Kerala Syllabus 8th Standard Basic Science Solutions Chapter 6 Chemical Change

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Kerala State Syllabus 8th Standard Basic Science Solutions Chapter 6 Chemical Change

Chemical Changes Questions and Answers

All the changes in nature is divided into physical changes and chemical changes. During a physical change, only a change in the arrangement of molecules occurs. Hence it can be easily brought back to its original state. In the case of chemical changes, new molecules are formed.

The alkali formed by the reaction of sodium with water is sodium hydroxide. Because of its basic nature it becomes pink colour when phenolphthalein is added to it.

In the reaction of magnesium burns in air heat and light are produced.

Thermo chemical reactions

The reaction of magnesium with dil. hydrochloric acid, hydrogen is produced and heat is evolved. So we feel hot when we touch the test tube.
Mg + 2HCl → MgCl2 + H2 + heat

Take a few crystals of potassium permanganate in a dry test tube. Heat the test tube. Bring a glowing incense stick to the mouth of the test tube. When potassium permanganate is heated, it decomposes to form potassium mangnate, manganese dioxide. and oxygen. In this reaction, potassium permanganate decomposes with absorption of heat and oxygen is evolved.

Add hydrochloric acid to a concentrated solution of sodium hydroxide taken in a test tube. Heat is evolved. Chemical reactions which liberate heat are called exothermic reactions and those which absorb heat are called endothermic reactions.

Photochemical reactions

Photosynthesis is a chemical reaction that is responsible for sustaining life on earth. In this reaction, plants produce glucose by absorbing light. Note the chemical equation of this reaction.
6 H2 O + 6 CO2 + Light → C6 H12 O6 + 6 O2

The glucose thus formed is stored by plants in the form of starch.

Take some silver bromide in watch glasses. That will change to black because silver is precipitated.

Chemical reactions which liberate or absorb light energy are known as photochemical reactions.

Electrochemical reactions

Kerala Syllabus 8th Standard Basic Science Solutions Basic Science Solution

The change in colour of copper sulphate solution and the deposition of copper at the electrode proves that a chemical change has occurred.

Kerala Syllabus 8th Standard Basic Science Solutions Chapter 6 Basic Science Class 8 Chapter 6

In this reaction, copper sulphate decomposed with the absorption of electrical energy. The process in which a substance undergoes decomposition by the absorption of electrical energy is known as electrolysis.

Kerala Syllabus 8th Standard Basic Science Solutions Kerala Syllabus 8th Standard Chemistry

Here, electricity is produced as a result of a chemical reaction between the acid and the metals kept immersed in it. Such arrangements which produce electricity as a. result of chemical reaction are known as electrochemical cells.
We can make cells using a variety of fruits and different metals like zinc. Chemical reactions in which electrical energy is consumed or produced are known as electrochemical reactions.


Kerala Syllabus 8th Standard Basic Science Solutions Changes in Nature Basic Science

You are familiar with the gold plated ornaments that are available in the market. Electricity is used to obtain a thin coating of a particular metal on other metallic objects. This process is called electroplating.

Now, you might have understood that forms of energy like heat, light and electricity are exchanged during chemical reactions. There are chemical reactions which involve the absorption or liberation of energy. Those which absorb energy are known as endoergic reactions and those which liberate energy are known as exoergic reactions.

Energy transfer occurs during any chemical reaction. A chemical reaction will be known based on the major energy form which gets absorbed or liberated.

Different types of cells

The cells are the electric source commonly used. Following is the table of cells and their uses

Cells Equipment used
Dry cell • Radio
• Camera
• Clocks
• Toys
Mercury cell • Watches
• Calculators
• Electric instruments
Nickel-cadmium cell • Rechargeable torches
• Cameras
Lithium-iron cell • Mobile phones
• Laptops

Environment friendly changes

Many natural and manmade chemical changes that occur around us are not nature – friendly. Nature tries its best to adapt these changes But after a limit it destroy the harmony of nature. The chemicals coming out from the factories will make the problem serious. Hence, accumulation of those materials, which cannot be biodegraded may become a threat even to the existence of life itself. For the sake of posterity, it is the duty of each one of us to protect the earth’from getting polluted.

Chemical Changes Textbook Questions and Answers

Electrolytic water Equipment Factory Question 1.
Assess the chemical reactions given below and answer the questions.
1. Calcium carbonate + heat → Calcium oxide + Carbon dioxide
2. Calcium oxide + water → Calcium hydroxide + heat
a. Mention the reactants and products in each case.
b. Which among these is endothermic? Which one is exothermic?
1. Calcium carbonate
2. Calcium oxide, water
1. Calcium oxide, carbon dioxide
2. Calcium hydroxide
b.First reaction is endothermic and second is exothermic reaction.


Chemical Bottle Email Question 2.
Some chemical reactions are given below. Identify the energy change involved and write down what type of chemical reaction takes place here.
a. Burning of a candle
b. Glowing of a fire fly
c. Plating a copper ring with gold
d. Reaction between potassium hydroxide and sulphuric acid
e. Burning of fuels
a. Thermochemical reactions
b. Photochemical reaction
c. Electrochemical reaction
d. Thermochemical reactions
e. Thermochemical reactions

Acid Fly Kerala Question 3.
A student tries to plate an iron nail with copper. Draw its arrangement by selecting the required materials from the list given below.
Silver nitrate, iron nail, copper sulphate, silver rod, copper rod, silver plate, iron sulphate, battery, wire, water, beaker.
Kerala Syllabus 8th Standard Basic Science Solutions 8th Standard Chemistry Textbook

Question 4.
List out the instruments that use electrochemical cells. What are the merits and demerits of using such cells?

Cells Equipment used
Dry cell • Radio
• Camera
• Clocks
• Toys
Mercury cell • Watches
• Calculators
• Electric instruments
Nickel-cadmium cell • Rechargeable torches
• Cameras
Lithium-iron cell • Mobile phones
• Laptops

Question 5.
Classify the following into physical changes and chemical changes.
1. Melting of ice.
2. Heating magnesium in water.
3. Silver bromide kept exposed to sunlight.
4. Change happening to soda water on opening its bottle.
1. physical change
2. chemical change
3. chemical change
4. physical change


Question 6.
Give an example each for exothermic and endothermic reactions.
Exothermic reaction:
Magnesium + hydrochloric acid → magnesium chloride + hydrogen + heat
Endothermic reaction:
Potassium permanganate + heat → potassium manganate + manganese dioxide + oxygen

Question 7.
Making a volcano
Heap up some ammonium dichromate powder on a tile. Deposit on it the chemical present on a match stick, and ignite. Write down the changes happening there.
Change in colour: …………………………..
Change in amount: …………………………..
Exchange of energy: …………………………..
Change in colour – orange-red Change in quantity – decreases Energy exchange – exothermic

Question 8.
Kindling fire by pouring oil. Heap up some potassium permanganate on a tile. Keep a piece of dry cotton wick on top of it. Pour one or two drops of glycerine on the wick. Observe the changes.
potassium permanganate heap burns

Question 9.
Take a magnesium ribbon and clean it by scrubbing. Then burn it in air. Collect the product formed and dissolve it in water. Dip litmus papers and pH paper in this solution and observe. Find out the reason for the results of the observations.
Red litmus changes to blue. This is because of the magnesium hydroxide produced when magnesium oxide dissolved in water is alkaline.


Question 10.
Take sodium chloride solution in a beaker. Add to it a few drops of phenolphthalein. With the help of two carbon rods, let electricity pass through the solution. Record your observations. What is your inference?
Sodium chloride is decomposed into sodium and chlorine

Question 11.
Take some silver nitrate solution in a test tube and add some sodium chloride solution to it. What is the colour of the precipitate formed? Filter the precipitate using a filter paper, place it on a watch glass and keep it exposed to sunlight. What do you observe?
Try to write down the equation of your observation with the help of your teacher.
Identify the change of energy involved in the second chemical reaction and write down what type of chemical reaction it is.
White precipitate. It will become black when it is opened in sunlight.
Energy change: photo chemical reaction

Chemical Changes Additional Questions and Answers

Question 1.
i. sodium + water → sodium hydroxide + hydrogen
ii. Magnesium + oxygen → magnesium oxide
Write the reactants and products of these reactions.
i. Reactants: sodium, water
Products: sodium hydroxide, hydrogen
ii. Reactants: magnesium, oxygen
products: Magnesium oxide

Question 2.
What is the observation when two drops of phenolphthalein is added to the solution obtained after the reaction of sodium with water? Justify your answer.
The solution becomes pink in colour. The product of the reaction is sodium hydroxide which is alkaline.


Question 3.
Tabulate the following changes into physical and chemical changes

  • Water changes to water vapour
  • burning wood
  • rusting the iron
  • explosion of crackers
  • cutting the wood
  • Magnesium burns in air
  • Ice is melting

Water changes to water vapour – Physical change
Burning wood – Chemical change
Rusting the iron – Chemical change
Explosion of crackers – Chemical change
Cutting the wood – Physical change
Magnesium burns in air – Chemical change
Ice is melting – Physical change

Basic Science for Class 6 Chapter 6 Question 4.
Tabulate the important energy change in the following reactions.

  1. Magnesium burns in air
  2. Heating potassium permanganate
  3. Photosynthesis
  4. Electroplating
  5. Reaction between Sodium hydroxide and Hydrochloric acid
  6. Glittering the fire fly
  7. Reaction in electrochemical cells


  1. Liberates heat energy
  2. Absorbs heat energy
  3. Absorbs light energy
  4. Absorbs electric energy
  5. Liberates heat energy
  6. Liberates light energy
  7. Liberates electric energy

Iron Sulphate Question 5.
Examine the following substances and draw the figure which shows the arrangement of coating silver on an iron ring Gold wire, silver rod, iron ring, copper sulphate, solution of silver cyanide and gold cyanide battery, beaker
Kerala Syllabus 8th Standard Basic Science Solutions Chapter 6 Chemical Change 4

Question 6.
Some medicines and chemicals are kept in dark bottles, why?
To prevent the chemical change due to the absorption of light

Question 7.
What is elecroplating? How are the electrode and the electrolyte selected when a metallic object is to be plated with another special metal?
To obtain a thin coating of a particular metal on another metallic object using electricity is known as electroplating.

The metal to be plated is taken as the positive electrode. The electrolyte is a solution of salt containing the metal to be plated. The negative electrode is the metallic object to be plated.


Question 8.
What is your observation when a glowing in inscent stick is brought to the mouth of test tube containing potassium permanganate which is heated? Give reason.
The stick flares up. When potassium permanganate is heated, it decomposes to form potassium manganate, manganese dioxide and oxygen.

Question 9.
Give an example each for the reaction of liberation of heat and absorption of heat from daily life.
Exothermic reaction: Burning of wood
Endothermic reaction: Marble is heated to quick lime

Question 10.
How electrical energy is produced by batteries?
Tn a battery, electrical energy is produced by the chemical reaction between zinc and sulphuric acid or by zinc and carbon. Electrical energy is produced by chemical reactions. Here chemical energy is converted to electrical energy.

Question 11.
Photosynthesis is the reaction that responsible to sustain life on the earth. Write the equation of the reaction. What is the energy change in it?
6H2O + CO2 → C6H12 O6+ 6O2
Light energy is absorbed in it.

Question 12.
Complete the figure given below
Kerala Syllabus 8th Standard Basic Science Solutions Chapter 6 Chemical Change 6

Kerala Syllabus 8th Standard Basic Science Solutions Chapter 6 Chemical Change 7

Question 13.
Acid is added to water for its electrolysis. Why?
Pure water is not an electric conductor. But if some acid, alkali or salt is added, lot of ions are formed in it and water will become an electric conductor.


Question 14.
When electricity is passed through copper sulphate solution using carbon electrodes kept in the solution, it decomposes.
A. By what name this process in known?
B. In which electrode is copper deposited dining the reaction?
C. Is there a colour change for the solution?
a. Electrolysis
b. Negative electrode
c. The colour of the solution decreases.

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