Oil and fat are collectively referred to as fats and oils, and the main components of fats and oils are glycerides of various fatty acids. Vegetable oils are easily decolorized by bromine, but not fats (animal oils are definitely not) because they sometimes have unsaturated bonds. The fatty tissues of animals and the seeds and kernels of oleaginous plants are the main sources of fats and oils. Those that are solid or semi-solid at room temperature are called fats, and those that are liquid are called oils. Fats contain more glycerides of higher saturated fatty acids, oils contain more glycerides of higher unsaturated fatty acids, and most of the natural fats and oils are mixed glycerides (i.e., R, R′, and R″ are not the same or not exactly the same). All kinds of fats and oils are mixtures of multiple advanced fatty acid glycerides. The average molecular weight of a fat can be reacted by its saponification value (the number of milligrams of KOH required to saponify 1g of fat). The smaller the saponification value, the larger the average molecular weight of the fat. The degree of unsaturation of fats and oils is often expressed by the iodine value (the number of grams of I2 required for the addition reaction between 100g of fat and iodine). The greater the iodine value, the greater the degree of unsaturation of the fat. The content of free fatty acids in fats and oils is often expressed in terms of acid value (the number of milligrams of KOH required to neutralize 1g of fat and oil). Fresh fats and oils have a very low acid value. Improperly preserved fats and oils will increase the acid value due to oxidation and other reasons. Some oils can form a hard and elastic film in the air, and oils with this property are called dry oils (iodine value greater than 130), such as tung oil and linseed oil. Waxes, like fats and oils, are esters that are widely found in nature. The main components of waxes are generally esters of high saturated fatty acids and high monohydric alcohols containing an even number of carbon atoms, e.g., the main component of white wax is wax ester of wax (C25H51COOC26H53), the main component of beeswax is beeswax stearate (C15H31COOC30H61), and the main component of cetaceous wax is cetaceous stearate (C15H31COOC16H31). ). Due to customary reasons, some of the called waxes are not esters. Example female paraffin is a high-level alkane, high polyethylene glycol is a synthetic wax.