Miscible Liquids and Boiling Points

Introduction

Water and ethanol mix perfectly: pour one into the other and you get a single clear liquid with no layers. That makes a separating funnel useless. Their boiling points are different — 100 °C for water and 78 °C for ethanol — so distillation seems the obvious answer. But if you try simple distillation, the liquid you collect is still a mixture. This page explains why, and sets up the need for a better method.

Core explanation

Miscible liquids. Miscible liquids mix at the particle level. Water and ethanol particles attract each other strongly, so they spread evenly through one another. There is no interface and no layers to run off, so any separation must rely on a different property. The obvious one is boiling point .

Boiling points of some liquids (°C, at normal pressure):

Liquid Boiling point (°C) --- --- Propanone (acetone) 56 Methanol 65 Ethanol 78 Water 100

Heating a mixture. A mixture of two miscible liquids does not boil at the boiling point of either pure liquid. A mixture of ethanol and water boils at a temperature between 78 °C and 100 °C, depending on its composition. As it boils, both substances evaporate. Particles of each liquid are escaping from the surface at the same time.

The vapour is richer in the more volatile liquid. The vapour above the boiling mixture contains a higher proportion of ethanol than the liquid does, because ethanol is more volatile — it escapes more easily. But the vapour still contains a significant amount of water.

Why simple distillation falls short. In simple distillation, the vapour goes straight into the condenser. The distillate is therefore just the condensed vapour: richer in ethanol than the starting mixture, but far from pure. As distillation continues, the ethanol in the flask is used up, the boiling temperature rises and the distillate contains more and more water. The thermometer does not stay steady at a single value, which is a sign that a mixture is distilling.

When the gap is large. If two liquids have very different boiling points (for example, a difference of more than about 80 °C), the vapour at first contains almost only the more volatile one, and simple distillation can give a reasonable separation. For closer boiling points, such as the 22 °C gap between ethanol and water, a better method is needed: fractional distillation .

Step-by-step reasoning

To predict the outcome of simply distilling two miscible liquids:

1. Compare the boiling points of the two liquids. 2. Recognise that both evaporate when the mixture boils. 3. The vapour is richer in the liquid with the lower boiling point. 4. If the gap is small, the distillate is still a mixture.

Visual explanation

Picture a particle diagram of a boiling ethanol and water mixture: red dots (ethanol) and blue dots (water) evenly mixed in the liquid. In the vapour space above, there are more red dots than blue, but blue dots are clearly still there. The condensed distillate shows the same mixture of colours.

Real-world analogy

Imagine a crowd leaving a stadium through the same gate: fast walkers and slow walkers. Early on, more fast walkers get out, but plenty of slow walkers leave too. If you only count the first group through the gate, it is mostly fast walkers, but it is not pure. One gate is not enough to sort them.

Real-world example

Early spirit makers heated fermented liquids in a simple pot still. The distillate was stronger in alcohol than the original liquid but still contained a lot of water, so the process had to be repeated to raise the concentration. This shows why simple distillation is not efficient for liquids with close boiling points.

Why?

Why do both liquids evaporate, when the mixture is below 100 °C? Evaporation happens at any temperature: some water particles at the surface always have enough energy to escape. At the boiling temperature of the mixture, a lot of water particles escape along with the ethanol particles, so both appear in the vapour.

Common misconception

"Heating an ethanol and water mixture to 78 °C boils off pure ethanol and leaves pure water." In fact the mixture boils above 78 °C, and water evaporates alongside ethanol. The vapour is a mixture, only enriched in ethanol.

Worked example

Question: Which pair would be separated more successfully by simple distillation: methanol (65 °C) and ethanol (78 °C), or propanone (56 °C) and a liquid boiling at 190 °C?

Reasoning: The first pair differs by 13 °C, so the vapour would contain lots of both. The second pair differs by 134 °C, so the vapour would be almost all propanone at first.

Answer: Propanone and the liquid boiling at 190 °C.

Quick check

1. Why can a separating funnel not separate ethanol from water? Answer: They are miscible, so they form a single liquid with no layers.

Exam focus

Examiners want you to state clearly that both liquids evaporate when a mixture boils, so simple distillation gives an impure distillate when boiling points are close. Name fractional distillation as the method needed.

Advanced insight

Ethanol and water can never be completely separated by any distillation at normal pressure. A mixture of about 95.6% ethanol by mass boils at a constant temperature (about 78.2 °C), and its vapour has the same composition as the liquid. Such a constant-boiling mixture is called an azeotrope . Other methods, such as drying agents, are needed to remove the last water.

Summary

Miscible liquids mix completely and form no layers, so they are separated using differences in boiling point. When a mixture boils, both liquids evaporate, although the vapour is richer in the more volatile liquid. Simple distillation therefore gives an impure distillate when boiling points are close, and the temperature does not stay steady. Fractional distillation is needed.

Practice questions

1. Define miscible. Answer: Miscible liquids mix completely to form a single liquid with no layers. 2. Explain why the distillate from simply distilling ethanol and water is not pure ethanol. Answer: Both liquids evaporate at the boiling temperature of the mixture, so the vapour, and the distillate, contain water as well as ethanol. 3. A mixture of ethanol and water boils at 85 °C. Explain why it does not boil at 78 °C. Answer: It is a mixture, not pure ethanol; mixtures boil between the boiling points of the pure components, depending on composition. 4. How does the thermometer reading show that a mixture is distilling? Answer: The temperature does not stay steady at one value but rises gradually during the distillation.