Changes Around Us — for Class 6

Everything Around Us Is Changing — All the Time

Look around for thirty seconds. The chai cooling in your cup, the rust forming on an old iron gate, paper burning in a bonfire, dough rising in the kitchen — change is happening everywhere, constantly.

In Class 6 Science, we study these changes systematically. The big question we need to answer: are all changes the same? The answer is no — and understanding the difference between reversible and irreversible changes, physical and chemical changes, is the foundation for everything you’ll study in chemistry right up to Class 12.

This topic carries reliable weightage in CBSE Class 6 board assessments and SA exams. Questions usually test whether students can classify a given change correctly and justify their reasoning. Let’s build that skill properly.

Key Terms and Definitions

Change — Any alteration in the physical or chemical properties of a substance. Simple enough, but the devil is in the details.

Reversible change — A change that can be undone; the original substance can be recovered. Example: melting ice. The water can be frozen again to get ice back.

Irreversible change — A change that cannot be undone; the original substance cannot be recovered. Example: burning paper. You cannot “unburn” paper and get paper back.

Physical change — A change in the size, shape, colour, or state of a substance without any new substance being formed. The chemical composition stays the same.

Chemical change — A change where one or more new substances are formed with different chemical properties. This is usually irreversible.

Slow change — Takes a long time to occur. Example: rusting of iron (days to months).

Fast change — Happens very quickly. Example: burning crackers (fraction of a second).

Periodic change — Repeats at regular intervals. Example: the changing of seasons, day and night.

Non-periodic change — Does not follow a regular pattern. Example: a flood, an earthquake.

The Core Framework: How to Classify Any Change

This is the method. Apply it to every change you encounter and you’ll get the classification right.

Step 1 — Ask: Is a New Substance Formed?

This is the most important question. If yes → chemical change. If no → physical change.

When milk turns sour, curd (a new substance with different taste and smell) is formed. Chemical change.

When you tear a paper, you still have paper — just in smaller pieces. No new substance. Physical change.

Step 2 — Ask: Can the Original Be Recovered?

If yes → reversible. If no → irreversible.

Here’s the key insight students often miss: most chemical changes are irreversible, and most physical changes are reversible. But this isn’t a hard rule — some physical changes are irreversible too.

Stretching a rubber band — physical change, reversible (it snaps back). Cutting your hair — physical change, irreversible (you can’t reattach it).

Step 3 — Ask: What Clues Do You See?

Signs that a chemical change has occurred:

Colour change (iron turning orange-red when it rusts)
Gas is produced (bubbles when vinegar and baking soda mix)
Heat or light is produced (burning wood)
New smell (egg being cooked)
Precipitate formed (a solid appearing in a liquid reaction)

Remember these five signs as “C-G-H-S-P” — Colour, Gas, Heat/Light, Smell, Precipitate. If you spot any of these, you’re likely looking at a chemical change.

Methods and Concepts in Depth

Physical Changes — Going Deeper

Physical changes affect only the form of a substance, not its identity. The molecules don’t change — they just rearrange or separate.

Changes of state are the most common physical changes:

Solid → Liquid: melting (ice melting)
Liquid → Gas: evaporation or vaporisation (water boiling)
Gas → Liquid: condensation (water vapour forming droplets on a cold glass)
Liquid → Solid: freezing (water freezing into ice)
Solid → Gas: sublimation (camphor disappearing when left open)

All of these are reversible because the water molecule ( $\text{H}_2\text{O}$ ) is the same in ice, water, and steam.

Size and shape changes — Cutting, tearing, grinding, bending are all physical changes. A copper wire being drawn into thin wire (wire drawing) is a physical change — copper is still copper.

Dissolving — When sugar dissolves in water, it seems to disappear. But we can recover the sugar by evaporating the water. So dissolving is a reversible physical change. No new substance is formed.

Chemical Changes — Going Deeper

In a chemical change, the original substances (called reactants) are converted into new substances (called products) with different properties.

Burning — This is the most common chemical change in Class 6 examples. When wood burns, it produces carbon dioxide, water vapour, and ash. None of these can be converted back into wood.

Rusting — Iron reacts slowly with oxygen and water in air to form iron oxide (rust). The rust has completely different properties — it’s brittle, brownish-red, and weak.

Cooking — Raw egg and cooked egg have completely different properties. The protein structure changes permanently. You cannot “uncook” an egg.

Ripening of fruit — Unripe mango slowly becomes ripe. This is a chemical change (new flavour compounds form), and it’s irreversible.

CBSE Class 6 SA papers frequently ask: “Is dissolving sugar in water a physical or chemical change? Justify.” The expected answer: Physical change, because no new substance is formed and the sugar can be recovered by evaporating water.

Slow vs Fast Changes

This classification is independent of physical/chemical. Any change can be slow or fast.

Change	Speed	Type
Rusting of iron	Slow	Chemical
Burning of paper	Fast	Chemical
Melting of ice	Slow/Medium	Physical
Bursting of a cracker	Fast	Chemical
Dissolving of sugar	Fast	Physical

Periodic vs Non-Periodic Changes

Periodic changes happen at regular, predictable intervals:

Day and night (approximately 24 hours)
Seasons (yearly cycle)
Phases of the moon (approximately 29.5 days)
Heartbeat (regular rhythm)
Tides (twice daily)

Non-periodic changes have no fixed pattern:

Earthquakes
Floods
Volcanic eruptions
A tree falling in a storm

Solved Examples

Example 1 — Easy (CBSE Class 6)

Q: Classify the following as physical or chemical changes: (a) Melting of butter (b) Souring of milk (c) Bending a copper wire (d) Burning of a candle

(a) Melting of butter — Physical change. The butter changes from solid to liquid form, but it’s still butter. The change is reversible (you can solidify it again in a refrigerator).

(b) Souring of milk — Chemical change. New substances (lactic acid, curd) are formed. The taste and smell change completely. You cannot turn sour milk back into fresh milk.

(c) Bending a copper wire — Physical change. The shape changes but the composition stays the same. It’s still copper.

(d) Burning of a candle — Tricky one! This is both physical and chemical. The wax melting is a physical change. The wax vapour burning and producing $\text{CO}_2$ and water is a chemical change. In CBSE Class 6, we typically say burning a candle is a chemical change because the dominant process is the burning.

Example 2 — Medium (CBSE Class 6 SA)

Q: Paheli kept some water in the freezer and converted it to ice. Later, she melted the ice and got water again. Her friend says this is not a change at all since we’re getting water back. Is her friend right? What do we call this type of change?

Paheli’s friend is partially right but using wrong reasoning. A change did occur — water changed its state from liquid to solid and back. However, since we recovered the original substance (water), this is a reversible physical change.

The friend should not say “no change occurred” — the state (form) of water definitely changed. The correct statement is: this is a reversible change.

Example 3 — Hard (CBSE Class 6, concept application)

Q: Iron is used to make many objects. Blacksmiths heat iron and beat it into different shapes. (a) Is heating iron to make it soft a physical or chemical change? (b) The iron objects kept outside get rusted over time. Is rusting a physical or chemical change? (c) Can you reverse both these changes? Explain.

(a) Heating iron — Physical change. Iron becomes soft and malleable when hot, but it’s still iron. Once cooled, it returns to hard iron. No new substance forms.

(b) Rusting — Chemical change. Iron reacts with oxygen and water in air to form iron oxide ( $\text{Fe}_2\text{O}_3$ ), which is rust. The rust has completely different properties from iron — it’s brittle, reddish-brown, and non-magnetic.

(c) Reversibility:

Heating iron: Reversible. Cool it down, it becomes hard iron again.
Rusting: Irreversible in normal conditions. Once rusted, you cannot chemically restore the iron to its original state simply by reversing the process. (In industries, iron is extracted from iron ore again, but that’s a completely different process.)

Exam-Specific Tips

For CBSE Class 6

The CBSE marking scheme expects two things in change-classification questions:

The classification (physical/chemical, reversible/irreversible)
The justification (1-2 sentences explaining why)

Never write just “physical change” — always add “because no new substance is formed” or “because the original substance can be recovered.”

2-mark questions on this topic almost always require a reason. If you write only the classification without justification, expect to lose 1 mark.

Common Question Formats in CBSE SA1/SA2

“Give one example each of a fast and a slow chemical change”
“Classify the following changes as physical or chemical”
“Is dissolving common salt in water a physical change? Justify”
“A candle burns and decreases in size. What type of change is this?”
“Give two signs that indicate a chemical change has occurred”

In CBSE Class 6, the term “chemical change” and “irreversible change” are often used interchangeably in exam contexts — but they’re not exactly the same. Chemical changes are always irreversible. But some physical changes (like cutting hair) are also irreversible. Know this distinction.

Common Mistakes to Avoid

Mistake 1: Thinking dissolving is a chemical change. When sugar or salt dissolves in water, students often say “it disappeared, so it must have changed.” But the sugar molecules are just spread throughout the water — evaporate the water and you get sugar back. No new substance formed. It’s a physical change.

Mistake 2: Saying “reversible = physical” and “irreversible = chemical” as absolute rules. Cutting hair is physical but irreversible. Know that reversibility and physical/chemical are two separate classification systems. A change can be physical+irreversible or chemical+irreversible (most common).

Mistake 3: Forgetting that burning a candle involves both physical and chemical changes. Wax melting = physical. Wax burning = chemical. For CBSE purposes, classify it as chemical overall.

Mistake 4: Confusing slow changes with reversible changes. Rusting is slow — but it’s also irreversible and chemical. Speed of change has nothing to do with reversibility. These are independent properties.

Mistake 5: Writing incomplete answers in exams. “It is a physical change” → 1 mark. “It is a physical change because no new substance is formed and the original substance can be recovered” → 2 marks. Always justify.

Practice Questions

Q1. Is rolling chapati dough a physical or chemical change?

Physical change. The shape of the dough changes but no new substance is formed. The dough is still dough, just flatter. This change is also reversible — you could roll the dough back into a ball.

Q2. What type of change occurs when milk is converted into curd?

Chemical change. New substances are formed — lactic acid is produced by bacteria acting on milk. The taste, smell, and consistency change completely. You cannot convert curd back into milk, making it irreversible.

Q3. Classify: Stretching a rubber band / Burning of magnesium ribbon / Cooking of rice / Making lemonade.

Stretching a rubber band — Physical, reversible (it returns to original shape)
Burning of magnesium ribbon — Chemical, irreversible (magnesium oxide is formed, a completely new white powder)
Cooking of rice — Chemical, irreversible (the starch structure changes permanently)
Making lemonade — Physical, reversible (you’re dissolving sugar and mixing lemon juice — no new substance formed; you could separate the components)

Q4. Give two examples each of: (a) periodic changes (b) non-periodic changes

(a) Periodic changes: Day and night; changing of seasons (also: tides, phases of the moon, heartbeat) (b) Non-periodic changes: Earthquakes, floods (also: volcanic eruptions, a tree falling in a storm)

Q5. Paheli notices that a wet cloth dries in the sun. Her friend says this is a chemical change because water “disappears.” Who is correct?

Paheli should correct her friend. Drying of a wet cloth is a physical change (specifically, evaporation). Water doesn’t disappear — it turns into water vapour and mixes with the air. No new substance is formed. The water can be recovered by condensation. The cloth itself doesn’t change chemically.

Q6. An iron nail is heated until it glows red-hot. (a) What change occurs in the iron? (b) If the same nail is left outside in rain for two weeks, what change occurs?

(a) Heating the iron nail causes a physical change — the iron expands slightly and becomes more malleable, but it’s still iron. The change is reversible when cooled.

(b) Leaving it in rain causes rusting — a chemical change. Iron reacts with water and oxygen to form iron oxide (rust). This is irreversible and the nail becomes weak and brittle.

Q7. Why is the process of making sugar from sugarcane juice called a physical change, but the process of burning sugarcane is called a chemical change?

When sugarcane juice is evaporated and crystallised to get sugar, no new substance is formed — we’re just separating sugar from water. The sugar was always present in the juice. This is a physical change (evaporation/crystallisation).

When sugarcane burns, it reacts with oxygen to produce carbon dioxide, water vapour, and ash. These are completely different substances from sugarcane. New substances are formed, making it a chemical change.

Q8. A blacksmith heats iron ore and extracts pure iron from it. Later, he heats the iron and beats it into a horseshoe shape. Are both processes the same type of change?

No, they are different types of changes.

Extracting iron from iron ore — This is a chemical change. Iron ore (iron oxide) is chemically reduced to get iron metal. New substances are formed and the process involves chemical reactions.

Heating iron and beating it into shape — This is a physical change. The iron is the same substance; only its shape changes. When cooled, it retains the horseshoe shape — still iron, just differently shaped.

FAQs

Is evaporation a physical or chemical change?

Evaporation is a physical change. Water converts from liquid to gas (water vapour), but the water molecule ( $\text{H}_2\text{O}$ ) remains unchanged. Condense the vapour and you get liquid water back. No new substance is formed.

Why is burning always a chemical change?

When something burns, it reacts with oxygen in the air. New substances are formed — typically carbon dioxide, water, and ash. Since new substances with different properties are formed, burning is always classified as a chemical change.

Is cooking a chemical or physical change?

Cooking is a chemical change. When food is cooked, proteins denature, starches change structure, and new flavour compounds form. The cooked food has permanently different properties from the raw ingredients — you cannot “uncook” food.

Can a physical change ever be irreversible?

Yes. Cutting paper, tearing cloth, and cutting hair are all physical changes (no new substance formed) but they’re irreversible in practice. This is why we need two separate classification systems — physical/chemical AND reversible/irreversible.

Is rusting the same as burning?

Both are chemical changes, but they’re very different in speed. Burning is a fast chemical change — it produces light and heat rapidly. Rusting is a slow chemical change — it takes days or months and produces no light or significant heat. However, both involve iron or other materials reacting with oxygen.

What happens to the mass when a chemical change occurs?

Mass is conserved — it doesn’t increase or decrease. When wood burns, the ash weighs less than the wood because gases (CO₂ and water vapour) escape into the air. But if you could collect all the gases, you’d find the total mass of products equals the mass of reactants. This is called the Law of Conservation of Mass (you’ll study this formally in Class 9).

Is mixing substances always a physical change?

Not always. Mixing vinegar and baking soda produces carbon dioxide gas — a new substance — making it a chemical change. But mixing sugar and water produces a sugar solution without any new substance — a physical change. Whether mixing causes a chemical or physical change depends on whether the substances react with each other.

How do we know a chemical change has occurred if we can’t see a new substance?

We look for indirect signs: a change in temperature (heat released or absorbed), a change in smell, gas bubbles forming, a colour change that wasn’t expected, or a solid forming in a liquid. If you observe any of these, a chemical change is likely occurring.