Density
Learn how much mass is packed into a volume, measure regular and irregular objects, and use density to predict floating. This is Cambridge International IGCSE Physics 0625, separate from Egypt’s Egyptian Baccalaureate / Thanaweya Amma and not a Year 3 course. Supplement / Extended liquid layering is labelled.
Density is mass packed into space
Mass tells us how much matter an object contains. Volume tells us how much space it takes up. Density compares them: how much mass is packed into each unit of volume. A compact metal block can have more mass than a same-sized piece of wood.
Open the worked example
If two equal-sized cubes have masses 80 g and 20 g, the first cube has four times the density: the same volume contains four times the mass.
Quick check: Two objects have the same volume. Which is denser: the one with more mass or less mass?
Think, then reveal: The one with more mass. Density is mass divided by volume, and the volumes are equal.
Explore mass per volume
Interactive model · change mass and volume
Density is the same mass distributed through a volume. Move either slider and watch the ratio change. This is a model, not a measurement of a real sample.
ρ = m ÷ V = 120 g ÷ 100 cm³ = 1.20 g/cm³
Compared with water (1 g/cm³), this sample is denser than water: it would sink.
Density laboratory · choose the measurement method
Liquid
Weigh the empty cylinder and then the cylinder with liquid. Subtract to find the liquid mass. Read the volume at eye level, at the bottom of the meniscus. Divide mass by volume.
Regular solid
Measure the dimensions with a ruler. Calculate the geometric volume (for a cuboid, length × width × height), weigh the solid, then calculate density = mass ÷ volume.
Irregular solid that sinks
Measure its mass. Record the starting water volume V₁. Lower the solid gently until fully submerged and read V₂. Its volume is V₂ − V₁. Avoid splashing; wipe any spill promptly.
Practise, then read the explanation
Try first, open the optional hint if useful, then submit to see the worked reasoning. Questions on immiscible-liquid layers are Supplement / Extended.
Optional hint
Compare equal-sized pieces of different materials: one may contain more mass in the same space.
Original learning material aligns to Cambridge IGCSE Physics 0625 syllabus objectives 1.4.1–1.4.4 as grouped in the supplied Volume A. Density equation, units, liquid/regular/irregular volume measurement and float-or-sink comparisons are Core. The relative layering of immiscible liquids and an object resting at the boundary are marked Supplement / Extended. Source reading: Physics-0625 Volume A, PDF pp. 49–53 (printed pp. 41–45). Values in the interactive model are illustrative, not experimental measurements.