The comprehensive approach in SCERT Class 8 Basic Science Textbook Solutions Chapter 4 Chemistry of Changes Important Questions ensure conceptual clarity.
Chemistry of Changes Extra Questions and Answers Class 8 Basic Science Chapter 4 Kerala Syllabus
Chemistry of Changes Class 8 Important Questions
Question 1.
Burning of wax is a ……………………… change.
(Chemical change / Physical change)
Answer:
Chemical change
Question 2.
What is the form of energy exchanged during the state transition of matter?
Answer:
Heat energy
Question 3.
The amount of space required for matter to exist is …………………………
Answer:
Volume
Question 4.
What gas is produced when magnesium reacts with hydrochloric acid?
a) Chlorine
b) Hydrogen
c) Oxygen
d) Nitrogen
Answer:
b) Hydrogen
Question 5.
An example of a substance that undergoes sublimation is:
a) Water
(b) Talc
(c) Wax
(d) Camphor
Answer:
(d) Camphor
Question 6.
Find the relationship and complete.
Gas that aids combustion: Oxygen
Gas that burns: _____________
Answer:
Hydrogen
Question 7.
What is the energy change that occurs in a Dry cell?
Answer:
Chemical energy is converted into electrical energy.
Question 8.
What is the chemical product formed when electricity is passed through water containing a small amount of acid?
Answer:
Hydrogen and Oxygen
Question 9.
Which solution is used for electroplating copper onto an iron nail?
Answer:
Copper sulphate solution
Question 10.
To gold-plate a copper bangle, what should be connected to the positive terminal of the battery?
Answer:
Gold
Question 11.
Select the correct statements related to gases from the ones given below:
a) Less distance between particles.
b) More energy of particles.
c) Higher speed of particle movement.
d) More attractive force between particles.
Answer:
b) More energy of particles. and c) Higher speed of particle movement.
Question 12.
When a candle burns, both chemical and physical changes occur. What are these?
Answer:
Chemical Change: Burning of a candle
Physical Change: Melting of a candle
Question 13.
Magnesium is taken in a test tube, and a liquid is added to it. A vigorous chemical reaction occurs, and a gas is released. If a burning splinter is shown to the gas, it burns with a “pop” sound.
a) Identify the probable liquid added to the test tube.
b) Which gas was produced?
Answer:
a) Dilute Hydrochloric acid
b) Hydrogen
Question 14.
What is the energy change occurring in the following?
a) Combustion of fuel
b) Photosynthesis
Answer:
a) Chemical energy is converted into heat energy
b) Light energy is converted into chemical energy.
Question 15.
Classify the following into chemical changes and physical changes:
a) Wax melts
b) Wax burns
c) Milk turns into curd
d) Milk thickens (or solidifies) on cooling
Answer:
| Chemical changes | Physical changes |
| Wax burns | Wax melts |
| Milk turns into curd. | Milk thickens (or solidifies) on cooling. |
Question 16.
Two processes are given below:
Quicklime is produced by strong heating limestone (or shells).
Slaked lime is produced by adding water to quicklime.
a) Which of these is an endothermic reaction?
b) Which is an exothermic reaction?
Answer:
a) Quicklime is produced by strong heating limestone (or shells)
b) Slaked lime is produced by adding water to quicklime.
Question 17.
What are the suitable solutions for electroplating the metals given below?
a) Silver
b) Gold
Answer:
(a) Sodium cyanide + Silver cyanide
(b) Sodium cyanide + Gold cyanide
Question 18.
What are the forms of energy associated with chemical reactions?
Answer:
Heat, Light, Electricity
Question 19.
What are the main properties of matter
Answer:
It occupies space
It has mass
Question 20.
Quicklime is taken in a steel cup. A little water is added to it.
(a) What is the substance formed when water is added to quicklime?
(b) Why do people handling quicklime wear gloves?
Answer:
a) When water is added to quicklime (calcium oxide), slaked lime (calcium hydroxide) is formed.
b) The reaction of quicklime with water is highly exothermic. If quicklime comes into contact with moisture on the skin, it will react and release a lot of heat. This can cause severe burns to the skin.
Question 21.
Suggest an experiment to prove that matter requires space to exist.
Answer:
Aim: To demonstrate that matter occupies space.
Materials: Beaker, water, and a small stone.
Procedure & Observation:
- Fill a beaker partially with water and note the initial water level.
- Carefully lower the stone into the water.
- Observe that the water level rises.
Conclusion: The rise in water level proves that the stone, being matter, takes up space (has volume)
Question 22.
Write what type of chemical reaction each of the following is:
a) Fireflies emit light.
b) When zinc and copper nails are inserted into a lemon and connected to an LED through a copper wire, the LED glows.
c) Ammonium chloride decomposes when heated strongly.
Answer:
a) Bioluminescence
b) Electrochemical Reaction
c) Thermochemical reactions
Question 23.
Complete the following chemical reaction
Answer:
(a) Silver (Ag)
(b) Chlorine gas (Cl2)
(c) Water (H2O)
(d) Oxygen (O2)
(e) Water (H2O)
(f) Slaked lime (Ca(OH)2)
Question 24.
A substance is strongly heated in a test tube. When a burning incense stick was held near the mouth of the test tube, the incense stick flared up.
a) What caused the incense stick to flare up?
b) What might be the substance that was heated?
c) What type of chemical reaction is this?
Answer:
a) When a substance is heated, oxygen gas is released. Oxygen is a gas that aids in burning/combustion.
b) Potassium permanganate
c) Endothermic reaction
Question 25.
What is the reason for the following?
a) Water kept in the freezer turns into ice.
b) Silver salts are stored in colored bottles.
c) Some specific types of medicines are stored in refrigerators.
Answer:
a) Water turns into ice in a freezer due to the cooling/freezing process (Phase Transition). When water is cooled below its freezing point (0°C), its molecules lose kinetic energy and arrange themselves into a more ordered, crystalline structure. This change of state from liquid to solid is a physical process that occurs when enough heat energy is removed from the water.
b) Silver salts (like silver bromide, silver chloride) are stored in colored or opaque bottles because they are sensitive to light and undergo photochemical reactions when exposed to it. Light energy can decompose silver salts into elemental silver, causing them to blacken or lose their intended properties. Colored or opaque bottles block out light, preventing this decomposition and preserving the integrity of the silver salts.
c) Some specific types of medicines are stored in refrigerators because their chemical stability and efficacy are temperature-dependent. Many ingredients in these medicines are sensitive to heat. Higher temperatures can cause them to degrade, lose their potency, or even become toxic. Refrigeration slows down these chemical degradation processes, maintaining the medicine’s effectiveness and ensuring its safety for use throughout its shelf life.
Question 26.
Define the following
a) Thermochemical Reactions
b) Photochemical Reactions
c) Electrochemical Reactions
Answer:
a) The reactions in which heat energy is liberated or absorbed are known as thermochemical reactions.
b) The reactions in which light energy is absorbed or liberated are known as photochemical reactions
c) If electrical energy is absorbed or produced during a chemical reaction, it is known as an electrochemical reaction.
Question 27.
a) What is electrolysis?
b) Write any practical application of electrolysis.
Answer:
a) The process of dissociation of a substance by absorbing electrical energy is known as electrolysis.
b) Electroplating
Question 28.
Draw a diagram of the setup for electroplating gold onto a copper bangle and label its parts
Answer:
Question 29.
A picture of the arrangement of copper plating on an iron bangle is drawn. What are the mistakes in this?
Answer:
Copper should be connected to the positive terminal of the battery.
The iron bangle should be connected to the negative terminal of the battery. Copper sulphate solution should be used as the electrolyte (or solution).
Question 30.
Match the following
| A | B | C |
| Sublimation | Chemical energy is converted into electrical energy. | Naphthalene |
| Firefly | Change of state | Photochemical reaction |
| Dry cell | Bioluminescence | Electrochemical reaction |
Answer:
| A | B | C |
| Sublimation | Change of state | Naphthalene |
| Firefly | Bioluminescence | Photochemical reaction |
| Dry cell | Chemical energy is converted into electrical energy. | Electrochemical reaction |