Fats: preparation and properties
Fats are esters of the trihydric alcohol glycerol with higher fatty acids. Animal fats are mostly solid and contain saturated acids (palmitic C₁₅H₃₁COOH, stearic C₁₇H₃₅COOH); vegetable fats (oils) are liquid and rich in unsaturated acids (oleic C₁₇H₃₃COOH, linoleic C₁₇H₃₁COOH, linolenic C₁₇H₂₉COOH), so they melt at lower temperatures. Fats are insoluble in water, lighter than water and soluble in organic solvents such as petrol; in the body they give more than twice as much energy as carbohydrates. In hydrolysis a fat splits into glycerol and fatty acids. Hydrolysis with alkali (saponification) gives glycerol and the salt of the fatty acid — soap: tristearin + 3NaOH → C₃H₅(OH)₃ + 3C₁₇H₃₅COONa. Sodium soaps are hard, potassium soaps soft or liquid. Hydrogenating a liquid fat with hydrogen over a Ni catalyst gives a solid fat (raw material for margarine): triolein + 3H₂ → tristearin. Making soap at home with alkali is dangerous: caustic alkali is used only under the teacher’s supervision.
Compare “saturated fat” and “vegetable oil” on food labels and note in a table which is solid and which is liquid. No chemical experiments with soap or fat are done at home.