Question: The purity of gold can be tested by weighing it in air and in water. How? Do you think you could get away with making a fake gold brick by gold-plating some cheaper material?

When an object is immersed in a liquid, the buoyant force acting on the particular object equals the fluid displaced by that object. The weight of the liquid does not have to match the object's weight. The force depends only on the density of the liquid.

The purity of gold is known by its density measurement. There is a set standard of pure gold; after testing, the given gold has to be of the correct density and match the standard.

Gold is denser than all common metals, meaning it takes up less volume for its weight. When submerged in water, a pure gold brick of a certain weight will “lose” less weight than any brick made of impure gold metal because it will displace less water and experience a small buoyant force. It is possible to determine the purity of gold in the air simply by weighing the gold.

If $w_1$is weight of gold in water and $w_2$in air then, the density can be expressed as,

role="math" localid="1653385666215" $p=\left(\frac{W_2}{W_2-W_1}\right)\times p_w$

Here,$p_w$ is the density of water at temperature T.

If the measured density of gold matches the standard density of gold, then it is said to be pure gold.

Bricks made of cheap materials will not have the same quality as gold bricks and could not be fabricated as gold. Generally, costly materials, such as plutonium, platinum, or osmium, are used to contain high-density materials.

Thus, the gold bricks could not be faked by gold plating on cheap materials.