Activities 8.1 and 8.2 — Is Air a Mixture? — "Nature of Matter: Elements, Compounds, and Mixtures"

1. 13 marksCuriosity Grade 8, Chapter 8, page 118

   Is air a mixture? What kind of mixture is it, and what are its main components?

   Hint. Recall the composition from Grade 6, then apply this chapter's two categories.

   Yes — air is a uniform mixture.

   You have learnt that air is a uniform mixture of mainly nitrogen, oxygen, argon, carbon dioxide, and water vapour.

   Why it counts as a mixture: the gases are simply mixed, they do not react chemically with one another, and each keeps its own properties. Oxygen in air still supports combustion; nitrogen in the same air still does not.

   Why uniform: the gases are evenly distributed and cannot be distinguished from one another — no microscope will show you a region of pure oxygen in a room.

   ✦ The chapter gives one number and one property, and no more. Nitrogen, which constitutes about 78% of the air, does not take part in combustion, and oxygen is required by most of the living beings to stay alive. It also helps in combustion. Percentages for oxygen, argon or carbon dioxide are not given here, so do not invent them — write the components in the order the chapter lists them and give the one figure it supplies.

2. 24 marksCuriosity Grade 8, Chapter 8, pages 118-119

   Describe how lime water is prepared in Activity 8.1, and state the safety precaution the chapter attaches to it.

   Hint. Two chemical steps, then a filtration. The precaution is about the first step.

   Preparation.

   1. Take a glass tumbler and fill it half with water.
   2. Add a small amount of calcium oxide (quick lime) slowly to it.
   3. Calcium oxide reacts vigorously with water to form calcium hydroxide and releases heat.
   4. Stir continuously to make a solution of calcium hydroxide. This solution is called lime water.
   5. Filter it and observe its colour — it is colourless.

   Safety. The chapter's box says only Perform this step carefully, and the reason is in the sentence beside it: the reaction is vigorous and releases heat. That is why the quick lime goes in slowly and in a small amount.

   ✦ Two things are worth noticing about the design. The solution is filtered so that any undissolved solid is removed — otherwise the liquid would already look cloudy and the test that follows would prove nothing. And you are told to observe its colour at this stage, because the whole activity depends on being able to say later that something changed.

3. 34 marksCuriosity Grade 8, Chapter 8, page 119

   In Activity 8.1 the colourless lime water is left in a petri dish for a few hours and turns milky. Explain why, and write the word equation.

   Hint. Something in the air is reacting with the calcium hydroxide, and the product does not dissolve.

   What happens.

   Lime water turns milky when carbon dioxide reacts with calcium hydroxide to form calcium carbonate (insoluble tiny white particles) and water.

   Word equation:

   Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

   Why milky, specifically. The calcium carbonate formed is insoluble, so it does not disappear into the solution the way a dissolved substance would. It stays as tiny white particles suspended throughout the liquid, and a liquid full of suspended white particles looks milky. The milkiness is the calcium carbonate.

   ✦ And here is the inference the activity is really for: Since lime water turns milky when exposed to air, this activity demonstrates the presence of carbon dioxide in the air. Nobody blew into the dish. The only thing the lime water was exposed to was ordinary room air, so the carbon dioxide must have come from there — which is one more component of the mixture confirmed by experiment rather than taken on trust.

   Why keep stirring at regular intervals? Stirring keeps bringing fresh solution to the surface, where it can meet the air. Without it only the top layer would react and the change would be far slower and harder to see.

4. 43 marksCuriosity Grade 8, Chapter 8, page 119

   Describe Activity 8.2 and state what it shows about the air. Why is the paper required to be black and free of visible dust?

   Hint. The design of this activity is almost entirely about making a small effect visible.

   The activity. Take a black sheet of paper, ensure that it is free from any visible dust particles, place it undisturbed near an open window or in the garden for a few hours, then examine it — a magnifying glass helps.

   Observation. Tiny particles have settled on its surface.

   This shows that dust particles are suspended in the air. They are not an integral part of the air and are considered pollutants. The nature and the number of dust particles in the air may vary from time to time and from place to place.

   ✦ Both conditions on the paper are controls. It must be black because dust is pale and would be invisible on a white sheet — the contrast is what makes a barely visible effect visible. It must start free of visible dust because otherwise you could never say whether the particles you find at the end had arrived from the air or were there all along. Change either condition and the activity proves nothing.

   And 'undisturbed' matters too: the particles have to settle out of still air, so any movement of the sheet would sweep them off again.

5. 53 marksCuriosity Grade 8, Chapter 8, page 119

   Why does the chapter say dust particles are not an integral part of the air? What difference does that make?

   Hint. Compare the dust with nitrogen, and think about what varies and what does not.

   Because they are not always there in the same way, and the air is perfectly good air without them. The chapter's own reason is in the next sentence: the nature and the number of dust particles in the air may vary from time to time and from place to place. Nitrogen is about 78% of the air on a hilltop and in a classroom alike; the dust is not.

   ✦ The distinction is between a component and a contaminant. Nitrogen, oxygen, argon, carbon dioxide and water vapour are what air is. Dust is something that has got into it — which is why the same sentence calls it a pollutant.

   And that is why the chapter can move straight to air quality. The major pollutants present in the air are particulate matter (dust, soot) and gases like carbon monoxide, ozone, nitrogen dioxide, and sulfur dioxide. The air quality index (AQI) is a tool used to describe the air quality. You can measure and report pollution precisely because it is not part of the definition of air — there is a clean baseline to compare against.

6. 63 marksCuriosity Grade 8, Chapter 8, page 119

   Sunlight entering a dark room through a small opening shows tiny shining particles moving about. What are they, and why can you see them there and nowhere else?

   Hint. The chapter raises this observation just before Activity 8.2.

   They are dust particles suspended in the air — the same particles that settle on the black sheet in Activity 8.2.

   Why they show up in a sunbeam. Two conditions happen to be met at once. The narrow beam is very bright, so each particle scatters enough light to be seen; and the rest of the room is dark, so there is nothing bright behind them to drown that scattering out. Take away either condition — full daylight, or no beam — and the same particles are still there but invisible. The beam does not create the dust; it reveals it.

   ✦ They move because the air particles are moving. Chapter 7 established this with potassium permanganate and with incense smoke: the tiny particles of smoke suspended in the air are constantly hit by invisible particles of gases, and their movement helps us observe the motion of gas particles. The drifting specks in a sunbeam are that same demonstration, running for free in an ordinary room.

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.