NCERT Solutions for Class 7th Science Chapter 5 Projects and investigations at the end of the chapter — Exploratory Projects

Book page 71 Updated on2026-09-19

Q1.

Write a message on a piece of paper using lemon juice as ink and let it dry. The message will become invisible. Now use a warm iron over the paper (or hold the paper over the flame of a candle, taking care that it does not catch fire). The invisible letters turn dark brown as the paper gets warm. Can any of these changes be reversed?

Answer

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

Step	What happens	Type of change	Can it be reversed?
The lemon-juice writing dries and becomes invisible	The water of the juice evaporates, leaving a thin colourless film	Physical	Yes — wet the paper again and the mark reappears faintly
The warm iron makes the letters turn dark brown	The dried lemon juice is charred — new brown substances are formed	Chemical	No — the brown letters can never be made invisible again

Why it happens: Lemon juice contains carbon compounds (sugars and citric acid). These break down and char at a much lower temperature than paper does. So the written parts turn brown while the rest of the paper is still only warm — which is exactly why the message appears.

Caution: Perform this activity under the supervision of an adult. If you use a candle, keep the paper well above the flame and moving, so that it does not catch fire.

Q2.

We hear a lot of news about landslides and breaking of rocks in hilly regions these days, causing a lot of damage to life and property. Discuss what steps we can take to reduce landslides and rock erosion.

Answer

Method for the discussion: first list what makes a hillside slip — loose soil, heavy rain, steep cutting for roads and houses, and the loss of tree cover — then match one preventive step to each cause. A good answer should name at least four steps and say how each one works.

Step	How it helps
Afforestation — plant trees, grasses and deep-rooted shrubs on the slopes	Roots bind the soil together and leaves break the force of falling rain
Stop deforestation and unregulated grazing	Bare slopes wash away far faster than covered ones
Terrace farming instead of ploughing along a slope	Flat steps slow the water down so it cannot carry soil away
Retaining walls, gabions (wire cages filled with stones) and wire netting on cut faces	Hold loose rock and soil in place along roads
Proper drainage — build drains that carry rain water off the slope	Water soaking into the hill makes it heavy and slippery, which is the commonest trigger
Control blasting, quarrying and steep road cutting; avoid heavy construction on fragile slopes	Every cut removes the support at the foot of the hill
Early-warning systems and rainfall monitoring	People can be moved to safety before a slide happens

Sample answer: “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.”

Q3.

Observe the activities going on in the kitchen and note any changes that can be reversed. Are these physical or chemical changes?

Answer

How to do it: sit in the kitchen for half an hour with a notebook. For every change you see, write three things — what changed, whether the original can be got back, and whether a new substance was formed.

Change seen in the kitchen	Can it be reversed?	Physical or chemical?
Ice cubes melting in a glass	Yes	Physical
Water boiling; steam condensing on the lid	Yes	Physical
Sugar or salt dissolving in water	Yes (evaporate the water)	Physical
Making balls of dough and rolling them into chapatis	Yes (roll it back into a ball)	Physical
Butter melting on a hot tawa	Yes (it sets again on cooling)	Physical
Grinding soaked rice and dal into a paste	No	Physical, but irreversible
Milk setting into curd; food being cooked; LPG burning	No	Chemical

Conclusion: almost every change in the kitchen that can be reversed turns out to be a physical change — usually a change of state or of shape. The changes that cannot be reversed, such as cooking, curdling and burning of fuel, are the chemical ones.

Q4.

Yeast is added while baking bread to make it fluffy and soft. How does yeast work? Try and find out! Take a small bottle, some sugar, fresh yeast, water, and a balloon. Make a sugar solution in the bottle by mixing two teaspoons of sugar and a small amount of water. Now add a spoonful of yeast and cover the mouth of the bottle with a balloon. Leave it undisturbed for about an hour. What do you observe? Carefully take off the balloon, holding its mouth tightly closed and attach it to another small bottle containing freshly prepared lime water. Shake the bottle so that the contents of the balloon get mixed with lime water. What do you observe? What can you conclude from this experiment? Identify all the changes occurring in the experiment and state which of them are physical and chemical changes.

Answer

What you observe after an hour: the mixture becomes frothy and bubbly, gives off a faint sour, bread-like smell, and the balloon slowly inflates and stands up on the mouth of the bottle. A gas is clearly being produced inside.

What you observe with the lime water: when the gas from the balloon is shaken with freshly prepared lime water, the lime water turns milky.

What you can conclude: the gas made by the yeast is carbon dioxide. Yeast is a living fungus; it feeds on the sugar and breaks it down, releasing carbon dioxide. In dough, the same gas is trapped as thousands of tiny bubbles, which push the dough up and leave holes in it — that is what makes bread fluffy and soft.

Sugar + Yeast → Carbon dioxide + Other substances
Carbon dioxide + Lime water → Calcium carbonate (white, insoluble) + Water → milky

Change in the experiment	Physical or chemical?	Reason
Sugar dissolving in water	Physical	The sugar only spreads out; it can be recovered by evaporation
Yeast acting on the sugar and producing gas	Chemical	A new substance, carbon dioxide, is formed
The balloon stretching and inflating	Physical	Only the size and shape of the rubber change
Gas moving from the bottle into the balloon	Physical	Only the position of the gas changes
Lime water turning milky	Chemical	A new substance, calcium carbonate, is formed

Tip: Use lukewarm water, not hot. Yeast is alive — very hot water kills it and then the balloon will never inflate. Keep the bottle in a warm place for a faster result.

Q5.

Chameleons (Girgits) change colour to blend in with their surroundings and also when they are angry or sense danger (Fig. 5.12). Is this a change that can be reversed? Explore from the internet or from your school library.

Fig. 5.12, page 72 — a chameleon (girgit) resting on a branch, its green colour blending with the leaves (redrawn sketch of the textbook illustration).

Answer

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.

Why it happens: The colour is not a dye that is made and destroyed. The chameleon's skin carries special cells containing tiny crystals arranged in a regular pattern. When the animal is calm, the crystals lie close together and the skin reflects blue and green light; when it is excited, angry or frightened, the skin is stretched slightly, the crystals move apart, and now yellow, orange and red light is reflected instead. Pigment-carrying cells in another layer can also spread out or bunch up. Since no new substance is formed — only the spacing of the crystals and the spreading of pigment change — this is a physical and reversible change.

Did you know? Chameleons change colour far more for communication and temperature control than for hiding. A dark skin absorbs more heat on a cold morning, and a bright display warns off a rival. Cuttlefish, octopuses and some fish can change colour in the same reversible way.

Where to look: your school library's encyclopaedia of animals, the NCERT reference books on animal behaviour, or a trusted website such as a national museum or a zoological society page. Note down the source you used.

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