Chitin,β-1,4-poly-N-acetyl-D-glucosamine, molecular formula (C8H13NO5)n. 
Physical characteristics: White or off-white amorphous translucent solid. Insoluble in water, dilute acid, dilute alkali, concentrated alkali and ordinary organic solution etc. There are of α, β and γ three crystallographic forms. Widely existing in shell of shrimp and crab, insect shell, fungi cell water and plant cell wall.