Every one of the three familiar states of matter is made of the same basic ingredients — atoms and molecules. What separates a solid from a liquid from a gas isn't what they're made of, but how strongly and how rigidly those particles interactwith each other.
With that molecular picture in mind, the definition is simple: a fluid is a substance that has no fixed shape. Pour it, and it flows to fill whatever container holds it. Both liquids and gases qualify — despite behaving very differently in other ways, neither one holds a shape of its own.
Fluids — and really, any substance — can be characterized by density: how much mass is packed into a given volume.
Density is what actually determines whether one substance floats on another, not which one has more total mass. A huge volume of a low-density fluid can outweigh a small volume of a high-density one — but the low-density fluid still floats on top.
Adjust each fluid's mass and volume. Watch which one settles on top — it's always the less dense fluid, regardless of which one has more mass.
Fluid A (ρ = 0.80 kg/L) floats on Fluid B (ρ = 0.90 kg/L) — floating depends only on density, not on which fluid has more mass.
A sample of an unknown liquid has a mass of 0.850 kg and occupies a volume of 1.00 L. Find its density, and compare it to water (ρ_water = 1.00 kg/L).
Fluid A has a mass of 2.4 kg and a volume of 3.0 L. Fluid B has a mass of 1.8 kg and a volume of 2.0 L. Which fluid is denser, and would Fluid A float on Fluid B if they didn't mix?
Real fluids are complicated — they resist flow to varying degrees, and their volume can shift slightly under pressure. To make the physics in the rest of this unit tractable, we'll model fluids as ideal fluids, which have exactly two defining properties:
Its volume never changes, no matter how much pressure is applied.
No internal friction — nothing resists layers of the fluid sliding past each other.