Physical and Chemical Changes | ICSE Class 7 Chemistry Notes

This note covers changes in the shape, size and state of materials; formation of new substances; heating and cooling; dissolving; burning; changes that can be reversed; and useful and unwanted changes in everyday life.

What distinguishes physical changes from chemical changes?

A property is a characteristic of a material. Shape, size and state are physical properties. State means the solid, liquid or gaseous form of a substance. A change in appearance does not necessarily mean that the substance itself has changed.

Definition: A physical change changes physical properties, such as shape, size or state, without forming a new substance.

Folding paper changes its shape. Crushing chalk changes the size of its pieces. The folded material remains paper, and the crushed material remains chalk. These are physical changes because the original substances remain the same.

Definition: A chemical change forms one or more new substances. The process through which these substances form is called a chemical reaction.

The new substances formed in a reaction are called products. They have properties different from those of the starting substances. Burning paper and the formation of curd from milk are examples of chemical changes.

Which question should guide classification?

Ask whether a new substance has formed. Do not decide simply by how dramatic the change looks. A large change in shape can be physical, while the gradual formation of a new substance can be chemical.

Classification means placing changes into groups using a stated basis. Whether a change is useful and whether it can be reversed are separate questions. Neither replaces the test for formation of a new substance.

How do simple activities demonstrate physical changes?

Paper, a balloon and chalk show that physical changes can alter appearance in different ways. In each activity, compare the material before and after the change. The important observation is that the material remains the same substance.

What happens when paper is folded?

1. Take a sheet of paper and observe its original shape.
2. Fold it to make an object and compare its shape with the original sheet.
3. Unfold the object and observe that the paper can be opened again.
4. Identify the change as physical because folding has formed no new substance.

The words “new object” and “new substance” have different meanings. A folded paper object has a new shape, but it is still made of paper. A change in the object’s appearance is therefore insufficient evidence of a chemical reaction.

What do a balloon and chalk show?

Inflating a balloon means filling it with air. Its shape and size change. Letting the air escape returns an intact balloon to its uninflated form. If the inflated balloon is pricked, it bursts, but its material remains rubber.

Crushing a piece of chalk produces smaller pieces and powder. The powder remains chalk. Difficulty in restoring the original piece does not make this a chemical change. The classification depends on the substance, not on whether the original shape is easy to recover.

Note: A balloon bursting and chalk being crushed illustrate why “cannot easily restore the object” is not a sufficient reason to call a change chemical.

Cutting paper and chopping vegetables similarly alter size or shape. Record the specific property that changes, then explain why no new substance is formed in that operation.

Why are changes between ice, water and water vapour physical?

Ice is water in the solid state. Liquid water and water vapour, its gaseous state, are other forms of the same substance. Changing between these states changes a physical property without making a new substance.

What are the names of the changes?

Melting is the conversion of a solid into a liquid. Freezing is the conversion of a liquid into a solid. Ice melts on heating, while water placed in a sufficiently cold environment, such as a freezer, freezes.

Evaporation is the conversion of liquid water into water vapour. It takes place even at room temperature. Condensation is the conversion of water vapour into liquid water. Cooling water vapour can therefore bring it back to its liquid state.

How does this explain drying clothes?

Wet clothes dry because water in them evaporates into the surrounding air. The water has changed state. It has not become a new substance. Drying clothes is therefore a physical change, even though the liquid water is no longer visible on the cloth.

Boiling converts liquid into vapour with bubble formation throughout the liquid. Boiling water is also a physical change. Do not confuse water vapour produced by boiling with a new gas produced by a chemical reaction.

The sequence of heating ice and then cooling the resulting water illustrates changes of state. The names describe the direction of the change, while the word “physical” describes whether a new substance forms.

How do sublimation and dissolving differ from melting?

Sublimation is the direct change of a solid into vapour without passing through the liquid state. It differs from melting because melting produces a liquid first. Iodine provides an example for studying this direct change of state.

The direct conversion of vapour into solid on cooling is called deposition. Sublimation and deposition describe changes between states. In a change of state such as iodine subliming, a different physical form does not mean that a new substance has formed.

What happens when sugar dissolves?

Dissolving, also called dissolution, produces a solution when a substance mixes into a liquid. A solution is a uniform mixture of a solute and a solvent. A solute is the dissolved substance; a solvent is the component that dissolves it.

When sugar dissolves in water, sugar is the solute and water is the solvent. The resulting sugar solution is a mixture. Preparing it is a physical change because no new substance forms merely through this dissolving process.

Why should these words not be interchanged?

Sugar becoming invisible as separate grains in water is not the same process as ice melting. The first involves a solute and solvent; the second is a change of state of water itself. Both can be physical without being identical processes.

Ammonium chloride is also used in demonstrations of sublimation and of dissolving in water. These are distinct activities: the sublimation demonstration involves heating, whereas dissolving examines its interaction with water. Name the actual operation before interpreting the observation.

How does lime water demonstrate a chemical change?

Lime water is a solution of calcium hydroxide in water. Carbon dioxide is a gas present in exhaled air, meaning air breathed out. When this gas reacts with lime water, a new white substance called calcium carbonate forms.

Calcium carbonate is insoluble in water, meaning that it does not dissolve in it. Its formation makes the liquid appear milky or cloudy. After the mixture stands for some time, a white substance settles at the bottom.

How can two liquids be compared?

1. Label two transparent tumblers A and B. These letters identify the containers, not chemical substances.
2. Fill one-fourth of tumbler A with tap water and one-fourth of tumbler B with lime water.
3. Blow exhaled air through separate straws into the two liquids, one at a time. Do not suck either liquid into the mouth.
4. Compare the observations: tap water shows bubbles without a change in appearance; lime water shows bubbles and becomes milky.

The two drawings show a student blowing through a straw into a tumbler. The labels identify tap water in A and lime water in B. Both drawings label the straw.

See Fig. 5.2 in your NCERT textbook

A chemical equation is a short representation of a chemical reaction. A word equation uses the names of substances. In the equation below, the plus sign means “reacts with” on the left and “and” on the right; the arrow means “produces”.

Calcium hydroxide + carbon dioxide → calcium carbonate + water

The reaction also forms a small amount of water. The milky appearance is due to calcium carbonate, rather than bubbles alone. Turning lime water milky is used as a test for carbon dioxide.

Note: Bubbles may result simply from blowing an existing gas into water. In the lime-water activity, the formation of a new insoluble substance provides the evidence for a chemical change.

What happens when vinegar reacts with baking soda?

Baking soda is the common name for sodium hydrogen carbonate. Mixing it with vinegar produces carbon dioxide gas. Fizzing is the bubbling that accompanies the release of this gas. Because a new substance forms, the change is chemical.

How is the gas identified?

1. Take a teaspoonful of vinegar in a test tube.
2. Add a pinch of baking soda and observe the fizzing sound and bubbles.
3. Pass the gas through freshly prepared lime water in another test tube.
4. Observe the lime water becoming milky and identify the gas as carbon dioxide.

Lemon juice can be used in place of vinegar in this activity. The key reasoning remains the same: gas is produced, and its effect on lime water helps identify it. The gas test gives more information than the appearance of bubbles alone.

Part (a) draws two test tubes connected by a bent tube. One contains vinegar and baking soda; the other contains lime water and gas bubbles. Part (b) is a photograph of a version using two connected bottles.

See Fig. 5.3 in your NCERT textbook

Vinegar + baking soda → carbon dioxide + other substances

This word equation identifies carbon dioxide without naming every other product. It summarises the evidence needed here: a gas different from the starting materials has formed. Avoid describing the fizzing merely as the baking soda disappearing into the liquid.

The activity also contains a second chemical change when carbon dioxide reacts with lime water. Keep the gas-producing mixture separate in your explanation from the lime water used to test that gas.

Why is burning a chemical change?

Combustion is a chemical reaction in which a substance reacts with oxygen and produces heat and/or light. Oxygen is the component of air that supports combustion. A combustible substance is one that undergoes combustion; examples include wood, paper, cotton and kerosene.

When magnesium ribbon burns, it forms magnesium oxide, a white powder. Heat and light are also produced. The formation of magnesium oxide makes this a chemical change, rather than merely a change in the shape or size of magnesium.

Magnesium + oxygen → magnesium oxide + heat + light

What conditions are required?

A combustible substance is also called a fuel. Its ignition temperature is the minimum temperature at which it catches fire. Combustion requires fuel, oxygen and enough heat to bring the fuel to its ignition temperature.

Paper can remain in air without immediately burning. Oxygen being present is therefore not enough by itself. The paper must also receive sufficient heat. The conditions explain why the presence of a combustible material does not automatically mean that a fire will start.

What does covering a candle show?

Place two identical lighted candles on separate shallow dishes and cover one with a glass tumbler. The uncovered candle continues burning. The covered candle stops burning after some time because it does not receive a continuous supply of air.

Part (a) shows a candle burning on a dish. Part (b) shows another candle on a dish under an inverted glass tumbler, with smoke above its wick.

See Fig. 5.6 in your NCERT textbook

Burning fuels, paper, matchsticks and fireworks provides examples of chemical changes. Heat or light may be conspicuous, but the explanation must still connect burning with a reaction and formation of new substances.

How can a burning candle involve both kinds of change?

A burning candle contains a sequence of changes. Some concern the state of the wax, while another concerns its reaction with oxygen. Calling the entire sequence either physical or chemical loses this distinction.

What happens to the wax?

1. Heat from the flame melts solid candle wax into liquid wax.
2. The liquid wax is carried up the wick, the thread running through the candle.
3. The wax evaporates because of the heat of the flame.
4. The wax vapour burns, producing the flame and new substances.

The melting and evaporation of wax are physical changes. Wax that flows away and cools can become solid again. This conversion to a solid is called solidification and is also a physical change.

The burning of wax vapour is a chemical change. Carbon dioxide forms through the reaction of carbon in the wax with oxygen from the air. Thus, the same familiar event combines changes of state with a reaction that forms a new substance.

When explaining this example, name the particular part being classified. “Wax melts” and “wax vapour burns” refer to different processes. A correct explanation identifies both instead of treating the flame and melted wax as evidence of just one kind of change.

How do rusting and the reaction of iron with sulphur form new substances?

Rusting is the chemical change in which iron forms rust, a brown-coloured substance described as iron oxide. Rust is different from the original iron. The brown layer is therefore evidence of a new substance, rather than simply a different shape of the metal.

Compare rusting with cutting paper. Cutting changes the size of pieces without changing paper into another substance. Rusting changes iron chemically. Neither the speed of the change nor the size of the object supplies the defining distinction.

Is mixing iron and sulphur already a reaction?

A mixture contains substances together without their necessarily undergoing a chemical change. Mixing iron powder and sulphur powder makes a mixture. Simply putting the powders together must be distinguished from making them react.

When suitable quantities of iron and sulphur are heated together, they react to form iron sulphide. This is a new compound. A compound is a substance formed by the chemical combination of elements, which are basic substances such as iron and sulphur.

Iron + sulphur → iron sulphide

The important phrase is “react to form”. A mixture and a compound are not interchangeable descriptions. In both rusting and the iron-sulphur reaction, identify the product before explaining why the change is chemical.

What do heating, cooling and energy changes tell us?

Energy enables changes to occur; heat and light are forms in which energy can be transferred or released. Both physical and chemical changes can involve energy. Receiving or releasing heat does not, by itself, establish whether a new substance forms.

Can a physical change require heat?

Heat must be supplied to change ice into liquid water and water into water vapour. These remain physical changes. Evaporation has a cooling effect. A temperature effect is therefore not sufficient evidence of a chemical reaction.

Temperature describes how hot or cold something is. Observe a temperature change separately from changes in state, colour or the formation of a gas. Then ask what those observations reveal about the substances present.

What happens when quicklime meets water?

Quicklime is calcium oxide. It reacts vigorously with water to form calcium hydroxide, also called slaked lime, and releases a large amount of heat. This is chemical because a new substance forms as well as heat being released.

Calcium oxide + water → calcium hydroxide + heat

An exothermic reaction releases heat while forming its products. The quicklime reaction is an example. An endothermic reaction absorbs energy. These terms describe energy changes; they do not replace the need to establish that a chemical reaction occurs.

Dissolving ammonium chloride in water produces cooling as heat is taken in from the surroundings. Compare that observation with the heating produced by quicklime and water. Do not assume that all substances added to water behave alike.

Note: Keep observation and explanation separate. “The mixture became warm” is an observation. “Calcium hydroxide formed from calcium oxide and water” identifies the chemical change responsible in the quicklime example.

Heat and light accompany burning magnesium, but melting ice also involves heat. The decisive distinction remains whether the starting substance merely changes its physical properties or produces a new substance.

Are physical changes and reversible changes the same thing?

A reversible change allows the original object or substance to be obtained again in its starting form. An irreversible change does not allow that return. This grouping asks a different question from whether a new substance forms.

Melting ice can be reversed by freezing the water. Evaporated water can be condensed back into liquid water. Folding paper can be reversed by unfolding it. These examples show changes where a return to the original form is possible.

Which physical changes cannot restore the original object?

Chopped vegetables cannot return to their original size and shape. A burst balloon cannot be restored by simply allowing air to escape. Yet chopping and bursting remain physical changes because the materials do not become new substances.

How should a change be described accurately?

Give separate reasons for separate classifications. To call chopping physical, explain that the material remains the same. To call it irreversible, explain that the original size and shape cannot be recovered. These statements answer different questions about the same operation.

Avoid learning “all physical changes are reversible” as a rule. It fails for familiar changes in size and shape. Likewise, inability to recover the original object does not prove that a new substance has formed.

The most reliable explanation begins with the evidence requested: substance identity for physical or chemical classification, and recovery of the original form for reversible or irreversible classification.

How are everyday changes classified as useful or unwanted?

A desirable change is useful or wanted in a particular situation. An undesirable change is unwanted in that situation. This classification depends on the purpose and circumstances, rather than on whether the change is physical or chemical.

Drying clothes is a useful physical change. Preparing curd, ripening fruits and cooking food are useful changes involving chemical changes. Rusting iron and food decaying during storage are examples of changes that may be undesirable.

Why can the same process be judged differently?

Decomposition means breakdown into other substances. Decomposition of stored food is unwanted, but decomposition of food waste can help produce compost, a material made from decayed organic waste and used in growing plants.

Thus, a change that is undesirable in some situations may be desirable in other situations. Do not label all chemical changes harmful or all physical changes useful. First identify the change, then consider what it does in the situation described.

How do food and respiration fit the classification?

Respiration is the breakdown of food in cells to release energy. Cells are the small structural and functional units of living organisms. In respiration using oxygen, glucose, a simple sugar, reacts with oxygen to form carbon dioxide and water while releasing energy.

Glucose + oxygen → carbon dioxide + water + energy

This is a chemical change because new substances form. The equation describes respiration using oxygen; it should not be used to claim that every form of respiration needs oxygen.

Boiling an egg and boiling water must not be confused. The water changes state, whereas the egg undergoes cooking. State the material and the actual change before assigning a category.

Glossary

• Physical change — A change in physical properties without the formation of any new substance.
• Chemical change — A change in which one or more new substances are formed.
• Product — A new substance formed as a result of a chemical reaction.
• Melting — The change of a substance from its solid state into liquid.
• Freezing — The change of a substance from its liquid state into solid.
• Evaporation — The conversion of liquid water into its gaseous state, water vapour.
• Condensation — The conversion of water vapour into water in the liquid state.
• Sublimation — The direct conversion of a solid into vapour without an intermediate liquid state.
• Solution — A uniform mixture formed when a solute dissolves in a solvent.
• Combustion — A chemical reaction with oxygen that produces heat and/or light.
• Ignition temperature — The minimum temperature at which a combustible substance catches fire.
• Reversible change — A change after which the original object or substance can regain its starting form.
• Exothermic reaction — A chemical reaction that releases heat along with the formation of products.
• Endothermic reaction — A chemical reaction in which energy is absorbed during the change.
• Respiration — The process of breaking down food in cells to release energy.

Common errors and misconceptions

• Misconception: Every physical change is reversible. Correct: Chopping vegetables is physical although their original size and shape cannot be restored. Formation of a new substance is the classification test.
• Misconception: A large change in appearance proves a chemical reaction. Correct: Folding paper and crushing chalk alter appearance while the materials remain the same substances.
• Misconception: Bubbles always mean that a new gas has formed. Correct: Blowing air through tap water also makes bubbles. The lime-water reaction provides additional evidence through formation of calcium carbonate.
• Misconception: Heating always causes a chemical change. Correct: Heating ice can melt it without forming a new substance. Quicklime reacting with water is chemical because calcium hydroxide forms.
• Misconception: A burning candle demonstrates just one kind of change. Correct: Melting and evaporation of wax are physical; burning wax vapour is chemical because new substances form.
• Misconception: Mixing iron and sulphur immediately produces iron sulphide. Correct: Mixing makes a mixture. Heating suitable quantities so that they react produces the compound iron sulphide.
• Misconception: Chemical changes are necessarily undesirable. Correct: Preparing curd and cooking food are useful chemical changes. Whether a change is desirable depends on its purpose and circumstances.

Exam-style questions with model answers

Q1. A sheet is folded into a paper object and then unfolded. The material remains paper throughout. Classify the folding and explain your answer. [2 marks]

1. Folding the sheet is a physical change because it changes the shape of the paper object.
2. No new substance forms: the material remains paper, both in the folded object and after unfolding.

Q2. Iron develops a brown layer of rust, a new substance different from iron. Name the type of change and give the reason. [2 marks]

1. The rusting of iron is a chemical change, rather than simply a change in the shape of the iron.
2. A new substance, rust, forms; its formation distinguishes this change from a physical change.

Q3. Ice melts into water; the water evaporates; the vapour is cooled back into liquid water. No new substance forms. Name and classify each of these three changes. [3 marks]

1. The first change is melting, in which solid ice becomes liquid water. It is physical because the substance remains water.
2. The second change is evaporation, in which liquid water becomes water vapour. It is physical because no new substance forms.
3. The third change is condensation, in which water vapour becomes liquid water. It is physical because only the state changes.

Q4. Vinegar and baking soda produce bubbles of a new gas. Passing this gas into lime water makes it milky through formation of insoluble calcium carbonate. Identify the gas, classify the first reaction and explain the milky appearance. [3 marks]

1. The gas is carbon dioxide. Its reaction with lime water, producing the stated milky appearance, is the test used to identify it.
2. Mixing vinegar and baking soda causes a chemical change because a new substance, the carbon dioxide gas, forms during the reaction.
3. The lime water becomes milky because insoluble calcium carbonate forms. This new white substance does not dissolve in the water.

Q5. When a candle burns, solid wax melts, liquid wax evaporates, some melted wax cools into solid wax, and wax vapour reacts with oxygen to form new substances. Classify each of these four changes with a reason. [4 marks]

1. Melting wax is a physical change because the wax changes from a solid into a liquid without forming a new substance.
2. Evaporating wax is a physical change because wax changes into vapour while remaining the same substance.
3. Solidification of cooled wax is a physical change because the liquid wax returns to a solid state.
4. Burning wax vapour is a chemical change because its reaction with oxygen produces new substances.

Q6. Chopping vegetables leaves the material unchanged but cannot restore the original shape. Food decomposition is unwanted during storage but useful when making compost. Explain the physical/chemical and reversible/irreversible classification of chopping, then explain the two judgements about decomposition. [4 marks]

1. Chopping vegetables is a physical change because the material remains unchanged and no new substance forms during cutting.
2. Chopping is irreversible in the stated sense because the vegetables cannot regain their original size and shape.
3. Decomposition during storage is undesirable because the stored food spoils, which is unwanted in that situation.
4. Decomposition when making compost is desirable because the same breakdown helps convert food waste into a useful material.

Q7. Calcium oxide, called quicklime, reacts vigorously with water to form calcium hydroxide, called slaked lime, and releases a large amount of heat. State the starting substances, name the product, write the word equation, classify the change with a reason, and name its heat behaviour. [5 marks]

1. The starting substances are calcium oxide and water. Quicklime is another name for calcium oxide, rather than an additional starting substance.
2. The product is calcium hydroxide. It is also called slaked lime and is different from the calcium oxide used at the start.
3. The word equation is: calcium oxide + water → calcium hydroxide + heat. The arrow points from the starting substances to the result.
4. The change is chemical because a new substance, calcium hydroxide, forms. The temperature effect supports the description but does not replace this reason.
5. The reaction is exothermic because it releases heat while forming the product. In this case, a large amount of heat is released.

Q8. In an investigation, transparent tumbler A contains tap water and transparent tumbler B contains lime water, a solution of calcium hydroxide. Each tumbler is one-fourth full. Exhaled air, containing carbon dioxide, is blown through separate straws. A shows only bubbles; B becomes milky and later develops a settled white substance, calcium carbonate. Describe the setup, the blowing step and precaution, the observation in A, the observation in B, the cause of milkiness, and the chemical conclusion. [6 marks]

1. Use two transparent tumblers labelled A and B. Fill A one-fourth full with tap water and B one-fourth full with lime water.
2. Blow exhaled air into each tumbler through a separate straw, one at a time. Do not suck water or lime water into the mouth.
3. In A, bubbles appear but the appearance of the tap water does not change. These bubbles arise as air is blown through it.
4. In B, the lime water becomes milky as well as showing bubbles. On standing, a white substance settles at the bottom.
5. Carbon dioxide reacts with calcium hydroxide in lime water to form insoluble calcium carbonate. This white substance causes the milky appearance.
6. The change in B is chemical because a new substance forms. Bubble formation alone would not supply the same evidence of a reaction.

Key takeaways

• Physical changes alter properties such as shape, size or state without producing a new substance.
• Chemical changes form one or more new substances, whose properties differ from those of the starting materials.
• Melting, freezing, evaporation and condensation of water are physical changes because the substance remains water.
• Reversibility and formation of new substances are separate tests; chopping vegetables is physical although their original form cannot be restored.
• Carbon dioxide turns lime water milky by reacting to form insoluble calcium carbonate, a new white substance.
• A burning candle combines physical changes of wax with the chemical change of burning wax vapour.
• Combustion requires a combustible substance, oxygen and sufficient heat for the fuel to reach its ignition temperature.
• A change may be desirable in one situation and undesirable in another, as decomposition of food illustrates.

Test yourself

Why is crushing chalk physical even if the original piece is difficult to restore?

The powder remains chalk, so no new substance forms. Difficulty in restoring the original shape does not make a change chemical.

What is the difference between melting ice and dissolving sugar?

Melting changes solid water into liquid water. Dissolving sugar mixes a solute into water to form a solution. Both are physical changes, but they are different processes.

Why does drying a wet cloth count as a physical change?

The water evaporates into water vapour. It changes state without forming a new substance, leaving the cloth dry.

What makes lime water appear milky when carbon dioxide passes through it?

Carbon dioxide reacts with calcium hydroxide to produce insoluble calcium carbonate. The new white substance causes the milky appearance.

Why does mixing iron and sulphur differ from making iron sulphide?

Mixing the powders produces a mixture. Heating suitable quantities so that they react produces iron sulphide, a new compound.

Does the use of heat prove that a change is chemical?

No. Melting ice requires heat but is physical. A chemical classification requires formation of a new substance, as when quicklime reacts with water.

Why does a candle covered by a glass tumbler stop burning after some time?

The covered candle does not receive a continuous supply of air. Oxygen from air is required for its combustion.

How can decomposition of food be both undesirable and desirable?

Decomposition is undesirable when stored food spoils. It is desirable when food waste is deliberately converted into useful compost.