Q1(h). Brownian movement and Tyndall effect: why only colloids

Answer in brief

Both need the colloidal size, 1–1000 nm: particles small enough to be jostled visibly yet large enough to be seen and scatter light strongly; molecules are too small, suspensions too large.

Key points

• Size ranges: true solution < 1 nm; colloid 1–1000 nm; suspension > 1 µm.
• Brownian movement: zig-zag motion caused by unequal bombardment by solvent molecules; a colloidal particle is small enough for the impacts not to cancel and large enough to be seen (ultramicroscope).
• Einstein: ⟨x²⟩ = 2Dt, D = kBT/6πηr; r = 50 nm gives an r.m.s. shift of 3.1 µm in 1 s (visible), r = 5 µm only 0.31 µm.
• Tyndall effect: light scattered by particles of different refractive index; Rayleigh I ∝ d⁶/λ⁴, so a 10 nm particle scatters 6.4 × 10⁷ times more than a 0.5 nm molecule.
• Molecules also move and scatter (blue sky), but far too weakly to be seen in a beaker; suspensions are opaque and settle.