Exercises: Separation techniques for mixtures

When making coffee in a filter coffee machine, hot water passes through the ground coffee and the paper retains the coffee grounds, as shown in the figure.

What separation technique takes place inside the coffee maker?

In a workshop, iron filings have been mixed with sand. When bringing a magnet close, the filings stick to it and the sand does not, as shown in the figure.

What separation technique is being used? Why does it work?

In the laboratory, you want to recover the salt dissolved in a glass of salt water. You heat the solution: the water evaporates and the salt remains at the bottom, as shown in the figure.

What is this technique called?

After frying, water and oil remain in a saucepan forming two layers: the oil on top and the water at the bottom. If you let the saucepan stand and carefully remove the top layer, what separation technique are you using?

Grandmother has mixed chickpeas and lentils in the same drawer. Since chickpeas are larger, she uses a colander: the lentils pass through the holes and the chickpeas stay inside, as shown in the figure.

What is this technique called?

Water and alcohol mix perfectly, but alcohol boils at a lower temperature than water. To separate them, the mixture is heated: the alcohol evaporates first, then that vapour is cooled and collected as a liquid in another container, as shown in the figure.

What technique is it?

Complete the sentence with the name of the appropriate technique: 'To separate an undissolved solid from a liquid, we use $\dots$; to separate two immiscible liquids, $\dots$; and to recover a dissolved solid by letting the liquid evaporate, $\dots$.'

An aquarium filter passes water through a sponge that retains dirt particles, and clean water returns to the fish tank. What separation technique does this filter use?

Sand and salt have been mixed in the same jar. Put the steps in order to separate them and recover both:

1. a) Heat the water until it evaporates and salt remains.
2. b) Add water and stir so that the salt dissolves (the sand does not).
3. c) Pour the mixture over filter paper to retain the sand.

Nico tries to separate salt from salt water using a coffee filter, but the filter retains nothing. Explain why filtration works with sand but not with dissolved salt, and say which technique Nico should use to recover the salt.

After cooking a broth, the fat is left floating, forming a yellowish layer on the surface. Cooks let the pot stand and then remove that layer with a ladle. What separation technique do they use? What property of fat makes it possible for it to work?

In the local bakery, they sieve the flour before kneading: the fine flour passes through the mesh of the sieve and the lumps stay on top. What technique is this? On what property of the particles is it based?

In a recycling plant, a conveyor belt carries iron scrap mixed with plastic and aluminum, and at the end of the belt there is a giant magnet. What material does the magnet separate from the rest? Why does aluminum not stick to the magnet?

Evaporation and distillation start in the same way: heating the mixture so that the liquid turns into vapour. How do they differ afterwards? Think about what is recovered in each case: the dissolved solid or the liquid?

In salt pans, seawater is left in shallow pools exposed to the sun and wind for weeks, until a layer of salt appears at the bottom, as shown in the figure.

What separation technique takes place in salt pans? What is obtained and what has happened to the water?

You have collected rainwater in a drum and want to clean it with a homemade filter made with layers: coarse gravel, fine sand and cotton wool, as shown in the figure.

1. a) What separation technique does this invention imitate?
2. b) Give reasons why placing the coarsest gravel in the top layer and the fine sand below is useful.

In the laboratory, you have a mixture of sand, iron filings and salt, and you need to recover all three components separately. Design the complete procedure: indicate each step, the technique you use and the property of the materials on which you rely.

A bottle contains a homogeneous mixture of water and alcohol.

1. a) Explain why neither filtration nor decanting works to separate them.
2. b) On what differing property of water and alcohol is distillation based to achieve this?

You dropped salt into the rice jar. You think of two ways to fix it: separating them dry using a fine sieve, or tipping everything into water to dissolve the salt.

1. a) Describe each procedure and name the technique or techniques it uses.
2. b) Which one would you choose? Give reasons for your choice, thinking about what would happen to the rice with each method.

A hiking group wants to drink water from a river that is running murky.

1. a) Put in order what they should do: filter the water through a cloth, let it stand for a good while, boil it.
2. b) Explain what each step removes.
3. c) Why is it necessary to boil the water even if it already looks clean after filtering?

You mix water, oil and sand in a glass, shake vigorously and let it stand.

1. a) Predict what you will see after a while: how many layers form and what is in each one?
2. b) Design how to separate the three components using decanting first and then filtration.

Imagine that we distill seawater in the laboratory.

1. a) What substance is collected at the end, after cooling the vapour? What remains in the heated vessel?
2. b) With this method, drinking water could be obtained from the sea. Why do you think it is not done this way on a large scale everywhere? Think about the energy required.

Three classmates have chosen the wrong technique. Find each person's mistake and correct it:

1. a) Aitana wants to separate water and oil by bringing a magnet close.
2. b) Mario filters salt water to keep the salt in the filter.
3. c) Sofía tries to separate sand and sugar by sieving, even though both grains are the same size.

On a desert island you only have seawater, a wide container, a sheet of transparent plastic, a small cup and plenty of sun. You set up the device shown in the figure: you leave the container with seawater in the sun, place the small cup in the center (without letting any seawater get into it) and cover everything with the plastic sheet, with a small weight right above the small cup. After a few hours, the small cup contains clean water.

1. a) Which two processes (changes of state) take place inside the device? Which laboratory technique does it imitate as a whole?
2. b) Why does the salt never reach the small cup?
3. c) Which natural phenomenon does something very similar with seawater?