Diffusion: high → low concentration?

**The reasoning** Diffusion is the **net movement of particles from a region of high concentration to a region of low concentration** until equilibrium is reached. This happens naturally without any external energy input — it's driven by random molecular motion. Think of perfume spreading across a room: the perfume molecules are concentrated at the spray point (high) and gradually spread to areas with fewer molecules (low). This is true for **gases, liquids, and even solids** (though much slower in solids). The principle: particles naturally spread out to balance their distribution. **Why the wrong options tempt you** **B (No)** — Tempting if you confuse diffusion with active transport (which moves substances *against* concentration gradients and needs energy). **C (Sometimes)** — This tricks you into thinking external factors change diffusion's direction. No! External factors may speed up or slow down diffusion, but the direction remains high → low. **D (Only solids)** — Reverses the truth. Diffusion is *slowest* in solids, *fastest* in gases. **Quick takeaway** Diffusion always flows downhill — from crowded (high concentration) to spacious (low concentration) — like students rushing from a packed classroom into an empty corridor!