Evaporation to Recover a Dissolved Solid

Introduction

Filtration cannot separate salt from salt water, because the dissolved salt passes straight through the paper. So how do we get the salt back? We remove the water instead. When a solution is warmed, the solvent turns into vapour and escapes, while the dissolved solid, which does not evaporate at those temperatures, is left behind. This method, evaporation , is the natural next step after filtration when the substance you want is dissolved in the filtrate.

Core explanation

The principle. A solution contains a solute dissolved in a solvent . For evaporation to work, the solvent must be volatile — it turns into vapour easily — while the solute is not volatile and stays behind. Water boils at 100 °C, but sodium chloride melts only at about 801 °C and hardly evaporates at all. Heating salt water therefore drives off the water as steam and leaves solid salt.

Evaporation happens below the boiling point. Particles at the surface of a liquid with enough energy escape as vapour at any temperature, which is why puddles dry up on a cool day. Heating simply speeds this up by giving more particles enough energy and by raising the temperature towards boiling.

Typical apparatus. The solution is poured into a shallow evaporating basin . The shallow shape gives a large surface area, so the solvent escapes quickly. The basin can be heated: - over a beaker of boiling water (a water bath ), which is gentle and keeps the temperature at or below 100 °C; - on an electric hotplate; - directly with a Bunsen burner on a gauze, which is fastest but least controlled.

Evaporation to dryness and its problems. Heating until every drop of water has gone is called evaporating to dryness . It works for sturdy solids such as sodium chloride, but has drawbacks: - as the last of the liquid disappears, the hot solid can spit out of the basin, which is a hazard to eyes; - some solids decompose when overheated — hydrated salts such as blue copper(II) sulfate crystals lose their water and turn to a white powder; - any soluble impurities left in the solution end up mixed with the product, because they are left behind too.

For these reasons chemists often evaporate only part of the solvent and then let crystals form slowly — crystallisation, the subject of the next topic.

Safe practice. Eye protection is worn throughout. Heating is kept gentle near the end, the basin is handled with tongs once hot, and flammable solvents are never evaporated over a naked flame.

What is lost. Simple evaporation throws the solvent away into the air. If you want to keep the solvent — for example, to obtain pure water from salt water — distillation is needed instead.

Step-by-step reasoning

To recover salt from a sand, salt and water mixture:

1. Stir to make sure all the salt has dissolved. 2. Filter: sand is the residue, salt solution is the filtrate. 3. Pour the filtrate into an evaporating basin. 4. Heat gently until most of the water has gone and solid appears. 5. Allow the rest to dry slowly to avoid spitting.

Visual explanation

Picture a shallow white basin on a gauze. Wisps of steam rise from a clear liquid. Later, a crust of white crystals appears around the edge, and finally the basin holds only a dry white solid with no liquid left.

Real-world analogy

A drying puddle on a sunny day works in the same way. The water slowly evaporates into the air, and a ring of mud and dissolved minerals is left on the ground where the puddle was.

Real-world example

In hot, sunny coastal regions, seawater is let into shallow salt pans. The Sun evaporates the water over weeks and sea salt is left behind to be raked up. The white crust inside a kettle forms in a similar way: dissolved calcium and magnesium compounds stay behind as water boils away.

Why?

Why does the salt stay while the water leaves? The water molecules are held to each other by relatively weak forces and escape easily when heated. The sodium and chloride ions are held by strong attractions in a giant lattice, so they need far higher temperatures to become a gas.

Common misconception

"When salt water evaporates, the salt evaporates too." The salt remains; only the water leaves. You can check this by weighing: the solid left behind has the same mass as the salt that was originally dissolved.

Worked example

Question: 50 g of seawater is evaporated to dryness and leaves 1.75 g of solid. What percentage of the seawater's mass was dissolved solid?

Reasoning: Percentage = (mass of solid ÷ mass of seawater) × 100 = (1.75 ÷ 50) × 100.

Answer: 3.5%, a typical value for seawater.

Quick check

1. Why is an evaporating basin shallow and wide? Answer: To give a large surface area so the solvent evaporates quickly.

Exam focus

State that evaporation separates a soluble solid from a solution and that the solvent is lost. Mention spitting and decomposition as reasons for heating gently or not evaporating to dryness. In method questions, filtration usually comes before evaporation.

Advanced insight

Chemists often use a rotary evaporator: the flask is rotated under reduced pressure in a warm bath. Lowering the pressure lowers the boiling point, so solvents can be removed quickly at low temperature without damaging heat-sensitive products, and the solvent vapour is condensed and recovered.

Summary

Evaporation separates a dissolved, non-volatile solid from a volatile solvent by driving off the solvent as vapour. A shallow evaporating basin and gentle heating are used. Evaporating to dryness can cause spitting, decomposition and trapped impurities, so crystallisation is often preferred. The solvent is lost unless distillation is used.

Practice questions

1. What type of mixture can be separated by evaporation? Answer: A solution of a non-volatile solid dissolved in a volatile solvent. 2. Why is a water bath sometimes used instead of direct heating? Answer: It heats gently and cannot exceed about 100 °C, reducing spitting and decomposition. 3. Give one disadvantage of evaporating copper(II) sulfate solution to dryness. Answer: The hydrated crystals lose their water of crystallisation and become white anhydrous powder. 4. Describe how to obtain dry salt from a mixture of sand and salt. Answer: Add water and stir to dissolve the salt, filter to remove the sand, then evaporate the filtrate to leave the salt.