Simple Distillation

Introduction

Evaporation recovers the dissolved solid from a solution but loses the solvent into the air. What if you want to keep the solvent — for example, to get pure water from salty water? Simple distillation does exactly that. The solution is boiled, the vapour is led away and cooled, and pure liquid drips out at the other end. It combines two changes of state you already know, boiling and condensing, into one separation method.

Core explanation

The idea. In a solution such as salt water, the solvent (water) boils at 100 °C, but the solute (sodium chloride) has a boiling point above 1400 °C. When the solution is heated, only the water turns into a gas. If that water vapour is cooled somewhere else, it turns back into liquid water with no salt in it.

The apparatus. A typical laboratory set-up has:

- a flask containing the solution, heated from below; - a thermometer at the top of the flask, with its bulb level with the side arm, measuring the temperature of the vapour; - a condenser — a glass tube surrounded by a jacket of cold running water — sloping downwards from the side arm; - a receiver (beaker or conical flask) to collect the liquid.

What happens. As the solution is heated, the solvent boils. Its vapour rises, passes the thermometer and enters the condenser. The cold walls of the condenser remove energy from the vapour, so it condenses back into a liquid. This liquid, the distillate , runs down the sloping tube into the receiver. The dissolved solid stays behind in the flask, becoming more and more concentrated.

Checking purity. While pure water is distilling, the thermometer reads steady at 100 °C (at normal atmospheric pressure). A steady reading equal to the known boiling point is good evidence that the distillate is pure.

When to use it. Simple distillation works well when you want to separate a liquid from a dissolved solid , or two liquids whose boiling points are very far apart. It does not work well for two liquids with close boiling points, because both would boil off together; that needs fractional distillation.

Safety. The flask must never be heated to dryness, and the apparatus must be open at the receiver end so that pressure cannot build up.

Step-by-step reasoning

To explain how simple distillation separates salt water:

1. Heating makes the water boil; the salt does not boil. 2. Water vapour leaves the flask and travels into the condenser. 3. The condenser cools the vapour, so it condenses to liquid. 4. Pure water collects in the receiver as distillate. 5. Salt remains in the flask.

Visual explanation

Picture a round-bottomed flask of blue copper sulfate solution over a heater. A side arm leads into a long sloping tube wrapped in a water jacket. Colourless drops fall from the end of the tube into a beaker, while the solution left in the flask becomes a deeper and deeper blue.

Real-world analogy

Simple distillation is like steam from a boiling pan of pasta water collecting on a cold kitchen window. The starch and salt stay in the pan, but the droplets running down the glass are almost pure water. A condenser does the same thing, deliberately and efficiently.

Real-world example

Laboratories need water free of dissolved minerals for making up solutions and rinsing glassware. Tap water contains dissolved calcium and magnesium salts, so many labs use a still to distil it. The distilled water is pure enough that it leaves no residue when evaporated.

Why?

Why does the distillate contain no salt? Only particles that have escaped into the gas state can travel through the side arm. Sodium and chloride ions are held strongly in the solution and do not evaporate at 100 °C, so they cannot reach the condenser. Only water particles make the journey.

Common misconception

"Distillation destroys the dissolved solid." The solute is not destroyed or boiled away; it stays in the flask. If distillation continued until the flask was almost dry, the solid would be left behind unchanged.

Worked example

Question: Ink is a solution of coloured dyes in water. A student distils 50 cm³ of ink. What is collected in the receiver, and what reading would the thermometer show?

Reasoning: The dyes are solids with very high boiling points; water boils at 100 °C. Only water vapour reaches the condenser.

Answer: Colourless water is collected, and the thermometer reads about 100 °C.

Quick check

1. What is the name for the liquid collected in simple distillation? Answer: The distillate.

Exam focus

Examiners expect you to describe distillation as two changes of state: evaporation or boiling in the flask and condensation in the condenser. Label the flask, thermometer, condenser (water in at the bottom, out at the top) and receiver. State that the solute remains in the flask.

Advanced insight

Distillation can also be run at reduced pressure. Lowering the pressure lowers the boiling point, so heat-sensitive liquids can be distilled at a lower temperature without breaking down. This "vacuum distillation" is used for products such as some vitamins and flavour compounds.

Summary

Simple distillation separates a solvent from a solution by boiling it, then condensing the vapour in a cold condenser. The pure liquid collected is the distillate; the solute stays in the flask. A steady thermometer reading at the known boiling point indicates a pure distillate. The method suits a liquid with a dissolved solid, or liquids with very different boiling points.

Practice questions

1. Name the two changes of state that take place in simple distillation. Answer: Boiling (liquid to gas) in the flask and condensation (gas to liquid) in the condenser. 2. Why does salt not appear in the distillate when salt water is distilled? Answer: Salt has a very high boiling point, so it does not turn into vapour and cannot reach the condenser. 3. Give one sign that the distillate collected is pure water. Answer: The thermometer stays steady at 100 °C while it is collected (or the liquid leaves no residue on evaporation). 4. Why is simple distillation unsuitable for separating two liquids that boil at 78 °C and 100 °C? Answer: Their boiling points are too close, so both liquids evaporate together and the distillate is still a mixture; fractional distillation is needed.