Methods of Separation in Everyday Life · Lesson 3 of 7
Recovering a Dissolved Solid
“Use evaporation to understand why water can leave a salt solution while salt remains.”
• Distinguish a dissolved solid from a mixture of separate solid pieces. • Describe how evaporation can recover salt from salt water. • Interpret the dark-paper and heated-dish observations. • Explain why salt left after seawater evaporates may need further purification.
What if the solid is no longer visible?
Stir salt into water until you can no longer see separate grains. The salt is still present: the mixture is a salt solution. Handpicking and sieving cannot collect individual salt particles from this liquid. Instead, consider a property of the other component. Water can change into vapour and move into the air, leaving the dissolved solid behind.
A mixture in which one substance, such as salt, is dissolved in another, such as water. The dissolved substance has not vanished.
The change of a liquid into vapour. In this chapter it can separate a dissolved solid by letting the liquid leave while the solid remains.
A slow observation on dark paper
Prepare a salt solution by stirring a few teaspoons of salt into a half-filled bowl of water. Spread a few drops on thick dark paper. You could make a small pattern and let it dry. At first the paper looks wet; later pale salt patches appear. The water has evaporated into the air, while the salt remains on the paper. A plain-water comparison helps show that the new pale material comes from the salt solution.
White patches that can appear on dark clothing after sweat dries offer a related clue: water leaves while dissolved materials can remain. The exact substances in sweat are not necessarily only common salt. The observation supports an explanation of what happened to the liquid rather than suggesting that salt appeared from nowhere.
A quicker teacher demonstration
A teacher may warm salt solution in a suitable dish on a wire gauze above a spirit lamp. As the water boils away, a solid deposit is left after the dish cools. This gives faster evidence for the same basic separation. Hot equipment can burn skin, so students should not handle the dish or the flame. The vapour is allowed to leave in this method; it is not collected as liquid water. The chapter raises the question of how both water and salt might be recovered, but does not teach a full recovery apparatus here.
Problem
A few drops of salt solution dry on black paper, leaving a pale deposit. A student says the salt was created by sunlight. How would you respond?
- 1.The salt was already in the solution, even though it was not visible as grains.
- 2.Water from the drops evaporated and left the salt behind.
- 3.Sunlight can speed drying, but it does not create salt in this observation.
Salt from seawater
Seawater contains dissolved salts and other substances. In shallow pits exposed to sunlight and air, much water evaporates over days. A solid mixture is left, from which common salt is obtained by further purification. Do not claim that evaporation of seawater alone yields perfectly pure table salt. A salty lake such as Sambhar Lake is another source worth investigating. A poster about the Dandi March can connect the science of obtaining salt to the historical setting of the chapter without replacing the separation explanation.
Drying some plant parts in the shade also allows excess water to leave while solid material remains. Shade drying can be chosen to protect useful parts of the material from strong sunlight. It is another application of evaporation, although it is not the same mixture as salt dissolved in water.
If water vapour escapes into the air, evaporation gives you the solid residue but does not give you that water back. Filtering a clear salt solution will not remove its dissolved salt.
Quiz
What remains when a salt solution dries on dark paper?
Why does an ordinary sieve not recover salt from clear salt water?
What does evaporation of seawater leave initially?
In the open heated-dish demonstration, what happens to the water?
Which activity most directly shows that dissolved salt has not disappeared?
Practice Problems
- Explain why a clear salt solution still contains salt.
- Describe what to observe before and after drying drops of salt solution on dark paper.
- Explain how sunlight and shallow pits help collect solid material from seawater and why further purification may be needed.
- Contrast the paper activity with heating salt solution in a china dish; include the safety requirement.
- A student wants both salt and water from a solution. Explain the limitation of simply letting the water evaporate into open air.
- Suggest why white deposits can appear on dark clothes after sweat dries, while avoiding the claim that the deposit is pure common salt.
Key Takeaways
• Dissolved salt remains in a solution even when its grains cannot be seen. • Evaporation can leave a dissolved solid behind as liquid water becomes vapour. • Evaporating seawater leaves more than common salt, so further purification can be needed. • An open evaporation method does not recover the water that escaped.