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I. Principle of like dissolves like 1. Polar solvents (such as water) readily dissolve polar substances (polar substances in ionic crystals and molecular crystals, such as strong acids)； 2. Non-polar solvents (such as benzene, gasoline, carbon tetrachloride, alcohol, etc.) can dissolve non-polar substances (most organic compounds, Br2, I2, etc.) ； 3. Substances with identical functional groups are miscible with each other; for example, water, which contains hydroxyl groups (—OH), can dissolve alcohols, phenols, and carboxylic acids that also have hydroxyl groups. II. Solubility of Organic Compounds and Functional Groups 1. Solubility of functional groups: (1) Functional groups that are soluble in water (i.e., hydrophilic groups) include —OH, —CHO, —COOH, and —NH2. (2) Water-insoluble functional groups (i.e., hydrophobic groups) include: all hydrocarbon groups (—CnH2n+1, —CH=CH2, —C6H5, etc.), halogen atoms (—X), nitro groups (—NO2), etc. 2. The ratio of hydrophilic groups to hydrophobic groups in a molecule affects the solubility of the substance: (1) When the number of functional groups is the same, as the number of carbon atoms in the hydrocarbon group (hydrophobic group) increases, the solubility gradually decreases ； For example, in terms of solubility: CH3OH > C2H5OH > C3H7OH > …… Generally, alcohols with more than 5 carbon atoms are insoluble in water. (2) When the number of carbon atoms in the hydrocarbon group is the same, the greater the number of hydrophilic groups, the higher the solubility of the substance ； For example, solubility: CH3CH2CH2OH (3) When the effects of hydrophilic and hydrophobic groups on solubility are roughly equal, the substance is slightly soluble in water ； For example, common substances that are slightly soluble in water include phenol C6H5—OH, aniline C6H5—NH2, benzoic acid C6H5—COOH, and n-pentanol CH3CH2CH2CH2CH2—OH (in the structural formulas of these substances, the group on the left of the “—” symbol is a hydrophobic group, while the group on the right is a hydrophilic group) ； Ethyl acetate CH3COOCH2CH3 (where —CH3 and —CH2CH3 are hydrophobic groups, and —COO— is a hydrophilic group). (4) Substances composed of two types of hydrophobic groups are necessarily insoluble in water. For example, halohydrocarbons R-X and nitrocompounds R-NO2 are insoluble in water because the hydrocarbon group R–, the halogen atom –X, and the nitro group –NO2 are all hydrophobic groups.