NCERT Solutions for Class 7 Science Chapter 1 the Ever Evolving World of Science (PDF) – 2027

Yes. A paper plane is a simple glider: it has no engine, yet it stays in the air for a while. Asking why it stays up led people to study wings. Early inventors studied bird wings and built model gliders; later, engineers added engines and designed real aircraft. So the dream of flying grew out of simple observations and experiments.

A “how” question asks for the mechanism — the steps by which something works or happens.

To answer it we (1) break the thing into parts, (2) find out what each part does, and (3) follow the chain from the first part to the last, changing one part at a time to test it. Example: a torch works because the cell, switch, wires and bulb together make a complete path, so the bulb glows.

A “why” question asks for the cause — the reason an event happens the way it does. “How” describes the steps; “why” explains what lies behind them. For example, ice in a glass melts because heat always flows from the warmer air to the colder ice. A good “why” answer is one that can be tested.

A pattern is something that repeats in a regular way. From patterns in nature we learn to:

• predict — day follows night and the Moon’s shape changes in a cycle of about a month, so we can plan ahead;
• find the hidden rule — day and night repeat because the Earth spins on its axis;
• classify — materials that shine and conduct electricity are grouped together as metals.

Some fruits are sour because they contain acids. A sour taste tells us that an acid is present — for example, citric acid in lemon and orange, tartaric acid in tamarind and ascorbic acid (vitamin C) in amla. A raw mango is sour, but as it ripens much of its acid is used up and it turns sweet.

When we rub soap on a yellow haldi stain, it turns reddish-brown. Soap solution is basic, and turmeric is a natural indicator that turns red with a base. When the cloth is rinsed well with plenty of water, the stain turns yellow again and fades with more washing.

We need materials that allow electricity to pass through them — conductors, such as metals. A lamp glows only when the cell, the wires, the lamp and anything joined into the loop make a complete path. In the picture on page 3 a steel spoon is joined into the circuit; the spoon is a metal, so the bulb glows. A plastic scale or a rubber band in its place would not let it glow.

They are all changes that happen around us, and they are of two kinds:

• Changes that can be reversed — ice melting into water (cool the water and we get ice back).
• Changes that cannot be reversed — a battery running out, fruits ripening, rocks breaking into pebbles.

Some of these changes happen, or happen faster, when things are heated.

Because at this age the body begins to change from a child’s body into an adult’s. Height and weight increase quickly (a growth spurt), and the shape of the body, the voice and our feelings also change. These changes are started by chemical messengers in the body called hormones. They are normal and happen to everyone, though at different speeds.

Life processes are not found in animals alone — plants carry them out too. Plants are living things, so they also need food, respire, move materials inside their bodies, grow and reproduce. Only the way they do it is different: for example, plants make their own food in their green leaves, while animals eat food made by others.

Yes, plants also need food to grow. Food gives them the energy to grow and the material from which new leaves, stems, roots, flowers and seeds are built. The difference is that green plants make their own food.

Green plants make their own food by photosynthesis, mainly in their leaves. The leaves take in carbon dioxide from the air through tiny pores called stomata, and the roots take up water from the soil. Using sunlight trapped by the green pigment chlorophyll, the leaves make food (glucose) and give out oxygen.

Yes, plants also breathe (respire), day and night. They have no lungs or nose; gases pass in and out through small openings:

• leaves — through tiny pores called stomata;
• stems — through small openings in the bark called lenticels;
• roots — the root surface takes in air from the spaces between soil particles.

Time is what passes between two events. It tells us which event came first and how long something lasted. We cannot touch time, but we can measure it by counting a fixed unit. The SI unit of time is the second (s); 60 s = 1 minute and 60 minutes = 1 hour.

We measure time by counting an event that repeats after equal intervals. Long before electric clocks, people read the time from the position of the shadow of an object in the Sun — the idea of the sundial. Later came water clocks and sand clocks, pendulum clocks, and today’s quartz and digital watches.

“How fast” asks about the rate of a change — how much happens in a unit of time. For a moving object it is its speed:

Yes, it is amazing. We humans can wonder about things we can never touch or visit — why we have day and night, how the Moon goes around the Earth, why eclipses happen — and by observing carefully and asking questions we can find the answers. This sense of wonder, or curiosity, is where science begins.

“Just make it half!” can answer many very different questions. The book gives these:

• “How do we ensure getting equal shares of cake?”
• “My essay is too long.”
• “I can’t fit this in the envelope.”
• “I cannot dance to such a long song.”

Any question that this answer really solves is correct — there are no wrong questions here.

• “The tea has come out far too strong and bitter — what do I do now?”
• “The kheer has thickened too much while cooling. How do I loosen it?”
• “My coffee is too hot to sip and I have to leave in a minute!”

An answer that begins with “Because…” must belong to a “Why…?” question.

• “Why did the mouse manage to escape from the cat?”
• “Why did the cat leave half its fish untouched?”
• “Why could the cat not bite through the string?”

This is a calm reply to someone who is worried, so the question should describe a small emergency.

• “It has started raining and we are still half an hour from home — now what?”
• “The water bottle has leaked all over the inside of the bag!”
• “The bench at the sports ground is wet — where will we sit?”

Any question in which 42 is a real, interesting quantity will do — but not an obvious sum like “What is 32+10?”, which the book asks us to avoid.

• “How many students in our class took part in the science fair?”
• “What was the highest temperature in °C recorded in our city last May?”
• “How many laddoos were left after the whole class had eaten?”