Distillation
Simple distillation
Separation method used to separate a solvent from a solution. is used to separate a The liquid in which the solute dissolves to form a solution. from a Mixture formed by a solute and a solvent.. It is useful for producing water from salt solution.
Simple distillation works because the A substance is said to be dissolved when it breaks up and mixes completely with a solvent to produce a solution. The dissolved substance in a solution. has a much higher The temperature at which a substance rapidly changes from a liquid to a gas. than the solvent. When the solution is heated, solvent Gas formed below the boiling point of a substance. evaporates from the solution. The gas moves away and is cooled and Turned from a gas or vapour into a liquid, usually as a result of cooling.. The remaining solution becomes more The concentration of a solution tells us how much of a substance is dissolved in water. The higher the concentration, the more particles of the substance are present. in solute as the amount of solvent in it decreases.
Image caption, 1. Salty water is heated
1 of 3
Simple distillation can also be used to separate two liquids with a large difference in boiling points, such as The alcohol which is produced as a result of fermentation of sugars by yeast. and water. The mixture must be carefully heated and the temperature controlled, so that only the liquid with the lower boiling point evaporates. This can then be collected using a condenser.
Image caption, 1. Water and ethanol solution is heated
1 of 3
Learn more on distillation and chromatography in this podcast.
Listen to the full series on 成人论坛 Sounds.
Fractional distillation
In fractional distillation a mixture of several substances, such as crude oil, is distilled and the evaporated components are collected as they condense at different temperatures. is used to separate different liquids from a mixture of multiple liquids. Fractional distillation of a mixture of several liquids works because the liquids have different boiling points.
Fractional distillation can be used to separate different In fractional distillation, such as that of crude oil, the different parts of the original mixture are called fractions. The substances in each fraction have similar boiling points to each other. from Mixture of hydrocarbons, mainly alkanes, formed over millions of years from the remains of ancient dead marine organisms.. This uses a fractionating column that is hotter at the bottom, and cooler at the top. The different liquids are collected from different parts of the column.
In the laboratory a glass fractionating column is used. This is connected to a condenser. It is often filled with glass beads to increase the surface area available for condensing.
When the mixture is heated:
- vapours rise through a fractionating column which is hot at the bottom, and cold at the top
- vapours condense when they reach a part of the column that is below the temperature of their boiling point
- the liquid flows back down the column
- if the temperature at the top of the column is above the boiling point of a substance, its vapour can pass into the condenser and be collected
- as the mixture is heated, eventually all the substances (fractions) will be collected
Which type of distillation is best to separate a mixture of two liquids?
A mixture of two liquids can be separated by either distillation method. However, simple distillation requires very careful control of the temperature. Fractional distillation is better if the two liquids have very similar boiling points. If simple distillation is used instead, the distillate is likely to be a mixture of the two liquids as both will evaporate when heated.