Simple Classification of Substances and Separation of Mixtures: Separation of soluble solid-liquid mixture (solution)

2.0 Simple Classification of Substances and Separation of Mixtures

2.6 Separation of soluble solid-liquid mixture (solution)

How can we separate sodium chloride and water?

Study the diagram (set-up) for evaporation to dryness.

Open the video below, evaporation to dryness

(Courtesy Youtube-Separation Techniques: Evaporation to Dryness by No Whines On Chemistry) and observe the procedure keenly.

NB: More controlled heating can be done using steam as shown in the diagram. This protects glass from cracking, which is common in direct heating.

Questions 2.6(a)

1. Identify two disadvantages and one advantage of this method.
2. Why does solid sodium chloride salt explode under strong heating?

Answers to Questions 2.6(a)

Simple distillation

Simple distillation is an alternative to evaporation to dryness. It is suitable where the boiling points of the substances in the mixture are far apart. Generally, solutions of solids in liquids meet this requirement.

Materials and substances required

1. Distillation flask (or boiling tube)
2. Liebig condenser (or delivery tube with salt-ice mixture)
3. Running water
4. Bunsen burner, tripod stand, wire gauze, retort stand, beakers, mixture

Study the diagram (set-up) for simple distillation. Then observe the demonstration of simple distillation.

(courtesy Youtube-To separate a saltwater mixture by distillation by Simple Science and Maths)

Question 2.6(b)

Identify one advantage of this method over evaporation to dryness.

Answer to Question 2.6(b)

Crystallization

Crystallization is another method of separating soluble solids from their solutions in liquids. An example is copper (II) sulphate solution in water.

Open the video to observe the process of crystallization.

(Courtesy Youtube-How To Grow Your Own Crystals At Home by The Q)

Caution: Although copper (II) sulphate or nitrate is relatively safe to touch, this is not true with all substances. Avoid touching chemicals with bare hands.

Question 2.6(c)

1. Define a saturated solution.
2. Name at least five other soluble solid-and-liquid mixtures.
3. What is the name given to the dilute solution that remains after crystallization?
4. Why should the crystalizing mixture be covered?

Answers to Questions 2.6(c)

Question 2.6(d)

Describe how you would obtain dry copper (II) sulphate crystals from a solution of copper (II) sulphate in water.

Explain why (the) crystals should not be washed.

Answer to Question 2.6(d)

In industry

In sugar factory, the sugar-water mixture is heated to expel much of the water, then the concentrated solution is allowed to cool and crystallize. Molasses are the uncrystallizable sugar that remains in the mother liquor.

Crystallization is also used to obtain sodium chloride from sea water such as at Ngomeni in Mombasa, and sodium carbonate from lake Magadi.

Separation of pigments

Each of the mixtures considered in sections 3.1 and 3.2 consists of two components and are therefore called binary mixtures.

It is possible to have three or more components in a mixture. Here, we also apply differences in properties to separate the components. If the components have different colours, it is easy to see if we have actually succeeded in separating them.

To separate the pigments in a green nutrient dye

Open the video, chromatography.

(courtesy YouTube-Paper Chromatography of Textile Dyes by CEEA Erasmus) to watch how components of blue ink of marker pen can be separated. NB: This set-up uses a strip of chromatography paper held vertically, and acetone (propanone or nail polish remover) as solvent.

Questions 2.6(e)

1. What is observed on the chromatography paper when a solvent, in this case water, is added to the dye?
2. Each colour on the chromatography paper represents a component in the dye. How many components are there in the dye?
3. By what method does the solvent move or rise in the chromatography paper?
4. Which component (state the colour) moves fastest with the solvent?
5. Chromatography paper and the colours spread on it make up what we call a chromatogram. Suggest how you would proceed to obtain each component of the dye from the chromatogram.
6. What is the dictionary meaning of chroma or chromatic? Why is this method referred to as chromatography?
7. This method works whether the chromatography paper is held horizontally or vertically. Give a reason?
Bonus Questions
8. The line on the chromatography paper where the solvent has reached at any given time is called a solvent front. Can a component move faster than the solvent (or solvent front)? Explain.
9. For chromatography to succeed, components of a dye must be soluble in the solvent used, not necessarily water. From what you have observed, what else must be true about the components of a dye for them to separate?

Answers to Questions 2.6(e)

How can we separate components of green leaf extract?

A suitable solvent for the natural plant dyes is propanone, a colouless liquid sold as nail polish (cutex) remover).

Questions 2.6(f)

1. Suggest a procedure that may be followed in this process.
2. Which other method of separation is used here, and where does chromatography begin?
3. How is the separation of colours caused?

Answers to Questions 2.6(f)

Other mixtures that can be separated by paper chromatography include filtered juice extracted from flowers, filtered juice extracted from green leaves, and so on. The paper can be held horizontally or vertically as shown below.

Questions 2.6(g)

1. Which apparatus and materials are required for chromatography?
2. 1. How many components are there in the black ink used?
   2. What are their colors?
3. Why do some pigments deposit very near the point of application (baseline), while others deposit further away (That is, which property of substances does chromatography rely on)?
4. Chromatography helps us to achieve two main things. State them.
5. Study the following chromatogram (used chromatography paper) then answer the questions that follow. P and Q are mixtures applied at points P and Q on the chromatography paper, held vertically in a jar or beaker. Baseline is the level of water (solvent) in a chromatography jar or beaker, which is covered. Solvent front is the level reached by water rising by capillarity in the chromatography paper.
   
   
   
   
   1. Explain why the chromatography jar is normally covered.
   2. How many components does Mixture P have?
   3. How many components does Mixture Q have?
   4. What can you say about R?
   5. Which two components are the same substance? Explain your answer.
   6. Which component is the least soluble? Explain.
   7. Which component is the most soluble? Explain.
6. A drop of Mixture Q (in question 5) is placed at the center of a chromatography paper held horizontally. A solvent is then added dropwise to Q, causing the latter to spread out in all directions on the paper. Draw and label a diagram to show the chromatogram that would result.

Answers to Questions 2.6(g)

At home

Barks of certain trees are crushed into small pieces, mixed with water then heated strongly to extract the dye. The mixture is then cooled and filtered to retain the solution of the dye in water. It is normally used to decorate costumes. Find out if juices from kales, spinach, cabbage, or onions are mixtures of pigments.

What do you think is the major problem of using dyes from plants to decorate costumes or fabric?