Changes Around Us: Physical and Chemical

Why This Matters

Look around you right now. Things are changing all the time.

An ice cube left on a plate slowly turns into a pool of water. A banana you bought yesterday gets a few more brown spots today. The wet clothes on the line dry up in the sun. A candle burns and slowly gets shorter.

These are all changes. But are they all the same kind of change?

Here is something interesting. Melt an ice cube and you can freeze the water back into ice. Easy. But burn a piece of paper, and you can never get that paper back. It is gone forever. Why is one change so easy to undo, while the other is impossible?

That single question is the heart of this chapter. By the end, you will be able to look at any change around you and say, with confidence, which kind it is — and, more importantly, why. This is one of the most useful ideas in all of science.

The Big Idea

All changes around us fall into two big groups. In a physical change, the thing changes its look — its shape, size, or state — but it stays the same substance inside. No new material is made, so it can usually be undone (ice melts, then freezes back). In a chemical change, a brand-new substance is made that was not there before. Because it is genuinely new, you usually cannot get the old thing back (burnt paper stays ash forever). The one question that tells them apart is simple: Did a new substance form? If no, it is physical. If yes, it is chemical.

Let’s Break It Down

Changes around us

A change just means something becomes different from how it was before. The ice was solid; now it is liquid. The paper was whole; now it is torn. Something is different.

We notice changes using our senses — our eyes (sight), nose (smell), ears (hearing), skin (touch), and tongue (taste). You see the ice turn to water. You smell the banana going off. You hear the fizz when something bubbles.

There are countless changes around us. To make sense of them, scientists put them into two neat groups: physical changes and chemical changes. Let us meet each one.

Physical changes — no new substance forms

A physical change is a change in which a substance changes only in its physical look — its shape, its size, or its state (solid, liquid, gas) — but no new substance is formed. The stuff stays the same; only its appearance is different.

Let us see this with the most famous example of all: ice.

When ice melts, it becomes water. But what is ice? It is just water that has frozen solid. So when it melts, you do not get some new material — you get water, which is what it was made of all along. And here is the proof: put that water in the freezer, and it becomes ice again. You went there and came right back. The substance never changed; only its state changed.

Figure 5.1 below makes this clear. The same blue substance just keeps swapping between solid and liquid, and a sheet of paper folded into a shape unfolds back to the same sheet.

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This is why a physical change can almost always be undone. Here are more everyday physical changes:

• Tearing paper. You now have small pieces instead of one big sheet. But each piece is still paper.
• Dissolving sugar in water. The sugar seems to vanish, but it is still there — taste the water, it is sweet! Boil the water away and the sugar comes back. Nothing new was made.
• Crushing chalk into powder. The powder is still chalk, just in tiny bits.
• Making a ball of dough. Rolling the same dough into a different shape does not change what it is.
• Inflating a balloon. The rubber stretches and changes shape, but it is the same rubber.

So why does no new substance form? Because in a physical change, you are only pushing the substance around — bending it, breaking it, or warming it enough to melt. You are not changing what it is made of. Think of it like rearranging the furniture in a room. The room looks different, but it is still your same furniture.

Real life uses physical changes a lot. We learned in Class 6 that water can exist in three states. Let us refresh that, because it is the clearest example of a physical change.

Chemical changes — a NEW substance forms

Now for the other group. A chemical change is a change in which one or more brand-new substances are formed. The new substance has different properties from what you started with. The making of a new substance happens through a process called a chemical reaction.

Let us look at a clear example from the textbook: blowing your breath into lime water.

Lime water is a clear liquid (its real name is calcium hydroxide solution). Take two glasses. Fill one with plain tap water and the other with lime water. Now gently blow air from your mouth through a straw into each one.

In the plain water, you just get bubbles, and nothing else happens. But in the lime water, something different occurs: it slowly turns milky (cloudy white). If you leave it, a white powder settles at the bottom.

Where did that white powder come from? It was not there before! The carbon dioxide gas in your breath reacted with the lime water and made a new substance — a white solid called calcium carbonate, which does not dissolve in water. That is why the liquid looks milky. A new substance appeared. That is the signature of a chemical change.

We can write this reaction in a short word form:

Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

(lime water) (your breath) (the new white solid)

Now here is the most important idea in the whole chapter — and the question NCERT often skips: why can’t you get the old substance back from a chemical change?

Picture it like this. In a physical change, you rearranged the furniture in a room — easy to put back. But in a chemical change, you knocked the furniture apart and built something completely new out of the pieces — a new object that did not exist before. To get the old thing back, you would have to break the new object apart and rebuild the old one piece by piece, which is very hard. That is why chemical changes are so difficult to undo. A new substance is genuinely new.

Figure 5.2 below shows three everyday chemical changes. In each one, look at how different the new substance is — and notice that you can never easily get the first thing back.

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Here are common chemical changes from daily life, and the new substance each makes:

• Burning (paper, wood, a candle, a gas stove): makes ash, smoke, and gases. The wood or paper is gone for good.
• Rusting of iron: a shiny iron gate slowly grows brown, flaky rust. We will look at this closely soon.
• Cooking food: raw dough becomes a cooked chapati; raw vegetables become a sabzi. The cooked food is a new substance — you cannot un-cook it.
• Curdling of milk: milk turns into curd. Tasty, but you can never turn the curd back into milk.
• A cut apple turning brown: the white flesh reacts with the air and forms a new brown substance on the surface.

A handy clue: when you see bubbles of gas, take notice. Mix a little vinegar (or lemon juice) with a pinch of baking soda, and it fizzes loudly. That fizz is a new gas — carbon dioxide — being made. A new substance formed, so this too is a chemical change.

The signs of a chemical change

Sometimes it is hard to see whether a new substance has formed. Luckily, chemical changes leave behind clues. If you spot one or more of these signs, a chemical change has probably happened. Figure 5.3 below shows the five main clues as easy icons.

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Let us go through each clue:

1. A colour change. The substance turns a different colour. A shiny silver iron nail turns brown when it rusts.
2. A gas is given off. You see bubbles or fizzing, like with vinegar and baking soda. A new gas is being made.
3. Heat or light is produced. When wood or a candle burns, it gives out heat and light. When a magnesium ribbon burns, it makes a bright white light.
4. A new smell appears. Spoiled food smells bad because new, smelly substances have formed.
5. A new solid forms. A solid powder appears in a liquid, like the white powder that settles when lime water goes milky.

A word of care: not every clue means a chemical change every single time. For example, boiling water makes bubbles, but that is just water turning to steam — a physical change. So use the signs as helpful hints, then always ask the one true test: has a new substance formed?

Before we move on, let us make sure the deeper “why” behind these changes is clear.

Reversible vs irreversible

We can also sort changes another way: by whether we can undo them.

• A reversible change can be undone. You can get the original thing back. Melting ice (then freezing it again) is reversible.
• An irreversible change cannot be undone. You cannot get the original back. Cooking popcorn is irreversible — you can never turn it back into a corn kernel.

Here is the useful link, which you may have already guessed:

• Physical changes are usually reversible (you can undo them), because no new substance was made — you only changed the look.
• Chemical changes are usually irreversible (you cannot undo them), because a brand-new substance was made.

Be careful, though: “usually” is not “always”. A few physical changes are hard to undo (you cannot un-chop a vegetable), even though no new substance formed. So reversible vs irreversible and physical vs chemical are two different ways of sorting, and they line up most of the time — but not perfectly. The surest test is still: did a new substance form?

Here is a clean side-by-side comparison of the two main types of change to lock it in:

Let us put all of this together by sorting some real changes into the two boxes. Figure 5.4 below does exactly that.

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Now let us practise deciding, step by step, with a real example.

Rusting and how to prevent it

Let us look closely at one chemical change you have surely seen: rusting.

Leave an iron object — a nail, a gate, a cycle chain — out in the open for a while, especially in a damp place. Slowly, a rough, flaky, reddish-brown layer grows over it. This layer is called rust.

Is rusting a physical or chemical change? It is a chemical change. The shiny grey iron reacts with oxygen (from the air) and water (or moisture in the air) to make a brand-new brown substance called iron oxide — that is rust. Rust is different from iron: it is brown, weak, and flaky, while iron is grey, strong, and shiny. A new substance formed, so it is chemical.

Rusting is a problem. It eats away at iron and weakens it. Rusty gates crack, rusty tools become useless, and rusty pipes leak. So how do we stop it?

The trick is simple. Rusting needs two things together: water and oxygen. If we keep either of them away from the iron, rust cannot form. Figure 5.5 below shows why iron rusts and three common ways we keep water and air away from it.

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Here are the main ways to prevent rusting, and you see them everywhere:

• Painting. A coat of paint covers the iron and blocks air and water. This is why iron gates, grills, and railway bridges are painted.
• Oiling or greasing. A thin layer of oil or grease keeps moisture off. This is why we oil tools, door hinges, and the chain of a cycle.
• Galvanising. The iron is coated with a thin layer of another metal, zinc. This zinc layer protects the iron underneath. Buckets, water pipes, and roofing sheets are often galvanised.

In every method, the idea is the same: build a barrier so that water and oxygen cannot reach the iron.

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Summary

• A change means something becomes different. We notice changes using our senses, and we sort all changes into two big groups.
• A physical change changes only the look of a substance — its shape, size, or state — and no new substance is formed. Examples: ice melting, tearing paper, dissolving sugar.
• A chemical change forms one or more brand-new substances through a chemical reaction. Examples: burning, rusting, cooking, milk turning to curd.
• The signs of a chemical change are: a colour change, a gas given off, heat or light produced, a new smell, or a new solid forming. Use these as hints, then ask the true test: did a new substance form?
• A reversible change can be undone; an irreversible change cannot. Physical changes are usually reversible, and chemical changes are usually irreversible — because a new substance is hard to take apart and rebuild.
• Rusting is a chemical change: iron reacts with water and oxygen to make brown rust (iron oxide).
• We prevent rust by keeping water and air away from iron — by painting, oiling or greasing, or galvanising (coating with zinc).
• The single most useful question for any change is: Did a new substance form? No means physical; yes means chemical.

What’s Next

You have learned how the things around you change — from ice and paper to iron and food. But the most amazing change of all might be the one happening inside you.

In the next chapter, Adolescence: A Stage of Growth and Development, you will discover the big changes your own body goes through as you grow from a child into a young adult — why they happen, and how to stay healthy and confident through them. See you there!