An odd-shaped stone, and its density

A stone has no neat shape, so no ruler and formula can give its volume. Activity 9.7 uses water instead.

Fill a measuring cylinder to any easy level, say 50 mL, and write it down as the initial volume (Table 9.2).

Tie the stone (or a bunch of keys) with a thread and lower it slowly into the water: the thread lets it go in gently without splashing. The water rises. Write down the final volume, say 55 mL.

Final − initial = 55 − 50 = 5 mL. That's the water the stone pushed aside, so it is the stone's own volume. For a solid, write it as 5 cm³.

Now you have both numbers: from Activity 9.3 the stone's mass is 16.400 g, and its volume is 5 cm³.

Density = 16.400 g ÷ 5 cm³ = 3.28 g/cm³

That's how to find the density of any odd-shaped object: weigh it, sink it in water, divide.

Density explains something much bigger too. The Earth has layers: crust, upper mantle, lower mantle, outer core, inner core. The crust, where we live, is the lightest, and density increases towards the centre, because deep down the pressure and temperature are much higher, squeezing the materials heavier and more compact.