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NCERT Solutions Curiosity Chapter 8 Chapter opener — Probe and ponder

Book page 116 Updated on2026-09-05

Q1.
Which of the entities in the picture above consist of matter, and which of them do not?
Answer

Almost everything you can point to in the school picture is matter — it has mass and takes up space. What is not matter are the forms of energy in the scene and what is going on inside the children's heads.

Consist of matterDo not consist of matter
The trees, bushes, grass and flowers; the students themselves; their uniforms and shoes; the football; the books, lunch boxes, water bottle and school bag; the white cane; the steps, the ramp, the metal railing and the tactile yellow tiles; the building, the arches and the doors; and the air all around themThe sunlight falling on the ground; the heat of the evening sun; the shadows; the sound of the children playing; and the thoughts, ideas and feelings of the students
Why it happens: Matter is anything that has mass and occupies space, and everything made of matter is built from elements and compounds. Light, heat, electricity, sound, thoughts and emotions are real and important parts of our world, but you cannot weigh them or fill a bottle with them — so they are not matter.
Tip: Air is easy to miss. You cannot see it, but a filled balloon has mass and takes up space — air is matter, and it is a uniform mixture of gases.
Q2.
How can elements be combined to form a compound?
Answer

Elements form a compound only when they combine chemically in a fixed ratio — not when they are merely mixed. Usually some energy, most often heat, has to be supplied to make the combination happen.

Activity 8.5 in this chapter shows both halves of the idea:

  • Mixing only: 5.6 g of iron filings stirred with 3.2 g of sulfur powder gives Sample A. The black iron and the yellow sulfur are still visible, a magnet still pulls the iron out — nothing new has been made.
  • Chemical combination: heat the same Sample A and a black mass, Sample B, is formed. It is iron sulfide, the same colour and texture all through, not attracted by a magnet, and the iron and sulfur can no longer be taken apart.
Iron + Sulfur (on heating) → Iron sulfide
Element one kind of atom Compound two elements joined, fixed ratio Mixture particles loose, ratio can vary
In a compound the two kinds of particles are locked together in a fixed ratio; in a mixture they simply sit side by side.
Why it happens: In a compound the particles of the different elements are held to each other so tightly that no physical method — no magnet, no sieve, no filter, no evaporation — can pull them apart. That is exactly why the new substance behaves nothing like the elements it came from. Water is made of hydrogen, a fuel, and oxygen, which supports burning, in the fixed ratio 2 : 1, and yet water puts out fire.
Q3.
How could the discovery of a compound that absorbs carbon dioxide from the air contribute to solving environmental challenges?
Answer

Such a compound would let us take carbon dioxide out of the air itself, instead of only trying to release less of it. Carbon dioxide traps heat in the atmosphere, so removing it directly attacks the cause of global warming.

You have already met one substance that does this in Activity 8.1: calcium hydroxide (lime water). Left standing in a petri dish, it turns milky because the carbon dioxide of the air reacts with it.

Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

A good absorbing compound could be used to:

  • clean the exhaust gases of factories, power plants and cement kilns before they reach the open air;
  • keep the air breathable in closed spaces — submarines, spacecraft and mines;
  • lock the carbon dioxide away as a solid carbonate, so it cannot escape again;
  • slow down global warming, and with it the melting of glaciers and the rise in sea level.
Did you know? The chapter's graphene aerogel points to the other half of the problem — it is highly porous, so it has a very high absorbing capacity. A material that is both a good absorber and cheap to make in large amounts is what scientists are still searching for.
Q4.
Share your questions
Answer

This bullet asks you to add your own questions to the chapter. Write questions that a page of the chapter could actually answer, and keep them open — starting with why, how or what if rather than yes/no.

Sample questions:

  • Air is a mixture of gases, so why does the oxygen not settle at the bottom and the lighter gases at the top?
  • If a compound has a fixed ratio, what happens when I mix iron and sulfur in the wrong proportion and heat it?
  • Stainless steel is a mixture, yet it looks like one metal — how uniform can a mixture really get?
  • Why can dissolved salt be got back from water by evaporating, while hydrogen cannot be got back from water in the same way?
  • Only two elements are liquid at room temperature. What decides whether an element is a solid, a liquid or a gas?
Tip: Keep these questions in your notebook and tick them off as the chapter answers them. The ones still unticked at the end are the ones worth asking your teacher.
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