Practical: Finding the density of objects of different shapes
To find density we must measure the object’s mass and volume. Mass is weighed on a balance. The volume of a rectangular block is length × width × height: V = a · b · c (cm³). The volume of an irregular object (for example a key) is found with water: read the starting water volume V₁ in a measuring cylinder, lower the object on a thread until fully under, read the new volume V₂; the object’s volume is V = V₂ − V₁. Then density is calculated by ρ = m : V and compared with a table to name the material. An object with density below 1 g/cm³ floats in water. Density can even help check whether a piece of jewellery is real gold (gold – 19.3 g/cm³).
Practical work: 1) weigh a key (or metal nut); 2) pour 100 ml of water into a 250 ml measuring cylinder, lower the object on a thread and record V₂; 3) calculate V = V₂ − V₁ and ρ = m : V; 4) measure the block’s mass and sides and calculate the same way; 5) compare with the table. Safety: lower the object slowly on its thread (glass can break – a plastic cylinder is better), wipe spills, and do not use sharp-edged objects.