NCERT Solutions Curiosity Chapter 9 .5.1 Determination of density — In-text Questions

Book page 1439 Updated on2026-09-05

Q1.

A tetra pack says it contains 200 mL buttermilk (chach) (Fig. 9.14). What does that mean?

Answer

It means the buttermilk inside occupies a space of 200 millilitres. The number on the pack is a volume, not a mass — it tells you how much room the liquid fills, not how heavy it is.

1 mL = 1 cm³, so 200 mL = 200 cm³
1 L = 1000 mL = 1 dm³, so 200 mL = 0.2 L

Buttermilk is mostly water, and 1 mL of water is close to 1 g,
so 200 mL of buttermilk weighs roughly 200 g

Why volume is stated and not mass: A drink is sold by the space it fills because that is what the container has to hold and what you actually drink. For a liquid the two are easy to convert into one another only if you know the density — which is precisely the quantity this chapter is building towards.

Tip: Volume of solids is usually quoted in cm³ and of liquids in mL, but they are the same size of unit — 1 mL is exactly 1 cm³. That is why Table 9.2 records the water displaced in mL and the volume of the stone in cm³.

Q2.

How accurately can these measuring cylinders measure?

Answer

Only as accurately as their smallest division allows, and that depends on the size of the cylinder — a bigger cylinder has a coarser scale.

Capacity of the measuring cylinder	Smallest volume it can read
10 mL or 25 mL	0.1 mL
100 mL	1 mL
250 mL	2 mL
500 mL	5 mL

So choosing a cylinder is a choice between two faults. To measure 70 mL of water:

a 50 mL cylinder cannot do it in one go — you must measure 50 mL and then 20 mL, and measuring in two steps is inconvenient and adds error twice;
a 250 mL or 500 mL cylinder does it in one step but reads only to 2 mL or 5 mL, so the accuracy drops;
a 100 mL cylinder does it in one step and reads to 1 mL — it is the best choice.

Why bigger means coarser: A wider cylinder needs a wider tube to hold the same height of liquid, so each millilitre poured in raises the level by less. The marks would come impossibly close together if they were drawn every millilitre, so they are drawn further apart in volume instead.

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