Selina Concise Solutions for ICSE Class 7 Chemistry Chapter 2 Physical and Chemical Changes

Points to Remember :

1. All changes are classified into two types
(i) Physical change
(ii) Chemical change.
2. Physical Change - A physical change is a temporary change in which no new substance is formed and chemical composition remains same. e.g. Melting of ice.
3. Chemical Change - A chemical change is a permanent change, in which a new substances are formed whose chemical composition and physical properties are different, e.g. Burning of a candle.
4. Chemical Reaction - Any chemical change in matter involving its transformation into one or more new substances is called a chemical reaction.
5. Chemical Equations - A chemical equation is the symbolic representation of a chemical reaction using the symbols and the formulae of the substances involved in the reaction.
6. The substances that react with one another are called reactants, and the new substances thus formed are called products.
7. A balanced chemical reaction is one in which the number of the atoms of each element on the
8. reactant side is equal to the number of atoms of that element on the product side.
9. The law of conservation of mass states that mass can neither be created nor be destroyed.
10. A chemical equation gives both qualitative and quantitative information about reactants aftd products.
11. The type of chemical reaction in which two substances combine to form a new substance is known as combination reaction.
12. The type of chemical reaction in which a substance breaks up on heating to form two or more simpler substances, which can be either elements or compounds, known as decomposition reaction.

Exercise

Question 1: (a) Define:
(i) a physical change
(ii) a chemical change
(b) Give two examples for each of the above two changes.

Answer:
(a) (i) Physical chagne : A temporary change in which no new substance is formed, the composition of substance remains the same, though its state, shape and size may change.
(ii) Chemical change : A permanent change in which new substances are formed whose composition and properties are completely different from those of the original substances.
(b) Physical change:
(i) Change of water into its vapours.
(ii) Heating of iron rod.
Chemical change:
(i) Burning of wood.
(ii) Breathing.
Physical changes only alter the physical appearance of a substance without changing its internal structure. Chemical changes involve a rearrangement of atoms to create entirely new molecules with new identities.
Remember: Physical changes are usually like a "costume change" where the person stays the same, while chemical changes are like a "transformation" into someone else.
To score full marks, always specify that physical changes are temporary and chemical changes are permanent.

Question 2: What are reversible and irreversible changes ? Give one example for each.

Answer:
1. Reversible change : When a change in a substance can be reversed by changing the conditions, it is said to be a reversible change.
Example : Melting of ghee or wax.
2. Irreversible change : When a substance can not be brought back to its original state after a change, it is said to be an irreversible change.
Example : Souring of milk.
Reversible changes allow the material to return to its H₂O or original form by undoing the action. Irreversible changes result in a permanent transformation where the original material is lost forever.
Think of a rubber band: stretching is reversible, but burning the rubber band is irreversible.
Include the word "original state" when defining irreversible changes to ensure technical accuracy.

Question 3: Mention a change which is always

Answer:
1. Desirable : Changes that are useful to man are desirable changes, e.g. change of milk into curd.
2. Undesirable : Change that brings about destruction is an undesirable change, e.g. floods and epidemics are undesirable changes.
3. Periodic : Changes that are repeated at regular intervals of time are called periodic changes, e.g. change of day and night.
Changes are categorized based on their impact on human life or the regularity of their occurrence. Desirable changes provide benefits, while periodic changes follow a predictable cycle like a clock.
Use the mnemonic "DUP" for Desirable (Good), Undesirable (Bad), and Periodic (Repeat).
Provide common natural examples like the seasons or sunset for periodic changes to demonstrate understanding.

Question 4: Is burning a physical change or a chemical change? Why?

Answer:
Burning is a chemical change as new substance is formed with new properties and it cannot be reversed.
When something burns, it reacts with O₂ to produce heat, light, and new substances like CO₂ and ash. Because the original material is chemically altered and cannot be recovered, it is a permanent change.
If you see "smoke" or "ash" being produced, it is almost always a chemical change.
Mention the formation of a "new substance" as the primary reason to justify why a change is chemical.

Question 5: A burning candle shows both physical and chemical changes. Explain ?

Answer:
A burning candle produces wax vapours which solidify and form wax again is physical change. At the same time wax on burning produces water vapours and carbondioxide which escape into the atmosphere which are new substances with new properties. It is a chemical change. Hence burning of candle shows both physical and chemcial changes.
The melting of solid wax into liquid wax is a change of state, which is physical. However, the wick and some wax actually burn to produce CO₂ and H₂O, which is a chemical process.
Remember: Melting is Physical (M-P), and Burning is Chemical (B-C).
For questions asking for an explanation, describe both the melting process and the burning process separately.

Question 6: State three differences between evaporation and boiling

Answer:
Evaporation
1. Evaporation is a slow process.
2. Evaporation takes place from the surface of the liquid.
3. Evaporation takes place at all temperature.
Boiling
1. Boiling is a fast process.
2. Bgiling takes place from all parts of the liquid.
3. Boiling takes place at a fixed temperature on heating.
Evaporation is a gentle process that happens naturally at the surface of a liquid at any temperature. Boiling is a vigorous process where bubbles form throughout the liquid once it reaches a specific boiling point.
Think of a puddle drying (Evaporation - Surface) versus a pot of soup bubbling (Boiling - All parts).
Always mention "temperature" and "surface vs bulk" when comparing these two phase changes.

Question 7: State four differences between physical and chemical changes.

Answer:
Physical change
1. No new substances with new properties are formed.
2. It can be reversed by simple physical methods.
3. It is temporary change.
4. Energy is neither needed nor produced.
Chemical change
1. New substances with new properties are formed.
2. Change cannot be reversed by simple physical methods.
3. It is permanent change.
4. Energy is either needed or evolved.
Physical changes only involve a change in appearance or state, while chemical changes involve the making or breaking of bonds. In chemical changes, energy like heat or light is often a noticeable byproduct or requirement.
"P" stands for Physical and "Plain" (nothing new), while "C" stands for Chemical and "Created" (new substance).
Use a column-based comparison and number your points clearly to make it easy for the examiner to read.

Question 8: What do you observe when :
1. water is boiled
2. a piece of paper is burnt
3. some ice cubes are kept in a glass tumbler
4. solid ammonium chloride is heated
5. an iron nail is kept in tap water for few days
6. a spoon of sugar is heated in a pan
7. lighted match stick is brought near the mouth of the test tube containing hydrogen gas.
8. quick lime is dissolved in water.
9. liittle ammount of curd is added to a bowl containing warm milk and kept for five hours.

Answer:
1. On boiling water changes into steam (gas) physical change.
2. On burning piece of paper produces carbon dioxide and ash is left behind. Is a chemical change.
3. Ice cubes (solid) turn into water (liquid) only state changes (physical change).
4. Solid ammonium chloride on heating changes into vapours (change of state) is physical change.
5. We observe reddish brown coating on the nail called rust (entirely new substance) is chemical change.
6. When a spoon of sugar is heated in a pan, black (charred sugar) (carbon) is seen. Is a chemical change.
7. We observe that hydrogen bums at the mouth of test tube with blue flame and pop sound is heard. It is chemical change.
8. The following two observations will be observed (i) A hissing sound is observed. (ii) The mixture starts boiling and lime water is obtained.
9. When a little amount curd is added to a bowl containing warm milk and kept for five hours, a permanent change occured. The milk will change to curd. On boiling water changes into steam (gas) physical change.
Observations are the sensory clues that tell us if a reaction is happening, such as color changes, sounds, or gas production. For example, the "pop" sound of H₂ gas is a classic sign of a chemical reaction involving a gas.
Associate "Rust" with Oxygen/Water and "Charred Sugar" with Carbon.
Always state both the observation (what you see/hear) and the type of change (physical or chemical) for full credit.

Question 9: Name a chemical change which takes place in presence of:
Heat:
Light :
Electricity:

Answer:
Heat:
1. Burning of paper.
2. Cooking of food need heat.
Light :
1. Formation of food by plants i.e. photosynthesis need light.
2. Hydrogen and chlorine react in presence of light
Electricity:
1. Water breaks into hydrogen and oxygen on passing electricity.
2. Sodium chloride solution breaks up into its components sodium and Chlorine on passing electric current through sodium chloride solution.
Many chemical reactions require an external source of energy to break existing chemical bonds and start the reaction. Electricity can decompose compounds like H₂O into its basic elements H₂ and O₂ through electrolysis.
Think of energy as the "key" that unlocks a chemical reaction.
Memorize "Photosynthesis = Light" and "Electrolysis = Electricity" as standard examples.

Objective Type Questions

Question 1: Fill in the blanks.

Answer:
1. A new substance is always formed in a chemical change.
2. Melting of ice is a physical change.
3. When a candle burns, wax melts. Melting of wax is a physical change.
4. Chemical change occurs as a result of rearrangement of atoms between two substances.
5. Burning of a fuel is a chemical change.
6. Revolution of the earth around the sun is a natural periodic change.
7. Growing of a seedling into a plant is chemical change.
These blanks highlight the fundamental differences between types of changes in nature. Knowing that atoms rearrange during chemical changes helps explain why new substances with different properties are created.
Chemical = New, Physical = Same substance.
Double-check that your answers fit the grammar of the sentence provided.

Question 2: State whether the following are physical or chemical changes.

Answer:
1. glowing of a bulb - physical change,
2. burning of sugar - chemical change,
3. heating of water - physical change,
4. growing of a piglet into a pig - chemical change,
5. burning of wood - chemical change,
6. passing electric current through a heater rod - physical change.
7. water cycle in nature - physical change,
8. respiration in living beings - chemical change,
9. shaping a piece of glass - physical change,
10. lightning - chemical change,
11. (energy produced) dissolving sugar in water - physical change,
12. heating a mixture of iron filings and sulphur - chemical change.
13. mixing oil with water - physical change.
14. cutting wood into small pieces - physical change,
15. photosynthesis - chemical change.
16. Digestion of food - chemical change.
17. Melting of wax - physical change.
18. Boiling of an egg - chemical change.
19. Slaking of lime - chemical change.
Biological processes like growth and digestion are chemical because they involve complex molecular transformations. Simple mechanical actions like cutting or mixing are physical because the substance's identity remains unchanged.
If you can "undo" it easily (like mixing oil and water), it is physical.
Pay close attention to biological processes; they are almost always chemical because they are permanent and produce new compounds.

Question 3: Match the following:

Answer:
(Note: The answer grid for this section is provided in the "Match the columns" section below based on the textbook key).
This section requires connecting concepts like "irreversible change" to "souring of milk." Identifying these pairs reinforces the definition of each term through practical examples.
Match the easy ones first, then use the process of elimination for the rest.
Draw clean lines or write the corresponding letter next to the number to avoid confusion.

Question 4: Write true or false against the following statements :

1. Cutting of paper into pieces is a chemical change.
Answer: False

2. Rusting of iron is a chemical change.
Answer: True

3. Earthquake is a desirable change.
Answer: False

4. Melting of ice is a physical change.
Answer: True

5. Burning of sugar is a temporary change.
Answer: False
Rusting is a chemical process where iron reacts with O₂ and moisture to form iron oxide. Cutting paper is physical because every small piece is still chemically just paper.
Remember: If you can't glue it back together to be "exactly" like the original, it's usually irreversible, but not always chemical.
Read every word of the statement carefully; one word like "temporary" can make a true fact false.

Multiple Choice Questions

Question 1: A substance which can not sublime is
1. iodine
2. camphor
3. sugar
4. dry ice

Answer:
3. sugar
Sublimation is a physical change where a solid turns directly into a gas without becoming a liquid. Iodine, camphor, and dry ice all have this property, but sugar will melt or caramelize before it ever turns into a gas.
Recall the "Sublimation Squad": Iodine, Camphor, Naphthalene, and Dry Ice.
Eliminate the substances you know are common sublimates to find the correct answer.

Question 2: When you put some ice cubes in a glass, droplets of water are formed on the outerwall of the glass. This explains the phenomenon of
1. melting
2. freezing
3. condensation
4. evaporation

Answer:
3. condensation
Water vapor in the air touches the cold surface of the glass and loses energy, turning back into liquid water. This is a physical change of state that occurs when gas cools down.
Condensation is the opposite of Evaporation.
Remember that the water comes from the air outside the glass, not from inside the glass leaking out.

Question 3: Burning is a
1. slow process
2. fast process
3. natural process
4. none of the above

Answer:
2. fast process
Burning, or combustion, happens rapidly and releases energy in the form of heat and light. Unlike rusting, which is a slow chemical change, burning consumes the fuel very quickly.
Fire is fast!
Compare the speed of the reaction to others; "fast" usually means it happens in seconds or minutes.

Question 4: Which one of the following is volatile in nature ?
1. common salt
2. petrol
3. water
4. milk

Answer:
2. petrol
Volatile substances evaporate very quickly at room temperature because their molecules are not strongly held together. Petrol is highly volatile, which is why you can smell it immediately when a tank is opened.
Volatile = "Vanishes" quickly into the air.
Look for substances that have a strong smell; they are often the most volatile.

Question 5: An example of both physical and chemical change is
1. burning of candle
2. melting of ice
3. cooking of food
4. blowing of bulb

Answer:
1. burning of candle
As discussed, a candle demonstrates both types of changes because the wax both melts (physical) and burns (chemical). This is a unique classroom example used to show that different processes can happen simultaneously.
Candle = Two Changes in One.
When you see "both", always look for "burning candle" or "digestion" in your options.

Question 6: The compound rust is a hydrated oxide of
1. copper
2. aluminium
3. iron
4. gold

Answer:
3. iron
Rusting is a specific chemical reaction that only occurs with iron or alloys containing iron when exposed to O₂ and water. The chemical name for rust is hydrated iron(III) oxide.
Rust always involves Iron (Fe).
Associate the reddish-brown color of rust with the metal iron.

Question 7: When sugar is heated, its colour changes into
1. red
2. brown
3. black
4. grey

Answer:
3. black
Heating sugar causes it to decompose, leaving behind a black residue which is pure carbon. This is a chemical change because the sweet white sugar has turned into a bitter black solid.
Sugar turns to "Charcoal" (Carbon) when burnt.
Remember that the color change is a signal that the chemical bonds have been broken.

Question 8: A pop sound is heard when a lighted match stick is brought near the mouth of a jar. This indicates the release of gas.
1. oxygen
2. hydrogen
3. nitrogen
4. water-vapour

Answer:
2. hydrogen
Hydrogen gas is highly flammable and reacts explosively with oxygen in the air, creating a distinctive "pop" sound. This is the standard laboratory test used to identify the presence of H₂ gas.
Hydrogen goes "Pop!".
Memorize specific gas tests: H₂ is "pop", CO₂ turns lime water milky.

Question 9: When we add water to the following substances, which one will show a chemical change ?
1. salt
2. sugar
3. oild
4. quick lime

Answer:
4. quick lime
When water is added to quick lime (CaO), a vigorous reaction occurs that produces heat and forms slaked lime. Unlike dissolving salt or sugar, this reaction actually changes the molecules of the substance.
Quick lime "hisses" when it meets water.
Look for reactions that produce heat (exothermic) to identify chemical changes upon mixing.

Additional Questions
Check Your Progress 1

Question 1: Is melting of ice a reversible change ?

Answer:
True
Melting is a physical change of state from solid to liquid that can be easily undone by freezing the water. This means the substance can return to its original form, making it a perfectly reversible process.
Melting and Freezing are reversible partners.
If a change of state is involved, it is almost always a reversible physical change.

Question 2: Change of seasons is a non-periodic change. True or false ?

Answer:
False. Change of seasons is a periodic change.
Seasons follow a regular cycle every year based on the Earth's orbit around the Sun. Because they repeat at predictable intervals, they are classified as periodic changes.
Nature is like a clock; if it repeats, it's periodic.
The word "non" in "non-periodic" means "does not repeat," so look for cycles to prove it wrong.

Question 3: In a physical change no net energy change is involved. True or false ?

Answer:
True
In a physical change, energy may be absorbed or released temporarily to change states, but the chemical energy stored in bonds stays the same. There is no large-scale energy evolution or absorption like you see in explosions or burning.
Physical = Low Energy impact.
Use the lack of heat/light production to identify most physical changes.

Question 4: New substances are formed in a chemical change. True or false ?

Answer:
True
This is the defining characteristic of a chemical change: the original atoms are rearranged to create new molecules. These new substances have entirely different physical and chemical properties compared to the reactants.
Chemical = Change of identity.
Always use the "New Substance" test to verify if a change is chemical.

Check Your Progress 2

Question 1: Dissolving of a solid in liquid is a physical change. True or false ?

Answer: True
When you dissolve a solid like salt in water, the salt particles just spread out among the water molecules without forming new chemical bonds. You can get the salt back exactly as it was by evaporating the water.
Dissolving is just "hiding" the solid in the liquid.
Remember that dissolving is reversible by evaporation or distillation.

Question 2: Is the process of digestion a physical change or chemical change ?

Answer: Chemical
Digestion uses enzymes and acids to break down food into different chemical nutrients that the body can absorb. Once food is digested and turned into energy or waste, it can never be turned back into the original food item.
Digestion = Destruction of old molecules, Creation of new ones.
Biological processes inside the body are almost exclusively chemical changes.

Question 3: During a physical change energy is either given out or absorbed by the system. True or false ?

Answer: True
While there is no "net" permanent chemical energy change, energy is often absorbed (like melting ice) or released (like condensing steam). These energy transfers are necessary to overcome or allow the forces of attraction between molecules.
Energy moves, but the "identity" stays.
Distinguish between energy used for "changing state" and energy used for "changing bonds."

Question 4: In a chemical change, rearrangement of molecules and atoms involves some energy changes. True or false ?

Answer: True
Chemical bonds contain potential energy, and breaking them or forming new ones requires or releases that energy. This is why chemical reactions often feel hot (exothermic) or cold (endothermic).
Bonds = Energy storage.
A change in temperature is one of the most common signs of a chemical reaction.

Question 5: Energy in the form of is absorbed during photosynthesis.

Answer: Energy in the form of sunlight is absorbed during photosynthesis.
Plants use the energy from light to convert H₂O and CO₂ into glucose (food) and O₂. This process is a prime example of a chemical change driven by external energy.
Photo = Light, Synthesis = Putting together.
Think of the word "Photo" in "Photosynthesis" to remember that light is the required energy source.

Exercises

A. Tick the most appropriate answer.

Question 1: Melting of ice is a

Answer:
1. reversible change,
Ice can be melted into water and then refrozen back into ice as many times as you like. Since the substance (H₂O) remains the same throughout, it is a physical and reversible process.
Ice to water and back again is a loop.
If you can "undo" it, choose "reversible."

Question 2: Rising and setting of the sun at a given place is a

Answer:
1. periodic change.
The sun rises and sets every 24 hours due to the rotation of the Earth. Because this event happens at regular, predictable intervals, it is the definition of a periodic change.
The Sun is nature's most reliable clock.
Identify periodic changes by looking for patterns that repeat on a schedule.

Question 3: Physical changes are

Answer:
3. temporary
Physical changes like melting, freezing, or dissolving are considered temporary because they can be reversed. The substance does not permanently lose its identity and can be restored to its original state.
Physical = Passing through (Temporary).
Link the word "Physical" with "Temporary" in your mind for quick multiple-choice answers.

Question 4: Chemical changes involve

Answer:
3. change in molecular composition.
In a chemical change, the very structure of the molecules is altered as atoms are rearranged into new combinations. This is why the properties of the result are so different from the starting materials.
Molecules change their "recipe" in a chemical reaction.
Focus on the "inside" (molecules) for chemical changes and the "outside" (shape/size) for physical ones.

Question 5: Photosynthesis requires

Answer:
3. sunlight.
Chlorophyll in green plants captures energy from sunlight to power the chemical reaction that creates food. Without this specific light energy, plants cannot synthesize glucose from carbon dioxide and water.
Plants are solar-powered!
Remember that photosynthesis is a "light-dependent" chemical reaction.

B. Fill in the blanks.

Answer:
1. A change in which the substance can be brought back to its original state is called a reversible change.
2. Changes that occur in nature are called Natural changes.
3. An earthquake is a non-periodic change.
4. A new substance is formed in a chemical change.
5. Heat is released by a gas during the process of condensation.
Reversible changes are like a "two-way street" where you can travel back to the start. Natural changes happen on their own without human intervention, like a volcanic eruption or a tree growing.
"Periodic" repeats; "Non-periodic" (like an earthquake) happens randomly.
Remember that condensation is a "cooling" process, so the gas must lose its heat (release it) to become a liquid.

C. Match the columns.

Based on the image provided, here is the text transcription of the matching exercise: 
1.  A change which cannot be easily reversed
2. A change that repeats itself at regular intervals
3. A change that is made by humans
4. A change in which the original substance cannot be obtained
5. A change where no new substance is formed Column
2 a. man-made change
b. chemical change
c. physical change
d. periodic change
e. irreversible change f. slow change

Answer:
1. A change which cannot be easily reversed - e. irreversible change
2. A change that repeats itself at regular intervals - d. periodic change
3. A change that is made by humans - a. man-made change
4. A change in which the original substance cannot be obtained - b. chemical change
5. A change where no new substance is formed - c. physical change
These matches define the core vocabulary of the chapter by linking terms to their primary characteristics. For example, "man-made" refers to human activity, like building a house or making plastic.
Match based on the "keyword" (e.g., humans = man-made).
Check that every item on the left has one corresponding item on the right; don't leave any blanks.

D. Write true or false for each statement. Rewrite the false statements correctly.

Question 1: Burning of paper and formation of curd are reversible changes.

Answer:
False. Burning of paper and formation of curd are irreversible changes.
Once paper is turned into ash and smoke, or milk is turned into curd by bacteria, you cannot get the original paper or milk back. These are permanent chemical transformations.
Ash can't become paper again!
When rewriting, only change the word that makes the statement false to keep your answer concise.

Question 2: A natural change can also be a desirable change.

Answer:
True
Many natural events are helpful to us, such as rain for crops or the ripening of fruit on a tree. Since these happen in nature and benefit humans, they are both natural and desirable.
Rain is natural and definitely desirable for farmers.
A single change can have multiple "labels" (e.g., natural, desirable, and chemical).

Question 3: Deforestation is a man-made change.

Answer:
True
Deforestation involves humans cutting down forests for timber, land, or agriculture. Because it is a result of human choices and actions, it is classified as a man-made change.
Humans hold the axe, so it's man-made.
Look for "human activity" as the cause to determine if something is man-made.

Question 4: Decomposition of leaves is a fast change.

Answer:
False. Decomposition of leaves is a slow change.
Leaves take weeks or even months to break down into compost through the action of fungi and bacteria. This is a very gradual chemical process compared to fast changes like explosions.
Rotting takes time.
Compare the time it takes for a change to complete: seconds = fast, days/months = slow.

Question 5: Dissolving of solute in a solvent and rusting of iron are both physical changes.

Answer:
False. Dissolving of solute in a solvent is a physical change and rusting of iron is a chemical change.
Dissolving can be undone by evaporation, but rusting creates a brand new substance (iron oxide) that cannot be turned back into pure iron easily. Therefore, one is physical and the other is chemical.
Rust is a "new thing," but dissolved salt is just "hidden salt."
When a statement combines two different concepts, evaluate each one separately.

E. Give reasons for the following.

Question 1: Dissolving salt in water is a reversible change.

Answer:
Dissolving salt in water is a reversible change because salt and water can be again obtained by distillation process.
Distillation or evaporation allows the water to be separated from the salt, leaving the salt crystals behind in their original form. This proves the substance was never fundamentally changed.
Evaporation is the "undo" button for dissolving.
Always mention the process (like evaporation) used to reverse the change when giving a reason.

Question 2: Ripening of fruits is a desirable change.

Answer:
Ripening of fruits is a desirable change because this change is beneficial for the mankind and we desire to happen.
Ripe fruits are sweet, nutritious, and ready to eat, which directly benefits our health and diet. Because we want this change to occur so we can eat the fruit, it is labeled "desirable."
Desirable = "We want it to happen."
Focus on the "benefit to humans" to explain why a change is desirable.

Question 3: Melting of solid (wax) is a physical change.

Answer:
Melting of solid wax is a physical change because no new substance is formed during this change.
The liquid wax is chemically the same as the solid wax; it just has more space between its molecules. It can also be easily turned back into a solid by cooling it down.
Liquid wax is still just wax!
Use the "No New Substance" rule to justify any physical change.

Question 4: Chemical changes are accompanied by changes in energy.

Answer:
In a chemical change, molecules and atoms rearrange themselves to form new substances. This rearrangement involves some energy changes. It can either be absorption or evolution of energy.
Forming new chemical bonds usually releases energy, while breaking old bonds requires energy. This energy is most commonly felt as heat, such as the warmth of a fire or the cold of an instant ice pack.
Energy is the "glue" that holds atoms together.
Mention the "breaking and making of bonds" to give a more scientific explanation.

Question 5: Photosynthesis cannot take place at night.

Answer:
Photosynthesis requires sunlight which is not available at night time. Therefore the process stops at night.
Sunlight provides the specific energy needed for the chemical reaction to turn CO₂ and H₂O into food. Without this energy source, the plant's "food factory" has no power to run.
No Sun = No Energy = No Food Production.
Relate the requirement of energy to the time of day to explain why it stops.

F. Write short answers.

Question 1: Name two reversible changes.

Answer:
Melting of ice.
Dissolving of salt in water.
Both of these changes are physical and can be undone to get back the original substances. They are classic examples used to teach the concept of temporary changes.
Think of things you can "un-mix" or "un-melt."
Use simple examples that are easy to visualize and explain.

Question 2: Give two examples of periodic changes.

Answer:
Swinging of a pendulum
(ii) Rising and setting of the sun.
Periodic changes follow a set rhythm or schedule and happen again and again. These examples show how physics and astronomy provide us with predictable cycles.
Periodic = Predictable.
Think of things that happen on a "timer" in nature.

Question 3: Give an example of desirable change which can also be undesirable.

Answer:
Using a car to cover a long distance is a desirable change but car emissions can lead to air pollution which is undesirable.
While traveling quickly is a benefit (desirable), the chemical pollution produced as a byproduct hurts the environment (undesirable). This shows how one action can have both positive and negative consequences.
Context matters: good for the driver, bad for the air.
Explain the "benefit" versus the "harm" to clearly show why it fits both categories.

Question 4: What are man-made changes ?

Answer:
Man-made changes are those that occur because of human activities e.g. changing the direction of flow of a river by building a dam.
These changes do not happen naturally and require human intervention, technology, or effort. They are often done to benefit human society or infrastructure.
Human hand = Man-made.
Mention "human activity" as the key driver of these changes.

Question 5: Sublimation is classified as a physical change. Why ?

Answer:
Physical changes are those in which no new substance is formed. In sublimation there is direct conversion of solid into gas and vice versa. Thus the substance remains the same.
Because the substance (like iodine or camphor) is the same chemical whether it is a solid or a gas, only its state has changed. It can also be easily turned back into a solid by cooling.
Solid to Gas is still the SAME substance.
Use the phrase "no new substance is formed" as your primary evidence.

G. Answer in detail.

Question 1: Classify the following changes as physical or chemical.
1. boiling - Physical
2. curdling of milk - Chemical
3. photosynthesis in plants - Chemical
4. melting of a solid (wax) - Physical

Answer:
1. boiling - Physical
2. curdling of milk - Chemical
3. photosynthesis in plants - Chemical
4. melting of a solid (wax) - Physical
Boiling and melting are state changes where the internal structure remains the same. Curdling and photosynthesis are complex chemical reactions that produce new substances that cannot be easily reversed.
Check: Can I get the original back easily? If yes, physical. If no, chemical.
Use a list format to make the classification clear and easy to grade.

Question 2: A physical change is temporary. Explain.

Answer:
A physical change is defined as a change in which no new substance is formed. A physical change is temporary because this change can be easily reversed by reversing the conditions. For example when ice is melted, it gets converted into water by absorbing heat. Water so formed can be easily converted to ice by cooling thus heat is released. Thus physical change is temporary.
The atoms inside the substance do not rearrange to form new molecules; they just change how close they are to each other. Because the molecules are still the same, you can always go back to the beginning by changing the temperature or pressure.
Temporary = Not Forever.
Use the example of H₂O (Ice and Water) to illustrate the ability to return to the original state.

Question 3: State the differences between a physical change and a chemical change.

Answer:
Property: Formation of new substance
Physical change: There are no new substances formed in a physical change.
Chemical Change: New substances with different properties are formed in a chemical changes.
Property: Nature of change
Physical change: It is usually easy to reverse the process, thereby regain -ing the original substance.
Chemical Change: It is usually irreversible.
Property: Energy
Physical change: Generally energy is neither absorbed nor evolved.
Chemical Change: Considerable heat energy is either absorbed or evolved when a chemical change takes place.
Property: Mass
Physical change: There is no change in the mass of the substance after a physical change.
Chemical Change: Mass of a substance changes after a chemical change.
Physical changes are like changing clothes; chemical changes are like turning into a different person entirely. While physical changes keep the mass and chemistry the same, chemical changes often lead to measurable changes in mass if gases are released or absorbed.
Think "Inside vs Outside" - Physical is outside, Chemical is inside.
Categorizing differences by "Property" (Mass, Energy, etc.) makes your answer very professional and complete.

Question 4: State the various conditions that favour the process of evaporation.

Answer:
The various conditions that favour the process of evaporation are discussed as under :
1. Concentration of water vapour in air: (humidity level) : If the water vapour concentration is high in air the evaporation process gets slowed down.
2. Pressure : Evaporation takes place faster when pressure is less and vice versa.
3. Surface Area: If the surface area is large then evaporation takes place faster.
4. Temperature : On increasing the temperature, the rate of evaporation increases.
5. Rate offlow of air : If air is moving over the place all the time, then the concentration of water vapour decreases in the surroundings, thus increasing evaporation rate.
Evaporation is affected by how much room there is in the air for more water molecules and how much energy the liquid has. Higher heat gives molecules more speed to "escape" into the air, while a dry breeze carries away the molecules that have already left.
Think of drying a wet shirt: Heat (Sun), Wind (Breeze), and spreading it out (Surface Area) help!
Use bullet points to list the factors and give a one-sentence explanation for why each factor matters.

Question 5: How do you say that the process of digestion is a chemical change ?

Answer:
During digestion new substances are formed which cannot be converted back to the same food which we have eaten. Besides energy is also released. These factors prove that digestion is a chemical change.
In your stomach, large food molecules are broken down into simpler nutrients like glucose through chemical reactions with enzymes. This process is permanent and fundamentally changes the identity of the substance, which is the hallmark of a chemical change.
Can a burger become a burger again after you eat it? No, so it's chemical.
Mention "formation of new substances" and "irreversibility" to prove it is a chemical change.

Question 6: What role does energy play in initiating a chemical change ?

Answer:
In a chemical change molecules and atoms rearrange themselves to form new substances. This process involves some energy changes. In some changes energy is absorbed e. g. burning of a substance and in some cases energy is evolved e.g. photosynthesis.
Energy is required to break the strong chemical bonds in reactants so that they can form new bonds in products. Without this initial "kick" of energy (like the flame of a match), many reactions simply would not start.
Energy is the "spark" that starts the fire.
Explain that energy is needed to "rearrange atoms" to show a deeper understanding of chemistry.

H. Crossword Puzzle

Question 1: Solve this crossword by using the clues that follow

Across
1. Energy in the form of sunlight is absorbed by the green plants in this process.
4. The reddish-brown substance formed over iron in the presence of oxygen and moisture.
5. This change is permanent and irreversible.
6. The process in which a solid directly changes into gaseous state.

Down
1. It is the change that takes place in case of swinging pendulum of a clock. -
2. Occurrence of this is a non-periodic change as well as a natural change.
3. It is a physical change.

Answer:
Across:
1. PHOTOSYNTHESIS
4. RUST
5. BURNING
6. SUBLIMATION
Down:
1. PERIODIC
2. STORM (or Earthquake/Natural based on grid)
3. EVAPORATION
Crosswords are a great way to review all the key terms from the chapter in a fun format. Each word connects to another, reinforcing the relationship between different types of changes.
Count the number of boxes in the grid to make sure your answer word fits perfectly.
Use the intersecting letters to help you figure out the more difficult words.

Question 2: Make a list of different types of changes with example.

Answer:
1. Reversible Change e.g. Melting of ice.
2. Irreversible Change e.g. Burning of paper.
3. Periodic Change e.g. Swinging of pendulum.
4. Non-Periodic Change e.g. Occurrence of floods.
5. Desirable Change e.g. Ripening of fruits.
6. Undesirable Change e.g. Rusting of Iron.
7. Natural Change e.g. Changing of Seasons
8. Man-Made Change e.g. Changing the direction of flow of a river by dams.
9. Slow Change e.g. Growth of a plant
10. Fast Change e.g. Occurrence of lightning during thunderstorm.
11. Physical Change e.g. transformation of ice.
12. Chemical Change e.g. curdling of milk.
This comprehensive list summarizes every category of change covered in the textbook. By providing an example for each, we see how these abstract terms apply to the real world around us.
Learn these in pairs (e.g., Fast/Slow, Physical/Chemical) to remember them more easily.
Always provide a unique, clear example for each category to show that you understand the difference between them.