Detergent granules dissolve into the wash liquor and spread by Fick's second law, ∂C/∂t = D∇²C: concentration diffuses from where it is high (near a dissolving granule) toward where it is low, at a rate set by the diffusion coefficient D.

Why hot water helps

D rises with temperature because warmer water is less viscous (Stokes-Einstein: D ∝ T/η), so granules dissolve faster and detergent reaches the fabric sooner. Reaction kinetics for the surfactants and enzymes also speed up with heat — but only up to a point.

Enzymes have a sweet spot

Proteases, amylases and lipases are proteins: each is folded to work fastest around 30–50°C. Push much hotter and the enzyme itself starts to denature — its folded shape unravels and activity collapses. Blood is worst hit: above roughly 60°C the protein in the stain itself coagulates and sets, which is why cold or lukewarm water is recommended for blood, never hot.

Matching enzyme to stain

• Protease cleaves proteins — targets blood, egg, grass-protein and sweat stains.
• Amylase cleaves starches — targets grass, potato, gravy and starchy food stains.
• Lipase cleaves fats/oils, working alongside surfactants that emulsify grease into micelles — targets butter, cooking oil and sebum stains, and benefits most from extra heat.

Chemistry + mechanics

Chemical action (enzymatic breakdown and surfactant emulsification) loosens the stain from the fibres; mechanical agitation — the tumbling/sloshing of the wash — physically carries loosened soil away and keeps fresh, un-depleted detergent solution moving past the fabric surface, which also boosts the effective mixing rate beyond pure molecular diffusion.