Probe and Ponder — The Opening Questions — "Nature of Matter: Elements, Compounds, and Mixtures"

1. 13 marksCuriosity Grade 8, Chapter 8, page 116

   Look at the opening picture of the school ground. Which of the entities in it consist of matter, and which of them do not?

   Hint. Matter is anything that has mass and takes up space. The chapter names four things that are not matter.

   Matter — nearly everything you can point at. The students themselves, their uniforms and shoes, the tree and its leaves, the flowering bushes, the grass, the football, the school building and its arches, the metal railing and the ramp, the steps, the backpacks and lunch boxes, the steel water bottle, the books and notebooks, the white cane — and the air filling the whole scene, even though you cannot see it.

   Not matter — the things that have no mass and take up no space. The sunlight lighting the ground, the shadows it casts, the warmth of the morning sun, the sounds of the game, and the thoughts and feelings of the children in it.

   ✦ The chapter states this directly on page 130: Elements and compounds are the building blocks of matter — everything that has mass and takes up space ... However, not everything around us is matter. Light, heat, electricity, and even thoughts and emotions are important parts of our world, but they are not made of matter.

   The test is the two-part definition, not visibility. Air is invisible and is still matter, because it has mass and occupies space — you proved this in Chapter 7 by weighing an inflated balloon. Light is perfectly visible and is not matter, because it has neither. So can I see it? is the wrong question to ask; does it have mass and take up space? is the right one.

2. 23 marksCuriosity Grade 8, Chapter 8, page 116

   How can elements be combined to form a compound?

   Hint. The chapter answers this twice — once by taking water apart, once by putting iron sulfide together.

   By combining chemically, in a fixed ratio, to give something entirely new.

   Compounds are formed when different elements combine in fixed ratios to form something entirely new. The properties of compounds are different from those of elements forming that compound. The constituent elements of a compound cannot be separated by any physical method.

   ✦ Activity 8.5 is this question performed in front of you. Iron filings and sulfur powder simply stirred together stay a mixture — you can still see the black and yellow specks, and a magnet still pulls the iron out. Heat them, and a black mass forms that is not attracted by the magnet at all, cannot be pulled apart again, and gives a rotten-egg gas with acid where the mixture gave an odourless one.

   	Mixing	Combining chemically
   What you do	Stir them together	Supply heat (or another chemical push)
   Properties	Each component keeps its own	Entirely new properties appear
   Ratio	Any ratio you like	A fixed ratio
   Separation	Physical methods work	No physical method works

   Water is the same story read backwards. Activity 8.3 shows that passing electricity through water gives hydrogen and oxygen, so those two elements must have combined to make it — in the fixed ratio of 2 atoms of hydrogen to 1 of oxygen.

3. 34 marksCuriosity Grade 8, Chapter 8, page 116

   How could the discovery of a compound that absorbs carbon dioxide from the air contribute to solving environmental challenges?

   Hint. Think about what carbon dioxide does in the atmosphere, and then about what the chapter says a highly porous material can do.

   What such a compound would give you. Carbon dioxide released by burning fuels builds up in the air. A compound that soaks it up would let you take it out again — at a chimney or power-station outlet before it ever escapes, or from ordinary air afterwards. The captured gas could then be stored, or used to make other useful substances instead of being lost.

   ✦ The chapter puts a real example beside this question. Graphene aerogel ... is made from carbon and is said to be the lightest material on earth ... It is highly porous and therefore, has a high absorbing capacity. For this reason, it can potentially be used as an environmental cleaner, for example, to clean up oil spills in both seas and on land. Notice the shape of that argument: a property (highly porous) leads to a capability (absorbs a great deal) leads to a use (cleaning up spills). A carbon-dioxide absorber would follow exactly the same chain.

   Two cautions worth writing down. The chapter itself says potentially — the material exists, the large-scale use is still a possibility rather than a fact. And absorbing carbon dioxide treats the symptom; it does not stop the burning that produced it. Both are honest points to make in an answer, since an examiner is looking for reasoning, not enthusiasm.

4. 42 marksCuriosity Grade 8, Chapter 8, page 117

   The chapter's opening paragraph lists a staircase, air, water, food, clothes, shoes, a book, trees, a ball and a stick. What single point is it making with such an odd list?

   Hint. Ask what these ten things have in common, and then what they do *not* have in common.

   The point is that all ten are matter, and almost none of them are one substance.

   Most of the things around us are not made of just one substance; rather they are made up of two or more substances mixed together.

   ✦ The list is deliberately ordinary. A staircase, a shoe and a lunch box are not laboratory objects, which is exactly why they make the point — the mixtures in this chapter are not something a chemist creates, they are the normal condition of the world. A pure substance is the rare and special case, and getting one usually takes work.

   That sets up the whole chapter: if most things are mixtures, then to understand matter you first have to be able to say what a mixture is (§8.1), what a pure substance is (§8.2), and what the two kinds of pure substance are (§8.3).

5. 53 marksCuriosity Grade 8, Chapter 8, page 117

   Before reading further, predict: which of these is a mixture, and which is a single substance — poha, sugar dissolved in water, air, gold, lemonade?

   Hint. Ask in each case whether more than one substance is present, whatever it looks like.

   	Mixture or single substance	Why
   Poha	Mixture	Flattened rice, oil, onion, spices, peanuts — the chapter's own opening example
   Sugar in water	Mixture	Two substances, sugar and water, neither of which has changed into anything else
   Air	Mixture	Mainly nitrogen, oxygen, argon, carbon dioxide and water vapour
   Gold	Single substance — an element	Nothing else is present in it
   Lemonade	Mixture	Water, sugar, lemon juice, salt

   ✦ Four of the five are mixtures, and that is the chapter's point. Notice that three of them — sugar water, air and lemonade — look completely uniform. Sugar and water particles cannot be seen separately in their mixture. Looking uniform is not the same as being one substance, which is precisely why the chapter has to divide mixtures into uniform and non-uniform before it can go anywhere else.

   Gold is the odd one out because it is a single kind of matter all the way down. It is also, as §8.5 notes, a native mineral — one of the few substances found in the Earth already pure.

Solutions written by the tuition.in editorial team and checked against NCERT Curiosity — Textbook of Science for Grade 8, Chapter 8 (hecu108.pdf), Reprint 2026-27, pages 116-133. Questions are referenced from the NCERT textbook for identification.