Kerala Syllabus Class 8 Basic Science: Chapter 4 Chemistry of Changes - Questions and Answers
Std 8: Chemistry: Chapter 4: Chemistry of Changes: Questions and Answers
♦ Look at the picture.
• A candle burning
• Food is cooked
• Making sculpture from stone
• Ripening of the banana
Substances undergo different types of changes here.
These changes may lead to the production of new substances.
These substances are different forms of matter.
♦ matter/substance
Matter is anything which occupies space and has mass.
♦ Which of the following is not considered a matter?
a. Air
b. Water
c. Light
d. Stone
Answer: Light
♦ Experiment
In the experiment, the water level is raised because the stone needs space to occupy, and so it displaces water from the space it needs.
| One of the properties of matter is that it occupies space. The space occupied by matter is its volume. |
|---|
♦ Weigh both the stones using a balance. What difference do you observe?
In the former experiment, if the stones are put in a balance (common balance or digital balance) its mass is read. The mass shown by the two
stones will be different according to their size.
Matter has mass. This is another property of matter.
♦ Mass
Mass of a substance is the measure of the quantity of matter contained in it.
♦ Is air a form of matter? Does air need space to occupy?
Everything in this universe, including air are made up of matter. Air is also a matter. Air needs space to occupy.
♦ Fix a towel inside a glass and immerse it upside down into the water taken in a beaker, as shown in the figure.
Why doesn't water enter the glass?
What happens to the water level in the beaker?
There is air inside the glass. So water cannot enter this space because air itself needs space for its occupancy. The water level is raised because water is displaced from this space which is equal to the volume of air.
♦ How can we find out whether air has mass?
Will the mass of a football without air be the same as the mass of an inflated football? Check it out using a digital balance.
If the mass of a football is measured with a digital balance before and after filling it with air, we can see that the mass is greater when the ball is air-filled.
It is now clear that air has mass.
♦ Matter
Anything that occupies space and has mass is called matter.
Matter exists mainly in three states.
♦ Complete the given table with respect to the three states of matter. Put a ✔ mark appropriately.
• For liquids and gases, there is no fixed shape. They acquire the shape of the container.
♦ Different types of changes
Heat some wax in a steel vessel. What happens?
What happens when the liquid wax is cooled?
When wax is heated, it melts into liquid form. This liquid solidifies into wax again on cooling.
We have seen that wax can be converted from solid to liquid and vice versa. But if a piece of paper is burnt to ash, can it be changed to paper again? Do all the changes occur at the same speed?
The change that happens when we burn a piece of paper is not like this. We cannot convert the products obtained by burning to the paper again. This means the changes may be of different types. Also, the speed of changes may differ. A pea seed germinates within an hour or a day. But the plant to grow and flower takes many more days. That means some changes are fast and some others are slow.
♦ Find more examples for slow and fast changes.
| Slow changes | Fast changes |
|---|---|
| • Rusting of Iron • Rottening of vegetables • Ripening of fruits • Greying of coloured clothes • Changing milk into curd • Blackening of silver ornaments | • Burning of petrol • Burning of paper • Explosion of crackers • Ignition of cooking gas • Neutralisation • Reaction of sodium with water |
♦ What is the difference between the burning of a piece of paper and the melting of wax? Do the chemical properties of wax change when it melts? What happens when a piece of paper burns?
In the melting of wax, liquid wax can be made into solid again, because there is no change in the chemical properties during the process. The change takes place only in physical nature. But, when a paper is burned, its chemical structure and properties change. So it cannot be converted into its former state. Burning of paper is a chemical change, and melting of wax is a physical change.
♦ Physical Change and Chemical Change
No new substances are formed during a physical change. Only the molecular arrangement changes. But in chemical change, a substance is converted into another one. That means, new molecules are formed.
Physical changes are reversible and temporary. But chemical changes are usually irreversible and permanent.
♦ Classify the following changes into physical change and chemical change.
i) curdling of milk
ii) melting of wax
iii) burning of a candle
iv) formation of ice
v) melting of ice
vi) dissolution of salt in water
vii) rusting of iron
viii) burning of firewood
| Physical change | Chemical change |
|---|---|
| ii) melting of wax iv) formation of ice v) melting of ice vi) dissolution of salt in water | i) curdling of milk iii) burning of a candle vii) rusting of iron viii) burning of firewood |
♦ In the following activities, which of them have changed chemical properties?
a. Wax melting.
b. Paper burning.
c. Water becoming vapour.
d. Water becoming ice.
Answer: Paper burning
♦ Which of the following does not change in chemical properties?
a. rusting of iron.
b. curdling of milk
c. melting of ice
d. burning of camphor
Answer: Melting of ice
♦ Which of the following statements is incorrect? a. A change of state is a chemical change
b. A change of state is a physical change
c. In chemical changes, energy is changed.
d. In a change of state, energy is changed.
Answer: a. A change of state is a chemical change
♦ Take some ice in a bowl and heat it. What happens? What happens if heating is continued?
When ice (solid) taken in a vessel is continuously heated, it is first converted into water (liquid) and then into water vapour (gas). In this case, the distance between the particles, their attractive force and energy are changed.
♦ Can you write about the distance and force of attraction between the particles, speed of movement and the energy of the particles in each state in the following table?
When a solid is heated,
Distance between the particles: Increases
Energy of particles: Increases
When a solid is heated, the distance between its particles and their energy increases and gradually it attains the particle arrangement of a liquid. If it is heated further, energy and distance between the molecules increase again, and the gaseous state is reached.
♦ What happens when a gas is cooled?
When a gas is cooled (heat is removed), the energy of the molecules decreases, and they come closer and become a liquid.
Solid substances like Camphor and naphthalene, when heated, directly change to gases. This is known as Sublimation.
♦ Change of state
The change of substance from one physical state into another is known as change of state.
♦ Is change of state a physical change or a chemical change?
Physical change
♦ Which form of energy is absorbed or liberated during the change of state?
Heat energy
♦ What happens to the energy of particles when heat is absorbed or liberated?
• Absorbing heat → particles gain energy, move apart.
• Releasing heat → particles lose energy, come closer.
In chemical changes, new products/substances are produced.
♦ Reactants and Products.
The substances which take part in a chemical reaction are known as reactants, and the substances formed are known as products.
♦ Experiment
Place some potassium permanganate crystals on a tile and pour glycerine in the middle. Record your observation.
Observation: Potassium permanganate slowly catches fire. A large amount of heat is produced along with smoke. A dark substance is left over as residue.
Test the product left over on the tile, whether it has the same properties as potassium permanganate. For this, take water in two beakers and add two or three crystals of fresh potassium permanganate into one of them. Add two or three crystals of the product obtained in the second beaker. You can see, they show a colour difference. (The first one becomes purple in colour and the second is brown.)
♦ Thermochemical Reactions
Reaction between Magnesium and dilute Hydrochloric acid
Take a piece of magnesium in a test tube and add dilute hydrochloric acid to it.
Observation: Small gas bubbles are formed. Bring a lightened match stick at the mouth of the test tube. The gas burns with a 'pop' sound, and water drops are formed at that part.
The gas produced: Hydrogen.
Another product of the reaction is Magnesium Chloride, which is dissolved in water.
Reactants: Magnesium + Hydrochloric acid
Products: Magnesium chloride + Hydrogen + Heat
After the reaction, when the bottom of the test tube is touched, it feels hot. That means, along with the products, heat is also produced by the reaction.
♦ What are the reactants in magnesium and hydrochloric acid react and produce hydrogen?
Magnesium and Hydrochloric acid
♦ Experiment
Reaction between quick lime and water
Take quicklime in a steel cup. Add some water to it. Touch the cup after some time. What do you feel?
Observation: Quick lime reacts with water and is made into a paste or powder form. Slaked lime is the product formed. Along with the product, heat is also produced.
Quick lime + water → Slaked lime + Heat.
There are many reactions in which heat is produced. They are called exothermic reactions.
♦ Exothermic reactions
If heat is liberated as a result of a chemical reaction, such reactions are known as exothermic reactions.
♦ Experiment
During some other reactions, heat energy is absorbed.
Decomposition of Potassium permanganate
Take some potassium permanganate in a test tube and heat it. Hold a burning incense stick near the mouth of this test tube. What do you observe?
Observation: Incense stick burns with a glowing flame. This is because of the liberation of oxygen. (Oxygen is the gas that supports burning.)
In this reaction, when potassium permanganate decomposes to produce oxygen, heat energy is continuously absorbed. (Continuous heating is required)
♦ Experiment
Reaction between Ammonium chloride and Barium hydroxide.
Take some ammonium chloride in a watch glass, add some barium hydroxide into it and mix well with a glass rod. Touch the bottom of the watch glass.
Observation: It feels cold. That means heat energy is absorbed during this reaction.
♦ Endothermic reaction
If heat energy is absorbed during a chemical reaction, it is known as an endothermic reaction.
♦ Which of the following is an endothermic reaction?
a. The reaction between quicklime and water.
b. The reaction between magnesium and hydrochloric acid.
c. The reaction between glycerine and potassium permanganate.
d. The reaction between barium hydroxide and ammonium chloride.
Answer: d. The reaction between barium hydroxide and ammonium chloride.
♦ The decomposition of potassium permanganate is a ..................
a. Endothermic reaction
b. Exothermic reaction
c. Photochemical reaction
d. Electrochemical reaction
Answer: a. Endothermic reaction
♦ Heat some crystals of potassium permanganate in a dry test tube. Bring a burning incense stick near the mouth of the test tube.
i. What do you observe?
ii. Which is the gaseous product formed?
iii. Which type of reaction is this?
Answer:
i. Incense stick burns with a glowing flame
ii. Oxygen (O₂).
iii. Endothermic reaction
♦ Find more examples for such reactions.
| Exothermic Reactions | Endothermic Reactions |
|---|---|
| • Reaction between potassium permanganate and glycerine • Reaction between magnesium and hydrochloric acid • Reaction between quick lime and water • Reaction of sodium with water • Burning of firewood • Burning of petrol, kerosene, etc. Dissolution of sulfuric acid in water | • Decomposition of potassium permanganate • Decomposition of hydrogen iodide • Reaction between ammonium chloride and barium hydroxide • Heating of lime shells to produce quick lime • Dissolution of potassium nitrate in water |
♦ Thermochemical reactions
The reactions in which heat energy is liberated or absorbed are known as thermochemical reactions.
♦ Photochemical reactions
The reactions in which light energy is absorbed or liberated are known as photochemical reactions.
♦ Experiment
Take some silver nitrate solution in a watch glass and add sodium chloride solution into it. Dip two pieces of cotton in the product formed. Cover one of them with black paper and keep the other one open. Keep them aside for some time. Record your observations.
Observation: Cotton piece kept open becomes black (dark).
Reason: Silver nitrate and sodium chloride react to form Silver chloride.
This silver chloride decomposes by absorbing light to form silver. This is the reason for the blackening of the cotton, which is kept open.
light energy
♦ Which reaction is represented by the illustration?
Photosynthesis is another example for light absorbing reaction.
♦ What are the reactants here? What are the products?
Carbon dioxide and water are the reactants in this reaction. They are converted into glucose and oxygen by absorbing sunlight. This is converted to starch and stored in different parts of the plant.
Water + Carbon dioxide ----------------- glucose + oxygen
♦ What are the reactants in photosynthesis?
Carbon dioxide and Water
♦ What are the products of photosynthesis?
Glucose and Oxygen
♦ Do you know any reactions which liberate light energy?
The glowing of a firefly is an example of this type of reaction.
♦ How do fireflies produce light?
The chemical luciferin absorbs ultraviolet rays with the help of the enzyme luciferase present in the body of fireflies and emits visible light. This phenomenon is known as bioluminescence. Firefly can control the amount of oxygen entering its body and thereby regulate the intensity of light produced. Some species of marine organisms and worms exhibit bioluminescence. In these reactions, light energy is liberated.
♦ Photochemical reactions
The reactions in which light energy is absorbed or liberated are known as photochemical reactions.
♦ Some medicines are kept in brown coloured bottles. What could be the reason?
They contain components which undergo decomposition in the presence of light.
♦ Silver nitrate is not stored in transparent bottles. Why?
Silver nitrate decomposes on absorbing light.
♦ What is an electrochemical reaction?
In some chemical reactions, electrical energy may be produced or absorbed. The devices are called dry cells, which produce electrical energy from chemical reactions.
♦ What are the parts of a dry cell?
• In a dry cell, chemical energy is converted into electrical energy.
♦ Let us do an activity.
Take some concentrated salt solution (sodium chloride) in a beaker. Add three or four drops of phenolphthalein into it. Dip the ends of copper wires connected to both the terminals of a battery into the solution.
Observation: Gas bubbles are formed at the immersed ends of the copper wire. The solution gradually becomes pink.
♦ Which compound’s presence is indicated by the pink colour?
When sodium chloride undergoes dissociation, sodium hydroxide is formed as one of the products. This turns the solution pink.
♦ Which form of energy is responsible for this reaction?
Here, electrical energy is absorbed for the chemical reaction.
♦ Electrolysis
The process of dissociation of a substance by absorbing electrical energy is known as electrolysis.
• If electricity is passed through water containing a small amount of acid, it dissociates into hydrogen and oxygen.
• This is another familiar example of electrolysis.
♦ When electricity is used from a dry cell, ........takes place in the cell.
a. Physical change
b. Heat absorption
c. Combustion.
d. Electrochemical reaction
Answer: d. Electrochemical reaction
♦ A white cloth dipped in silver nitrate darkens when it is kept in sunlight.
i. What form of energy is responsible for the chemical change here?
ii. What is the common name for such chemical reactions?
Answer:
i. Light energy
ii. Photochemical reaction
♦ When electricity is passed through a concentrated salt solution, one of the products formed is
a. Acid
b. Alkali
c. Nitrogen
d. Carbon dioxide
Answer: b. Alkali
♦ To decompose water into hydrogen and oxygen, it needs to be --------.
a. Boiled
b. Exposed to sunlight
c. Passed air through
d. Passed electricity through
Answer: d. Passed electricity through
♦ What is electroplating?
Coating a metal with another metal using electricity is known as electroplating.
♦ Let us coat copper on an iron bangle.
♦ Plating Silver:
An iron bangle (any metallic articles to be plated) is connected to the negative terminal, and a clean silver plate is connected to the positive terminal of the battery. The solution to be used is a mixture of silver cyanide solution and sodium cyanide solution.
♦ Gold plating: The metallic object to be coated should be connected to the negative terminal of the battery. A thin gold plate is to be connected to the positive terminal. A mixture of gold cyanide and sodium cyanide solution is taken in the beaker.
♦ For electroplating a metal,
• The metallic object to be coated is connected to the negative terminal of the battery.
• The metal for coating (the metal whose coating is to be made) is connected to the positive terminal of the battery.
• A solution of the coating metal is used as a solution.
♦ When making a battery using a lemon and lighting a bulb, what should you pay attention to when installing the batteries?
As shown in the figure, take some lemon fruits and put a zinc nail and a copper nail in each. Connect each copper nail to the zinc nail of the other in series, leaving behind the zinc nail and copper nail at the ends (first and last lemons). Now connect the zinc nail of the first and copper nail of the last to an LED by using connecting wires in such a way that the positive of the LED (long lead) to the copper nail and the negative of LED (short lead) is connected to the zinc nail.
Observation: LED glows.
Reason: Zinc and copper metals react with the acid contained in the lemon, producing electricity.
♦ Different forms of energy are absorbed or liberated in every chemical reaction. Reactions are known by the major form of energy involved in that reaction.
| Chemical reaction | Major energy change |
|---|---|
| Burning of substances | Liberates heat |
| Decomposition of substances on heating | Absorbs heat |
| Bioluminescence | Liberates light |
| Cell made up of lemon | Liberates electrical energy |
| Electrolysis of sodium chloride solution | Absorbs electrical energy |
♦ In the process of plating copper on an iron spoon
i. To which terminal is the iron spoon connected?
ii. Which solution is used for this?
iii. To which terminal is the copper plate connected?
Answer:
i. The iron spoon is connected to the negative terminal (cathode).
ii. The solution used is copper sulfate (CuSO₄) solution.
iii. The copper plate is connected to the positive terminal (anode).
♦ Which solution is used in plating silver on an iron ring?
Silver nitrate (AgNO₃) solution
♦ Which solution is used in plating gold on a silver spoon?
Gold chloride (AuCl₃) solution
♦ Sodium reacts with water to form new substances.
i. What are the reactants in this reaction?
ii. What are the products of the reaction?
Answer:
Reactants → Sodium + Water
Products → Sodium hydroxide + Hydrogen gas
♦ Let us assess
1. What changes in the arrangement of particles occur during the following situations?
a) Solid changes to liquid
b) Liquid changes to gas
c) Gas changes to liquid
Answer:
a) Distance between particles increases. Their movement increases
b) Distance between particles increases. Their movement increases
c) Distance between particles decreases. Movement also increases
2. Which is the main form of energy liberated/ absorbed during the given reactions? Write the type of each of these chemical reactions.
a) Ammonium chloride and barium hydroxide react.
b) Coating an iron bangle with copper.
c) Glowing of Firefly.
d) Decomposition of potassium permanganate.
e) Lighting an LED using lemons.
Answer:
a) Heat energy is absorbed - Thermochemical reactions
b) Electricity is absorbed - Electrochemical reactions
c) Light is emitted - Photochemical reactions
d) Heat is absorbed - Thermochemical reaction
e) Electricity is produced - Electrochemical reaction
3. Heat some crystals of potassium permanganate in a dry test tube. Bring a burning incense stick near the mouth of the test tube.
a) What do you observe?
b) Which is the gaseous product formed?
c) Which type of reaction is this?
Answer:
a) Incense stick glows with a shining flame
b) Oxygen
c) Thermochemical reaction (Endothermic)
4. A white cloth dipped in silver nitrate darkens when it is kept in sunlight.
a) Which form of energy is responsible for this chemical change?
b) What is the general name for this type of reaction?
Answer:
a) light
b) Photochemical reactions
5. Sodium metal reacts with water to give new substances.
a) Which are the reactants in this reaction?
b) Which are the products formed?
Answer:
a) Sodium, water
b) Sodium hydroxide, Hydrogen
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