Properties of Alcohols (SQA National 5 Chemistry): Revision Note

Miscibility of alcohols

• Solubility describes how well a solute dissolves in a solvent

  • For example:

    • Sugar is soluble in water

    • Sand is insoluble in water

• Miscibility describes how well two liquids mix or dissolve together

  • It essentially has the same meaning as solubility, but only applies to liquids

  • For example:

    • If two liquids are miscible, they will mix completely to form a single solution

    • If two liquids are immiscible, they will not mix

• The miscibility / solubility of an alcohol in water changes as its carbon chain gets longer

Miscibility of small alcohols

• Methanol, ethanol and propanol are miscible with water

• Their small size and -OH group allow them to mix completely with water

Miscibility of larger alcohols

• From butanol onwards, the solubility decreases significantly as the carbon chain gets longer

• The long hydrocarbon part of the molecule does not mix well with water

• This effect becomes more significant as the chain grows

Solubility of alcohols in water

Melting & boiling points of alcohols

• As you go up the alcohol homologous series, the molecules get larger

• This has a direct effect on their melting and boiling points

Melting points of alcohols

• The melting points of straight-chain alcohols generally increase as the number of carbons in the chain increases

  • In general, a bigger molecule means a higher melting point

• The trend for melting points of alcohols is less regular than boiling points

Boiling points of alcohols

• The boiling points of straight-chain alcohols show a much clearer and more predictable trend:

  • The boiling points increase as the molecule gets bigger

Explaining the general trend

• The overall trend for both melting and boiling point is that they increase as the molecule gets bigger

• The melting and boiling points increase because:

  1. As alcohol molecules get larger, the strength of the intermolecular forces increases

     • Intermolecular forces are the weak attractions between molecules

  2. More energy is needed to overcome these stronger forces to allow the substance to melt or boil

  3. Therefore, the melting and boiling points generally increase