EN हिं

NCERT Solutions Curiosity Chapter 8 .1.1 Is air a mixture? — Activity 8.1: Let us experiment

Book page 1188 Updated on2026-09-05

Q1.
Take a glass tumbler and fill it half with water. Add a small amount of calcium oxide (quick lime) slowly to it. What do you observe?
Answer

The calcium oxide reacts vigorously with the water. You will see bubbling and hissing, the lump breaks up into a white powdery paste, and the tumbler becomes distinctly warm to the touch.

Calcium oxide + Water → Calcium hydroxide + heat released
Why it happens: This is a chemical change — a completely new substance, calcium hydroxide, is formed, and heat is given out during the reaction. That released heat is what you feel through the glass.
Safety first: The book warns you to perform this step carefully. Add the quick lime slowly and in a small amount, keep your face away from the tumbler, and do not touch the mixture with bare hands.
Q2.
Stir continuously to make a solution of calcium hydroxide. This solution is called lime water. Filter it and observe its colour.
Answer

After filtering, the lime water is a clear, colourless liquid. The milky-white look of the unfiltered liquid disappears with the undissolved calcium hydroxide left behind on the filter paper.

Why it happens: Only a little calcium hydroxide dissolves in water; the rest stays as fine solid particles that scatter light and make the liquid look white. Filtering removes those particles and leaves behind only the dissolved part — and a solution of a colourless substance in water is itself colourless.
Tip: Starting with a clear, colourless solution is the whole point. Any milkiness you see later must then have come from something new, not from what you began with.
Q3.
Leave this colourless solution in a petri dish for a few hours (Fig. 8.5a). Keep stirring the solution at regular intervals. What do you observe (Fig. 8.5b)? Does it turn milky?
Answer

Yes. Over a few hours the clear solution slowly turns milky white, and a thin white film may appear on the surface.

StageWhat you see
Just after filtering (Fig. 8.5a)Clear, colourless lime water
After a few hours in the open petri dish (Fig. 8.5b)Milky white — turbid throughout
Why it happens: The petri dish is open, so a large surface of the lime water stays in contact with the air. Stirring keeps bringing fresh solution to that surface, so the reaction with the air goes on steadily instead of stopping at the top layer.
Q4.
Can you explain why the solution has turned milky?
Answer

Because the carbon dioxide present in the air has reacted with the calcium hydroxide of the lime water and formed calcium carbonate — insoluble tiny white particles — along with water.

Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
Why it happens: Calcium carbonate does not dissolve in water. It stays suspended as very fine white particles that scatter light, and that scattering is what your eye reads as milkiness. Since nothing was added to the dish except air, the carbon dioxide must have come from the air — so this activity proves that air contains carbon dioxide.
Try This: Blow gently through a straw into fresh lime water. It turns milky in seconds instead of hours, because exhaled air has far more carbon dioxide than ordinary air.
Was this helpful? Report an error