NCERT Solutions for Class 7 Science Chapter 5 Changes Around Us Physical and Chemical (PDF) – 2027

They are talking about everyday changes that we notice with our senses. Two of them are physical changes (nothing new is formed) and two are chemical changes (a new substance is formed).

Here is Table 5.1 filled in. Look carefully at what exactly changes in each case — that is what the observation column is asking for.

Yes. Some other changes that happen around us:

• In the kitchen: milk setting into curd, kneading atta into dough, baking a chapati on the tawa, sugar dissolving in tea, a cut apple turning brown.
• At home and school: sharpening a pencil, folding a paper boat, an iron gate rusting, a candle burning.
• In nature: a seed germinating, fruits ripening, dew forming on grass, dead leaves rotting into compost.
• In our body: hair and nails growing, a wound healing, digestion of food.

Yes. Changes can be grouped in more than one useful way, and the same change can appear in several groupings.

The grouping this chapter uses is the first one: physical change and chemical change.

Yes. When the boat, the aeroplane, the bird or the butterfly is unfolded, we get back the same sheet of paper — the same size, the same colour, the same weight. Only some crease lines remain.

Yes. As soon as the air rushes out, the rubber shrinks back and we get the same uninflated balloon in our hand. Only its size and shape had changed, so this is a physical change.

The balloon bursts with a loud bang and tears into pieces of rubber. No — we cannot get the whole uninflated balloon back. The pieces are still rubber, so this is a physical change that cannot be reversed.

No — the powder cannot be pressed back into the original stick of chalk. But the powder is still chalk: it is the same white colour and it still writes on the blackboard. Only its size has changed, so this is a physical change.

Yes — in all three cases only the appearance changed, while the material stayed exactly the same.

Since no new substance is formed in any of them, all three are physical changes. This is exactly the definition the book gives: changes in which only physical properties like shape, size and state change are called physical changes.

Yes — but only in tumbler B.

The white solid that settles is a new substance — calcium carbonate — which is insoluble in water. Because one or more new substances are formed, this is a chemical change, brought about by a chemical reaction:

The mixture fizzes at once. We hear a fizzing, bubbling sound and see a froth of gas bubbles rising and filling the upper part of the test tube. The test tube also feels slightly cool.

The clear, colourless lime water turns milky, exactly as it did in Activity 5.3 when we blew air into it.

We infer that the gas is carbon dioxide, because turning lime water milky is the test for carbon dioxide. This gas was not present in the vinegar or the baking soda, so it is a new substance and a chemical change has taken place:

No fizzing and no stream of gas bubbles are seen. The baking soda simply dissolves in the water and the liquid stays clear. (A few small air bubbles that were clinging to the powder may float up, but the fizzing sound is absent.) This is a physical change.

It is a chemical change, because a new substance, magnesium oxide (a white powder), is formed. Heat and light are also given out.

The open candle keeps burning. The covered candle grows dim, flickers and goes out after a short while.

Because the covered candle does not have a continuous supply of air. The oxygen inside the tumbler is soon used up, and without oxygen the flame is extinguished.

By the lime water test. Pour a small amount of freshly prepared lime water into the petri dish (or into the tumbler as soon as the candle goes out) and swirl it gently. The lime water turns milky, which confirms that carbon dioxide is present.

Wrap a blanket or a thick cloth tightly around the person. This cuts off the supply of air, and the fire is extinguished. Use a cotton or woollen blanket — never a synthetic one, as it can melt and stick to the skin.

Heat — enough heat to raise the substance to its ignition temperature, the minimum temperature at which it catches fire.

So combustion needs three things together, shown as the fire triangle:

1. A combustible substance, also called fuel — paper, wood, cotton, kerosene.
2. Oxygen — supplied by the air.
3. Heat that allows the fuel to reach its ignition temperature.

No. What we really need is heat, not a flame. A flame is only one convenient way of supplying that heat.

For a while nothing seems to happen. Then the bright spot turns brown, the paper starts to emit smoke, and finally it catches fire (Fig. 5.7b) — even though no flame was ever brought near it.

By the idea of ignition temperature.

Focusing the sunrays heats the paper. Its temperature keeps rising with time. After some time the paper becomes so hot that it starts burning. The minimum temperature at which a substance catches fire is called its ignition temperature.

All the students are partly right — and taken together they describe the whole story. The burning of a candle involves both physical and chemical changes at the same time.

What actually happens: the wax of the candle melts, is carried up the wick, and evaporates due to the heat of the flame. It is this vapour of wax that burns to produce the flame.

Sometimes yes, sometimes no. Changes fall into two groups — those that can be reversed and those that cannot.

Here is Table 5.2 completed with twelve of the changes discussed in this chapter.

Yes. Some other desirable changes around us:

• At home: curd being churned into butter, dough rising with yeast, rice cooking, wet clothes drying in the sun, water being boiled for drinking.
• In the garden: seeds germinating, buds opening into flowers, kitchen waste and dry leaves rotting into compost.
• In industry: clay being baked into pots and bricks, sugarcane juice being made into jaggery, cotton being spun into thread.
• In our body: digestion of food, healing of a wound.

Yes. These sediments are formed by the slow breaking down of large rocks — a process called weathering, which involves both physical and chemical changes.

Yes. That fine sand is formed when rock pebbles, soil and sediments are broken down and moved from one place to another by natural forces like wind and flowing water. This process is called erosion.

(c) (i) and (iii)

Both are physical changes.

Three reasons:

1. No new substance is formed. Ice, water and steam are all the same substance in three different states.
2. Only the state changes, which is a physical property.
3. Both changes can be reversed — ice melts back to water, steam condenses back to water.

Curdling of milk is a chemical change.

Justification:

1. A new substance, curd, is formed. Its taste (sour), smell and texture (thick, set) are completely different from those of milk.
2. The change cannot be reversed — curd can never be turned back into milk.

It is both — the formation of soil from rocks, called weathering, involves physical as well as chemical changes.

A suitable title: “From Kitchen Waste to Colourful Flowers”.

The lime water turns milky in (a) and (d) only.

Only the first change can be reversed; the second cannot.

“Landslides in our hills have increased mainly because trees are being cut and slopes are being cut steeply for roads and hotels. We should plant deep-rooted trees on bare slopes, build proper drains so that rain water does not soak into the hillside, protect road cuttings with retaining walls and wire netting, practise terrace farming, and follow strict rules about construction in landslide-prone areas.”

Conclusion: the kitchen changes that can be reversed — melting, boiling, dissolving, rolling dough — are physical changes. Cooking, curdling of milk and burning of fuel cannot be reversed; they are chemical changes.

Observation after an hour: the mixture becomes frothy and the balloon slowly inflates — a gas is being produced.

Observation with lime water: the lime water turns milky.

Conclusion: the gas made by the yeast is carbon dioxide. Yeast breaks down the sugar and gives out carbon dioxide; in dough this gas forms tiny bubbles that make bread fluffy and soft.

Changes in the experiment:

Yes — this change can be reversed. A chameleon that has turned bright and dark can return to its ordinary green or brown within minutes, and it may change back and forth many times in a single day.