Selina Concise Solutions for ICSE Class 8 Chemistry Chapter 2 Physical and Chemical Changes

Points to Remember:

• All changes are classified into two types
  (i) Physical change (ii) Chemical change.
• 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.
• 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.
• Chemical Reaction- Any chemical change in matter involving its transformation into one or more new substances is called a chemical reaction.
• 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.
• The substances that react with one another are called reactants, and the new substances thus formed are called products.
• A balanced chemical reaction is one in which the number of the atoms of each element on the reactant side is equal to the number of atoms of that element on the product side.
• The law of conservation of mass states that mass can neither be created nor be destroyed.
• A chemical equation gives both qualitative and quantitative information about reactants and products.
• The type of chemical reaction in which two substances combine to form a new substance is known as combination reaction.
• 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.

Activity 2
Take a carrot and cut it into pieces.
Now answer the following questions:

Question 1: Is the change temporary?
Answer: No.
Even though it is a physical change, we cannot join the cut pieces back to make a whole carrot. This shows that some physical changes cannot be easily reversed to their original state.
Teacher's Tip: Remember that "physical" refers to the substance's identity, not just if you can undo the action.
Exam Tip: Use the term "irreversible physical change" for actions like cutting or breaking.

Question 2: Is the change reversible?
Answer: No.
Once the carrot is sliced into pieces, you cannot perform a simple physical action to turn it back into a single, whole carrot. The shape and size have been permanently altered even though the carrot material remains the same.
Teacher's Tip: Think of a broken glass; it is still glass, but you cannot easily "un-break" it!
Exam Tip: Always check if the original form can be restored to determine reversibility.

Question 3: Why is cutting of carrot into pieces a physical change?
Answer: Because there is no change in the property or chemical composition of the original substance i.e., carrot, and change is only in the state, size, shape, colour, texture or the smell of some or all of the substances that undergo physical change.
The carrot pieces still taste like carrot and have the same vitamins and minerals as the whole carrot. No new substance like ash or gas was created during the cutting process.
Teacher's Tip: If the "DNA" or chemical formula of the object hasn't changed, it's a physical change.
Exam Tip: Mention "no new substance is formed" to get full marks for physical change definitions.

Exercise

Question 1: Define: (a) a physical change, (b) a chemical change.
Answer:
(a) Physical Change: A physical change is a temporary change in which no new substance is formed and the chemical composition of the original substance remains the same, even though its physical properties like colour, state, shape, size etc. might change.
(b) Chemical Change: A chemical change is permanent change in which new substances are formed whose chemical composition and physical and chemical properties are different from those of in original substance.
Physical changes usually involve changes in state or appearance, while chemical changes involve breaking and making bonds to create something entirely new. For example, melting ice is physical, but rusting iron is chemical.
Teacher's Tip: Physical = Same Stuff; Chemical = New Stuff!
Exam Tip: Always provide an example like "Melting of ice" or "Burning of paper" when defining these terms.

Question 2: Classify the following as a physical or a chemical change. (a) Drying of wet clothes (b) Manufacture of salt from sea water (c) Butter getting rancid (d) Boiling of water (e) Burning of paper (f) Melting of wax (g) Burning of coal (h) Formation of clouds (i) Making of a sugar solution (j) Glowing of an electric bulb (k) Curdling of milk
Answer:
Physical change
(a) Drying of wet clothes
(b) Manufacture of salt from sea water
(d) Boiling of water
(f) Melting of wax
(h) Formation of clouds
(i) Making of a sugar solution
(j) Glowing of an electric bulb
Chemical change
(c) Butter getting rancid
(e) Burning of paper
(g) Burning of coal
(k) Curdling of milk
Physical changes in this list involve changes of state like evaporation or melting where the substance doesn't turn into something else. Chemical changes like burning or curdling result in new substances that cannot be turned back easily.
Teacher's Tip: If you see the word "burning" or "cooking," it is almost always a chemical change.
Exam Tip: Create a T-chart in your exam paper to organize classification questions clearly.

Question 3: Fill in the blanks.
Answer:
(a) The process of a liquid changing into a solid is called freezing.
(b) A change, which alters the composition of a substances, is known as a chemical change.
(c) There is no change in the composition of the substance during a physical change.
(d) The reaction in which energy is evolved is called exothermic reaction.
Freezing is a change of state from liquid to solid, like water turning to ice. Exothermic reactions are special because they release heat or light into the surroundings.
Teacher's Tip: "Exo" means exit (heat exits), and "composition" means what something is made of.
Exam Tip: Double-check your spelling for technical terms like "composition" and "exothermic."

Question 4: Given reason:
(a) Freezing of water to ice and evaporation of water are physical changes.
(b) Burning of a candle is both a physical and chemical change.
(c) Burning of paper is a chemical change.
(d) Cutting of a cloth piece is a physical change, though it cannot be reversed.
Answer:
(a) Freezing of water to ice and evaporation of water are physical change because water can be brought back to its original (liquid) form by 1. We can heat the ice to bring it back to water. 2. We can cool down the vapours to bring it back to water.
(b) When a candle is lighted, some of the solid wax first melts and turns into liquid, then it turns into vapours to produce a flame. New substances CO₂ and H₂O vapours are formed alongwith the evolution of light and heat energy. This shows a chemical change. When some of the molten wax drops to the floor, it again solidifies. Which shows a physical change. Thus the melting of candle wax is a physical change and the production of CO₂ and H₂O represents chemical change.
(c) When a piece of paper is burnt a new substance ash is produced. Even when the burning is stopped, the ash cannot be changed back into paper. This shows that the formation of the ash from paper is a permanent and irreversible change.
(d) Because it does not change chemical composition of cloth and the change is only in the state, size, shape, colour, texture or the smell of some or all of the substances that undergo physical change.
These reasons highlight that physical changes focus on state and appearance while chemical changes focus on internal structure. Even if a change like cutting cloth is permanent, it is physical because the material itself remains cotton or silk.
Teacher's Tip: For the candle question, remember: Melting = Physical, Burning = Chemical.
Exam Tip: When giving reasons, always mention whether a "new substance" is formed or not.

Question 5: Give four difference between physical and chemical changes.
Answer: The differences are Physical and Chemical Changes:
Physical change
1. In a physical change no new substance is formed and the chemical composition of substance remains same. There are changes only in physical properties and state.
2. Temporaiy change which can be reversed by simple physical methods.
3. Weight of original substance doesn’t change
4. Energy like heat, light etc. may or may not be absorbed or released
Chemical change
1. In a chemical change new substance with entirely different chemical composition and properties is formed.
2. Permanent change and irreversible
3. Weight of original substances may increase or decrease
4. Energy like heat, light etc. are given out or absorbed.
Physical changes are generally easy to undo, like melting and refreezing chocolate. Chemical changes result in a total transformation, like baking a cake where you can't get the eggs and flour back.
Teacher's Tip: Use the "New Substance" rule as your primary way to distinguish the two.
Exam Tip: Present differences in a table format for better clarity and higher marks.

ADDITIONAL QUESTIONS

Check Your Progress 1

Question 1: Is melting of ice a reversible change ?
Answer: True
Melting is reversible because you can turn the resulting water back into ice by freezing it. This cycle can be repeated many times without changing the water molecules.
Teacher's Tip: Think of "reversible" as a two-way street!
Exam Tip: Always explain "why" it is true by mentioning the reverse process (freezing).

Question 2: Change of seasons is a non-periodic change. True or false ?
Answer: False. Change of seasons is a periodic change.
Seasons occur at regular intervals every year because of the Earth's orbit around the Sun. This makes them predictable and periodic, unlike an earthquake.
Teacher's Tip: Periodic changes happen like a clock—on a regular schedule!
Exam Tip: If a statement is False, always provide the corrected statement to earn full credit.

Question 3: In a physical change no net energy change is involved. True or false ?
Answer: True
While energy is used to melt ice or boil water, it is usually released back when the process is reversed. No large amount of energy is permanently stored or released from chemical bonds.
Teacher's Tip: "Net" means the total balance after everything is finished.
Exam Tip: Use this point when comparing physical and chemical changes in long answers.

Question 4: New substances are formed in a chemical change. True or false ?
Answer: True
This is the most important rule of chemistry: chemical changes create entirely different materials. For example, when hydrogen and oxygen combine chemically, they form water.
Teacher's Tip: No new substance? Physical! New substance? Chemical!
Exam Tip: This is the standard definition you should use in all related questions.

Check Your Progress 2

Question 1: Dissolving of a solid in liquid is a physical change. True or false ?
Answer: True
When you dissolve salt in water, the salt is still there; you can get it back by evaporating the water. No new chemical compounds are created in the solution.
Teacher's Tip: If you can get the solid back by drying it out, it's a physical change.
Exam Tip: Evaporation and distillation are common ways to prove dissolving is physical.

Question 2: Is the process of digestion a physical change or chemical change ?
Answer: Chemical
Digestion involves enzymes breaking down food into nutrients that the body can absorb. These nutrients are chemically different from the food we originally ate.
Teacher's Tip: Digestion involves "acid" and "enzymes," which are clues for chemical reactions!
Exam Tip: Remember that while chewing is physical, the stomach process is chemical.

Question 3: During a physical change energy is either given out or absorbed by the system. True or false ?
Answer: True
Even though there is no "net" change, energy is still moving; for example, ice must absorb heat to melt. Similarly, water releases heat to the environment when it freezes.
Teacher's Tip: Heat moving in or out is why we feel cold or hot near ice or steam.
Exam Tip: Don't confuse "energy involvement" with "new substance formation."

Question 4: In a chemical change, rearrangement of molecules and atoms involves some energy changes. True or false ?
Answer: True
Chemical bonds must be broken and new ones must be formed, which always requires or releases energy. This is why many chemical reactions get hot or produce light.
Teacher's Tip: Rearranging atoms is like taking apart LEGOs and building something new!
Exam Tip: Mention "bond breaking and forming" when discussing energy in chemical changes.

Question 5: Energy in the form of ........ is absorbed during photosynthesis. Energy in the form of sunlight is absorbed during photosynthesis.
Answer: sunlight.
Plants use the energy from sun rays to convert water and carbon dioxide into food (glucose). This process is a vital chemical reaction for all life on Earth.
Teacher's Tip: "Photo" means light—so photosynthesis literally means "making with light."
Exam Tip: Identify photosynthesis as a chemical change because it produces a new substance (glucose).

EXERCISES

A. Tick the most appropriate answer.

Question 1: Melting of ice is a
1. reversible change
2. irreversible change
3. chemical change
4. none of these
Answer:
1. reversible change
You can melt ice into water and then freeze that water back into ice. Because you can return to the original state, it is classified as reversible.
Teacher's Tip: Changes of state (solid to liquid) are almost always reversible.
Exam Tip: Read all options before picking, as melting is also a "physical change" if that were listed.

Question 2: Rising and setting of the sun at a given place is a
1. periodic change
2. undesirable change
3. physical change
4. reversible change
Answer:
1. periodic change
The sun rises and sets every single day at a predictable time. This repetition at fixed intervals is the definition of a periodic change.
Teacher's Tip: Anything that happens on a schedule (like your school bell) is periodic!
Exam Tip: Distinguish periodic from non-periodic by looking for a regular time pattern.

Question 3: Physical changes are
1. permanent.
2. periodic,
3. temporary
4. irreversible.
Answer:
3. temporary
Most physical changes can be undone by changing conditions like temperature. They do not result in a permanent new identity for the substance.
Teacher's Tip: Temporary means "not for forever."
Exam Tip: Use "temporary" as a keyword when describing physical changes in short answers.

Question 4: Chemical changes involve
1. change in shape.
2. change in size.
3. change in molecular composition.
4. none of these
Answer:
3. change in molecular composition.
In a chemical change, the very molecules that make up the substance are rearranged into new molecules. This is why the properties of the substance change completely.
Teacher's Tip: Composition is the "recipe" of the substance; chemical changes change the recipe!
Exam Tip: Focus on the word "molecular" to identify chemical processes.

Question 5: Photosynthesis requires
1. heat energy.
2. wind energy
3. sunlight.
4. sound energy.
Answer:
3. sunlight.
Sunlight provides the specific type of radiant energy that chlorophyll in leaves can capture. Without this energy source, plants cannot produce their own food.
Teacher's Tip: Remember the Sun is the ultimate source of energy for plants!
Exam Tip: Photosynthesis is a classic example of an endothermic chemical reaction.

B. Fill in the blanks.

Question 1: A change in which the substance can be brought back to its original state is called a reversible change.
Answer: reversible change.
This term describes any process where you can go back to where you started. For instance, stretching a rubber band and letting it go is a reversible change.
Teacher's Tip: If you can "reverse" the action, it's a reversible change!
Exam Tip: Spelling counts; make sure to write "reversible" correctly.

Question 2: Changes that occur in nature are called Natural changes.
Answer: Natural changes.
These are events like rain, growth of a tree, or ripening of fruit that happen without human interference. They are part of the Earth's natural cycles.
Teacher's Tip: If humans didn't do it, it's Natural!
Exam Tip: Examples of natural changes include the phases of the moon and the aging of living things.

Question 3: An earthquake is a non-periodic change.
Answer: non-periodic change.
Earthquakes happen randomly and cannot be predicted on a fixed calendar or schedule. This lack of regular timing makes them non-periodic.
Teacher's Tip: "Non" means not—so non-periodic means no regular time pattern.
Exam Tip: Other examples of non-periodic changes are accidents and volcanic eruptions.

Question 4: A new substance is formed in a chemical change.
Answer: chemical change.
This is the defining characteristic of a chemical reaction, where the starting materials are transformed. For example, when wood burns, it turns into ash and smoke.
Teacher's Tip: If you see "new substance," immediately think "chemical!"
Exam Tip: This fact is the basis for most classification questions in this chapter.

Question 5: Heat is released by a gas during the process of condensation.
Answer: released
When steam turns into liquid water, the gas particles slow down and lose energy. That lost energy is released into the surrounding air as heat.
Teacher's Tip: Condensation makes things warm because the gas "gives away" its heat.
Exam Tip: Remember: Evaporation absorbs heat, but condensation releases it.

C. Match the columns.

Question 1: Match the items in the list below:
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
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
Matching helps connect definitions with their scientific terms. For example, knowing that "man-made" refers to human activity helps identify things like building a bridge as a change.
Teacher's Tip: Match the ones you are 100% sure of first!
Exam Tip: Draw straight lines or write the letters clearly next to the numbers to avoid confusion.

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.
You cannot turn ash back into paper, nor can you turn curd back into fresh milk. These are permanent chemical transformations.
Teacher's Tip: Once it's burnt or soured, you can't go back!
Exam Tip: When rewriting, underline the word you changed (e.g., irreversible).

Question 2: A natural change can also be a desirable change.
Answer: True
Rain is a natural change that is very desirable because it helps crops grow and provides drinking water. The ripening of fruit on a tree is another example.
Teacher's Tip: "Desirable" means something we want to happen.
Exam Tip: Remember that one change can fall into multiple categories (e.g., natural and desirable).

Question 3: Deforestation is a man-made change.
Answer: True
Deforestation happens when humans cut down large areas of forests for timber or land. Because it is caused by human activity, it is classified as man-made.
Teacher's Tip: If humans are the cause, it's man-made.
Exam Tip: Pollution and construction are other common examples of man-made changes.

Question 4: Decomposition of leaves is a fast change.
Answer: False. Decomposition of leaves is a slow change.
It takes many weeks or even months for fallen leaves to rot and turn into compost. Fast changes happen in seconds or minutes, like a firecracker exploding.
Teacher's Tip: Think about how long you have to wait to see the change happen.
Exam Tip: Rusting and growth are also classic examples of slow changes.

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 salt in water doesn't create a new substance, but rusting creates iron oxide, which is a new reddish material. Rusting is permanent, while dissolving can be reversed by evaporation.
Teacher's Tip: Rust is a whole new substance that didn't exist before!
Exam Tip: Be careful with compound statements; even if one part is true, the whole thing might be false.

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.
When you heat the salty water, the water turns to steam and can be collected, while the solid salt crystals stay behind in the container. This proves the two substances were just mixed, not combined into something new.
Teacher's Tip: If you can use heat to separate things, it's likely a reversible physical change.
Exam Tip: Mention the specific process "evaporation" or "distillation" to support your answer.

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 fruit is sweet, soft, and healthy for us to eat, whereas unripe fruit is often hard and bitter. We look forward to this natural process so we can enjoy the harvest.
Teacher's Tip: "Desirable" is all about whether humans like the result!
Exam Tip: Always relate "desirable" to human benefit or usefulness.

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.
Liquid wax is exactly the same substance as solid wax; it has just changed its state because of heat. When it cools down, it returns to being solid wax again.
Teacher's Tip: State changes (Solid -> Liquid -> Gas) are the best examples of physical changes.
Exam Tip: Emphasize that the chemical identity of the wax remains unchanged.

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.
Energy is needed to break old chemical bonds, and energy is released when new ones are made. This is why some reactions feel hot (release energy) and others feel cold (absorb energy).
Teacher's Tip: Energy change is like the "fuel" that makes chemical reactions happen.
Exam Tip: Use the terms "absorption" and "evolution" to sound more scientific in your answers.

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.
Plants need the specific energy from sun rays to power the chemical reaction that makes sugar. Without this light energy, the leaves cannot complete the "cooking" process of making food.
Teacher's Tip: Think of sunlight as the "stove" for the plant's kitchen!
Exam Tip: Remember that photosynthesis is a light-dependent reaction.

F. Write short answers.

Question 1: Name two reversible changes.
Answer:
(i) Melting of ice.
(ii) Dissolving of salt in water.
Both of these can be undone: water can be frozen back to ice, and salt can be recovered from water through evaporation. The starting materials are never permanently lost.
Teacher's Tip: Reversible means you can go "backwards."
Exam Tip: Always number your points as (i) and (ii) for clarity.

Question 2: Give two examples of periodic changes.
Answer:
(i) Swinging of a pendulum
(ii) Rising and setting of the sun.
A pendulum swings back and forth in a set rhythm, and the sun follows a precise 24-hour cycle. Both events repeat themselves at very regular, predictable intervals.
Teacher's Tip: If you can predict exactly when it will happen next, it's periodic.
Exam Tip: Use simple, clear examples like the phases of the moon or the ticking of a clock.

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.
Traveling quickly is great for us, but the chemical smoke produced by the engine harms the environment. This shows that many changes have both pros and cons.
Teacher's Tip: "Desirable" depends on who is looking at it—the driver or the planet!
Exam Tip: Use this specific example of car travel to explain the dual nature of changes.

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 wouldn't happen on their own in the wild; they require human planning and effort. Other examples include cooking food and making plastic from oil.
Teacher's Tip: If a person caused it, it's "man-made."
Exam Tip: Always provide a clear example like "building a house" or "burning fuels."

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.
When dry ice sublimates, it turns from solid carbon dioxide directly into carbon dioxide gas. Since it is still carbon dioxide in both forms, it is a physical change.
Teacher's Tip: Sublimation is just a shortcut from solid to gas, skipping liquid!
Exam Tip: Mention that it is a change of "state" to explain why it is physical.

G. Answer in detail.

Question 1: Classify the following changes as physical or chemical. a. boiling b. curdling of milk c. photosynthesis in plants d. melting of a solid (wax)
Answer:
a. boiling - Physical
b. curdling of milk - Chemical
c. photosynthesis in plants - Chemical
d. melting of a solid (wax) - Physical
Boiling and melting are just changes in how the molecules move and pack together (state changes). Curdling and photosynthesis create brand-new substances like curd and glucose through chemical reactions.
Teacher's Tip: If you can't turn it back into exactly what it was, it's usually chemical.
Exam Tip: Always double-check "boiling" vs "burning"—boiling is physical, burning is chemical!

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.
Temporary means it doesn't last forever if you decide to change it back. By simply changing the temperature or pressure, you can often restore the substance to its original look.
Teacher's Tip: Temporary = Undoable!
Exam Tip: Use the melting/freezing of water example as your primary proof for this explanation.

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 regaining 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 your clothes—you're still the same person inside. Chemical changes are like a caterpillar turning into a butterfly—everything about it has transformed into something new.
Teacher's Tip: Remember "REMI"—Reversibility, Energy, Mass, Identity!
Exam Tip: If the question asks for "state the differences," using a table format is always best.

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 happens faster on hot, dry, and windy days because those conditions help water molecules escape into the air. Think about how wet clothes dry much faster in the sun than in a damp room.
Teacher's Tip: High Temp + High Wind + Big Surface = Fast Evaporation!
Exam Tip: Explain how a wet cloth dries faster when spread out to show you understand "Surface Area."

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.
Inside our bodies, food is mixed with chemicals like hydrochloric acid and enzymes to turn it into simple nutrients. Once this process is done, the food is gone forever and has become part of our body's energy supply.
Teacher's Tip: You can't turn a digested apple back into an apple!
Exam Tip: Mention "new substance formation" and "irreversibility" to justify why it's chemical.

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 acts like the "spark" needed to get atoms to stop holding onto their old partners and start bonding with new ones. Without energy input, many chemical reactions just wouldn't have the power to start.
Teacher's Tip: Energy is the "push" that starts the atomic rearrangement.
Exam Tip: Use the terms "absorbed" and "evolved" to describe how energy moves during reactions.

H.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. STORMS
3. EVAPORATION
Solving a crossword helps reinforce vocabulary and definitions in a fun way. Each word represents a key concept from the chapter on how matter changes.
Teacher's Tip: Count the boxes if you're stuck; the number of letters is a big clue!
Exam Tip: Learn the spelling of long words like "photosynthesis" and "sublimation" as they often appear in fill-ups.

Question 2: Make a list of different types of changes with example.
Answer: Different types of changes are as under:
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 shows that one single event can be categorized in many ways. For example, melting ice is physical, reversible, and usually desirable.
Teacher's Tip: Try to find one example that fits three or more categories at once!
Exam Tip: Memorize one standard example for every category in this list for your finals.