrogen and Fluorine. In such a reaction, hydrogen gets oxidised and fluorine gets reduced. This reaction is shown below:
H2 + F2 → 2HF
So, these were Physical and chemical changes Class 7 Science Notes. Share this article with your friends.