Chhattisgarh State Board Class 8 Science Chapter 4 Chemical Reactions When and What Type Exercise Questions for Practice here.
1.) Choose the correct answer
1.) NaOH + HCL —- NaCl + H2O this is a
a) Decompostition reaction
b) Combination reaction
c) Neutralization reaction
d) Precipitation reaction
Ans- c) neutralization reaction
2.) On ignition when a substance combines with oxygen the process
a) Reduction
b) Oxidation
c) Decomposition
d) Displacement
Ans-
3.) Which of these is not a characteristic of chemical reaction?
a) Production of heat
b) Change in clour
c) Liberation of gas
d) Formation of no new substance
Ans- d) formation of no new substance
4.) 2KClO3—— 2KCl + 3O2 is a reaction
a) Combination
b) Decomposition
c) Displacement
d) Neutralization
Ans-b) decomposition
2.) Write the type of chemical reaction
1.) NH3 + HCL—- NH4Cl
Ans- Combination reaction
2.) NaOH + HCl—– NaCl + H2O
Ans- Neutralization reaction
3.) 2NH3—– N2 + 3H2
Ans- Oxidation reduction reaction
4.) CuSO4 + Zn—– ZnSO4 + Cu
Ans- Displacement reaction
3.) Answer the following questions
1.) When calcium carbonate is heated we get calcium oxide and carbon dioxide.
CaCO3—– CaO + CO2
What type of reaction is this? What are the reactants and products? Write it.
Ans- when calcium carbonate is heated and it forms calcium oxide and carbon dioxide it is the decomposition type of reaction.
The reactant for this reaction is calcium carbonate CaCO3
The product of this reaction are calcium oxide Cao and carbon dioxide CO2.
2.) Give two examples for each of the following reactions in which-
1.) precipitate is formed-
Ans-Ca(OH)2 + CO2—— CaCO3 + H2O
2.) heat is liberated-
Ans-when coal is burn in oxygen carbon dioxide is produced and heat is liberated.
CaO + H2O—— Ca(OH)2 + heat
3 .) colour is changed-
Ans-
4.) Some reactions are given below. Identify the chemical reaction and write the reasons.
- Melting of ice-
- Dissolution of sugar in water-
- Burning of wood
- Blackening of pieces of potatoes, brinjals and apples.
- Melting of wax
- Blackening of silver ornaments
- Sourcing of milk
- Synthesis of food by green plants in presence of sunlight
From the all above reaction some reaction has physical changes and some reaction has chemical changes following are the reaction in which chemical changes are seen-
Chemical changes reaction |
Reasons of change |
burning of wood |
It shows combustion reaction because heat is produced |
Blacking of silver ornaments |
Silver has some property when it contacts with air after some period it becomes black |
Blacking of pieces of potatoes, brinjals and apple |
It happens due to oxidation reaction |
Souring of milk
Synthesis of food by green plants in presence of sunlight |
Chemical reaction happened Photosynthesis reaction |
5.) Point out the difference between
1.) Oxidation and reduction-
Oxidation |
Reduction |
Addition of oxygen and removal of hydrogen is known as oxidation reaction. |
Addition of hydrogen and removal of oxygen is known as reduction reaction. |
For eg. S + O2— SO2 |
For eg. 2NH3—- N2+ 3H2 |
2.) Combination and decomposition
Combination reaction |
Decomposition reaction |
When two or more reactants combine in a reaction to form a single product it is called a combination reaction. |
The chemical reaction in which two or more products are formed from a single reactant is called decomposition reaction. |
For eg. S+ O2— SO2 |
2H2O—- 2H2+ O2 |
2Mg+ O2— 2MgO |
CaCO3——- CaO + CO2 |
6.) Write the characteristics of chemical reaction.
Ans-the processs or reaction in which new compound is formed is called chemical reactions.
Characteristics of chemical reaction-
1) In this reactions reactant formed product
2) Formation of precipitation
3) Change in colour
4) Liberation of heat
5) Formation of new product or substance
7.) Enlist various chemical changes that you come across in your daily life also classify or categorized them.
Ans- There are various chemical reaction are happened in our daily life like digestion of food in our body, combustion, fermentation reaction and the reaction of sops with water, detergents with water, cooking of food, turning of milk into curd etc are the various chemical reactions are happened in our daily life.
Following are the example of chemical changes
Chemical change in everyday life |
Burning of paper or wood |
Turning of milk into curd |
Digestion of food in the stomach |
Baking cake |
8.) When Seema tasted glucose, she felt cold in tongue. Why did she feel so? What is this reaction called?
Ans- when Seema tasted glucose she felt cold in tongue because heat is formed from our body and make reaction with surrounding i.e glucose therefore it felt as cold. This type of reaction in which heat is generated inside is known as endothermic reactions.