Solutions as special mixtures

When is a mixture also a solution?
Do all substances dissolve in water?
Is there a limit to how much of a substance can be dissolved in a given amount of water?
How can the components of a solution be separated?

The important message to convey in this section is that solutions ARE mixtures, albeit of a special type. The word mixture is used to describe any combination of two or more substances. A mixture can only be a solution if the particles (and here we are referring to the smallest possible particles, namely molecules) of the two substances are separate from each other and mingle freely with each other. Solutions are mixtures even at the level of molecules (or other fundamental particles).

The CAPS document includes the following two statements that are somewhat problematic:

~when substances dissolve, solute particles become dispersed in the spaces between the solvent particles

~a solution becomes saturated when enough solid solute has been added to fill up all the spaces in the solvent

The use of the following alternative statements is recommended:

~when substances dissolve, solute particles become dispersed (spread) throughout the solvent particles

~a solution is saturated when the maximum amount of solute has been dissolved in the solvent

The important idea to convey is that the particles of solute and solvent are closely mixed, in other words they are mixed on the level of atoms and molecules (particles is the preferred term here for learners at this level of development).

One possible challenge with this chapter and the one following (Dissolving) is that the concepts (solution, dissolving, soluble vs insoluble etc.) are almost inextricably linked, which makes it difficult to explain one concept without using words that are only explained in a later section. For instance, it is really difficult to explain the concept of a solute dispersing into a solvent without using the word dissolve (which is covered conceptually in Solutions, but is only named much later in the chapter Dissolving). The word dissolve will be introduced in this chapter, but used sparingly and only towards the end, so as to sensitise learners to its use, and the way in which the concept links with others in this chapter.

Solutions

Take note of the following definitions of the new words.

Solution: A homogeneous mixture of two or more substances, which may be solids, liquids, gases, or a combination of these.
Solvent: A substance in which other substances can dissolve.
Solute: A substance that dissolves.

In the last chapter we looked at mixtures. We are now going to look at a special case of mixtures, which are called solutions.

When is a mixture also a solution?

When two substances are mixed it will be possible to still see each substance in the mixture. Is sugar and sand a mixture? Yes!

A solution is a special type of mixture. What makes a solution so special? When is a mixture also a solution?

Often, the best way to answer a question is to ask it in a different way: When is a mixture NOT a solution?

In the next activity we are going to make a few mixtures and then decide which of them are solutions, and which of them are not. That should help us find the answer to the question: When is a mixture also a solution?

When is a mixture also a solution?

In this activity we are going to mix substances with water to see which ones make solutions.

How do you think will we know when a substance has made a solution with the water?

It will look as if it has disappeared. We will not be able to see the particles of the substance in the water.

MATERIALS:

small quantities of the following substances:
- sugar
- salt
- sand
- oil
- vinegar
- flour
- copper sulfate
tap water
clean yoghurt tubs (small)
plastic spoons for scooping and stirring

The accepted spelling is 'sulfate' but very few people are aware of this, and most textbooks still use the old spelling 'sulphate'. When working with copper sulfate, as with most chemicals, safety precautions must be adhered to, such as wearing safety goggles and avoiding contact with the skin, eyes and nasal passages. This is a good opportunity to discuss safety precautions in general when doing science investigations, including wearing protective clothing (lab coats, goggles, gloves), as well as acting cautiously and carefully when around chemicals, and especially not drinking or inhaling any substances.

INSTRUCTIONS:

Half-fill a yoghurt cup with tap water.
Place one small scoop of sugar in the water and stir it well.
Look at the mixture and discuss what it looks like.
At the top of the table below, a few possible observations are given. Choose the one that is the best description for what you observed, by making a cross in the matching column. (You may also choose more than one column.) The first substance (sugar) has been filled in to show you what you should do.
Once you have recorded your observation, you can empty the yoghurt tub.
Repeat steps 1 - 4 until you have tested all the substances on the list.

Table: Mixing substances with water

	Observations
Substance	It looks as though none of the substance has disappeared	It looks as if all or most of the substance has disappeared
sugar		X
salt		X
sand	X
oil	X
vinegar		X
copper sulfate		X

QUESTIONS:

Which of the substances seemed to disappear when they were mixed with the water?

Sugar, salt, vinegar and copper sulfate

Which of the substances in this activity did NOT form solutions with water? (Hint: which ones did not look as if they 'disappeared' into the water?)

Sand and oil

What is a solution?

When two substances make a solution, it will look as if the one substance has disappeared into the other:

The substance that looks as if it has disappeared is called the solute.
The substance that we can still see is called the solvent.
The solvent and solute together are called the solution.

The definitions provided here for solute and solvent may be a suitable distinction at this level for learners, but in general the distinction is not so clear. We tend to call the one there is more of the solvent, for example, brass is zinc dissolved in copper, and air is oxygen and other gasses dissolved in nitrogen.

Is sugar and sand a solution? (You may want to page back to Part 3 of the activity Mixing solids to remind yourself.)

No, because if we look carefully we can still see the individual sugar and sand grains.

Teacher note: One way to explain this would be as follows: If we could shrink ourselves down to the size of molecules (or 'particles', to use the language at this level) we would see clumps of sugar and sand, even if the sugar and sand crystals are really small.

Which mixtures are solutions?

In this activity we will use our observations from the previous activity (When is a mixture also a solution?) to decide which of the mixtures we made were solutions.

QUESTIONS:

In the activity When is a mixture also a solution?, we mixed different substances with water. We saw that some of the substances looked as if they had disappeared in the water.

What name do we give to the substance that looks as if it has disappeared?

Solute

What name do we give to the substance that we can still see?

Solvent

What name do we give to these mixtures?

These mixtures are solutions

Complete the table using the information about the sugar - water mixture as an example.

Mixture	Is the mixture a solution after stirring? (Yes or No)
Sugar and water	Yes
Salt and water	Yes
Sand and water	No
Oil and water	No
Vinegar and water	Yes
Copper sulfate and water	Yes

In the activity above, we mixed different substances with water.

Which substance is the solvent in all the mixtures?

Water

From the mixtures above, choose an example of a solution that consists of a solid solute and a liquid solvent.

Any one of the following examples: Sugar in water, salt in water or copper sulfate in water.

From the mixtures above, choose an example of a solution that consists of a liquid solute and a liquid solvent.

Vinegar and water

From the mixtures above, choose an example of a mixture of two liquids that is NOT a solution.

Oil and water

Wait! How is it possible for one substance to 'disappear' into another?

Good question Tom. We know that science is not magic, and that it is not possible for something to disappear!

How do we explain the observation that one substance (the solute) 'disappears' into the other (the solvent)?

In the next activity we will look more closely at a solution, in order to understand how it is possible for the solute to look as if it disappears into the solvent.

To emphasise the logic so far, we make observations of things around us and we try to explain them using the models we develop. The learners need to get the message that the particle view is not only a description of a reality we cannot directly observe, but also a tool to explain things that we can observe.

What is a solution?

MATERIALS:

copper sulfate crystals
tap water
clear container, such as a glass beaker or test tube (a water glass will also do)
plastic spoon for scooping and stirring

INSTRUCTIONS:

Look at the copper sulfate crystals and the water. Write one sentence to describe each substance in the table below.
Mix one small scoop of the copper sulfate with enough water to dissolve it completely (half a cup of water should be enough). Let it stand for a few minutes until it clears.

Teacher note: ensure that all of the copper sulfate has dissolved, or learners will be confused by some of the results of this activity.

Look at the copper sulfate solution and write a sentence to describe it in the table below. Save it for answering the questions that follow.

Table: Description of a copper sulfate solution in water

Substance or mixture	Description (what it looks like)
Water	The water is a clear, colourless liquid.
Copper sulfate crystals	The copper sulfate is a blue solid.
Copper sulfate solution	The solution is a clear, blue liquid.

QUESTIONS:

Look at the solution. How can you tell that there is copper sulfate in the water? Another way to ask this question would be: What evidence do you have that there is copper sulfate in the water?

The water is blue. The water is the same colour as the copper sulfate crystals.

Can you see any copper sulfate crystals moving about in the water?

No.

Explain your answer to question 2.

The crystals separated into their individual particles that are too small to see with the naked eye.

What do you think happened to the copper sulfate particles? Where are they now?

The copper sulfate particles have been mixed up with the water particles.

Draw a picture of the particles in the copper sulfate solution in the space. You can use the following symbols to represent each substance:

shaded circles to represent water particles
white hexagons to represent copper sulfate particles

Teacher note: The sketch should show the container with liquid particles at the bottom. In the liquid the black dots should be homogeneously dispersed among the clear ones, in the same way as the sugar particles are dispersed among the water particles in the following diagram:

In the video clip on "How water mixes with a solute to make a solution", the language may be a bit difficult to understand, but if you watch the clip you will get the general idea. However, there is no problem with selling this (and other) clips on the grounds that it uses ideas and language that they (the learners) can access when they get older. You, the teacher, can even challenge the learners to find out about some of the new ideas in this clip and report back to you personally or in class. Forces are also dealt with again in this video - this can be introduced by asking "What keeps the particles of a solid or liquid from moving away from each other?" The answer is that there are forces between the particles.

Soluble substances

NB: A very common misconception is that sugar or salt "melts" away when added to water. Dissolving (in the case of sugar and salt in water) requires two materials to be mixed together (a solute will dissolve in a solvent), whereas in melting (in the case of ice), there is heating of one material to change its state. A single substance melts when it changes from one state to another. In order to avoid introducing this misconception which then sticks with learners into the later grades, never use the word "melt" when describing dissolving. Emphasise to learners that the sugar does not melt and that melting is different - it is a change of state.

We have a word for substances that form solutions when they are mixed with water. These substances are called soluble substances.

Substances that do NOT form solutions when they are mixed with water are called insoluble substances.

In the next activity we are going to use some findings from a previous activity (Which mixtures are solutions?) to link this new idea to what we know about solutions.

Soluble or insoluble?

INSTRUCTIONS:

The table from the activity Which mixtures are solutions? has been copied below, and an extra column has been added.
Use the extra column to say whether the substance that was mixed with water in the activity is soluble or insoluble.

Table: Soluble and insoluble substances.

Mixture	Is the mixture a solution? (Yes or No)	Is the substance that was mixed with the water soluble or insoluble?
Sugar and water	Yes	Soluble
Salt and water	Yes	Soluble
Sand and water	No	Insoluble
Oil and water	No	Insoluble
Vinegar and water	Yes	Soluble
Copper sulfate and water	Yes	Soluble

QUESTIONS:

Complete the following sentences by writing soluble or insoluble in the open spaces.

Substances that do NOT form solutions when they are mixed with water are called _____ substances.

insoluble

Substances that form solutions when they are mixed with water are called _____ substances.

soluble

In the previous chapter, we saw how to separate mixtures. For example, we could hand sort the objects, sieve the larger grains out of the mixture and decant the oil from the top of the water. But what about a solution? Do you think you can separate the sugar from the solution once it has been dissolved? Let's try to find out the answer to this question!

How can we recover a solute (sugar) from the solution?

Prepare the sugar solution beforehand (or at the beginning of the class so that learners see you mixing) by mixing sugar into water and making sure it is dissolved. You can prepare enough for the whole class to use or do this experiment as a demonstration. Start off by asking learners if they can see the sugar. Revise the concept that it has dissolved into the water to form a solution. Before starting the investigation, ask the learners how they think you could separate or recover the sugar from the solution.

Ask learners if they think you could separate by sieving or filtering. After they answer you, demonstrate filtering by pouring the sugar solution through filter paper placed in a funnel into another beaker. Show the learners the filter paper so that they can then see that there is no sugar left behind on the filter paper. They can fill in their observations in the table.
Ask learners if they think settling will work. You could prepare a sugar solution the day before the class and then leave it overnight in the classroom and tell learners you want to see the next day whether the sugar has settled out of the solution. They can then see for themselves the next day during the next lesson that it has not settled out and that this technique cannot be used to separate a sugar solution. They can fill in their observations in the table.

Continued...

The next step is to see what will happen if we evaporate the water. This probably will not be a logical step for the learners to take. So ask leading questions such as, what do they think will happen to a dish of water if you leave it outside in the sun. Answer: It will evaporate. Note though, that evaporation does not need heat to take place, but the heat from the sun will speed up the process.
Ask learners what they think will happen to the sugar solution if you leave it outside in a dish to evaporate? Will the water evaporate? What will happen to the sugar? The learners should see the need to perform the investigation from asking these questions as the answer might not be obvious.
While you are waiting for the water to evaporate and leave behind the sugar crystals, you can demonstrate another way of recovering the sugar (solute) which is by boiling the sugar solution. Set up a beaker of the sugar solution on a stand over a Bunsen burner. Light the Bunsen burner and allow the solution to boil. Do not put a lot of the sugar solution into the beaker otherwise the demonstration will take too long. Boiling speeds up the process and the last remaining water can be left to evaporate until only sugar crystals are left at the bottom of the beaker. This is called crystallisation.

AIM (What do you want to find out?):

MATERIALS AND APPARATUS:

sugar solution
2 beakers
funnel
filter paper
evaporating dish
stand
Bunsen burner
matches

METHOD:

Pour a small amount of the sugar solution into an evaporating dish.
Place the dish outside, or on a windowsill, in a sunny spot.

Remind learners that evaporation does not require additional heat to take place, but heating speeds up the process.

Leave the dish outside and check regularly to observe what is happening to the sugar solution.
Your teacher will demonstrate whether you can also recover the sugar by boiling the solution.
Record all your observations in the table below.

RESULTS AND OBSERVATIONS:

Method	Result - Could you recover the sugar from the solution?
Sieving or filtering
Settling overnight
Evaporation
Boiling

Which methods worked to recover the sugar from the solution?

Boiling and evaporation.

What was left at the bottom after completing these methods?

Sugar crystals.

Why do you think this happens?

This is because the water evaporates or boils and turns into water vapour. The sugar cannot evaporate and is left behind as a solid in the form of crystals.

Which method do you think works best and why?

Learner dependent answer: They could say boiling as it is faster, or they could say evaporation as it does not require much equipment such as a Bunsen burner, etc.

CONCLUSION:

What can you conclude from this investigation?

Now that we have looked at how to separate a solute from a solution, have you ever wondered just how much sugar you could dissolve in the water? Do you drink tea, for example, and put sugar in? How many teaspoons of sugar do you think you can dissolve in a cup of tea? In the next section we will explore this idea.

Saturated solutions

Explanation of new words

Dissolve: When the particles of a solute spread throughout the particles of a solvent, we say the solute dissolves in the solvent to make a solution.
Saturated solution: When so much solute has dissolved that no more of it will dissolve, we say the solution is saturated.

Suppose we were to make a cup of tea and we put in 3 teaspoons of sugar. Mmm... lovely sweet, warm tea!

Now imagine you add three more teaspoons of sugar to the tea. How many teaspoons of sugar did we add?

Six, in total.

When the particles of a solute spread throughout the particles of a solvent, we say the solute dissolves in the solvent to make a solution.

Do you think 6 spoons of sugar will dissolve in the tea? Who has tried this at home? What did you find?

Now let us imagine 3 more teaspoons of sugar is added to the tea. Very sweet tea! Do you think all the sugar will dissolve?

How much sugar do you think we will be able to dissolve in the tea? An infinite amount? A cupful or less? Let's try it out.

How much solute will dissolve?

This investigation makes the ideal demonstration, and could even be given as a homework experiment. It also allows for the extension of learners' understanding of the concept of solubility and saturated solutions. You could heat the saturated sugar solution for the learners to show that more solute will dissolve when the solvent is at a higher temperature (this is mostly true, but not always!). If more sugar is added to the heated solution until it is saturated at the higher temperature, then the solution will be supersaturated when cooled. A sugar crystal can be suspended in the solution before it cools down and more sugar crystals will grow on the crystal and on the thread used to suspend the crystal. The less the solution is disturbed, the larger the crystals will grow.

MATERIALS:

clear container (a glass beaker would be best, although a large yoghurt tub would also work, but not if you decide to heat it later)
tap water
small packet of sugar
plastic spoon for scooping and stirring

INSTRUCTIONS:

Measure half a cup of water into the container.
Add a teaspoon of sugar to the water. Stir until all the sugar has dissolved.
Add another teaspoon and stir again.
Keep adding teaspoons of sugar until no more sugar can dissolve.

QUESTIONS:

How many spoons did you add until no more sugar dissolved?

How did you know that no more sugar could dissolve?

The sugar stopped dissolving and sank to the bottom of the container.

Complete the following sentences by writing saturated or unsaturated in the open spaces.

When no more solute can dissolve in a solution, we say it is a _____ solution.
When more solute can be dissolved in a solution, we say the solution is _____.

saturated
unsaturated

Now let's have some fun with saturated solutions!

Making sugar crystals

Sugar crystals can take a few days to a week to "grow", so set up this experiment and then leave the jars on the windowsill where they will not be disturbed. You can use different coloured food colouring so that learners have brightly coloured crystals at the end. Each learner can make their own or you can scale up the quantities below and make it in one big beaker, or possibly three different ones with different colours. The quantities listed below are for one crystal to grow in one jar. However, it would be ideal for each learner to have their own crystal.

MATERIALS:

1/2 cup water
1 cup table sugar
clean glass jar
food colouring
pencil
rough string (cooking twine works great)
beaker or pan for boiling water and making solution
spoon
stove or Bunsen burner and stand

INSTRUCTIONS:

Tie a length of string onto a pencil. The string should be long enough to reach almost to the bottom of your glass jar.
Make a saturated sugar solution by boiling the water in the pan, slowly adding sugar a teaspoon at a time. If you have a Bunsen burner and stand, you can do this in a beaker over the flame.
Stir after each spoonful and keep adding sugar until the sugar won't dissolve any more in the water. If you do not add enough sugar, your crystals will not grow quickly. If you use too much sugar, your crystals will grow on the undissolved crystals and not on the string.
Pour some food colouring into your saturated solution to give the crystals a colour.
Pour your solution into the clear glass jar. If you have undissolved sugar at the bottom of your container, avoid getting it in the jar.
Place your sting inside the glass jar.

A tip is to weight the string with something heavy so that it does not touch the sides of the glass jar.

Place your jar where it will not be disturbed and check on your string each day and observe the crystal growth.
Allow the crystals to grow until they have reached a size that you desire, or until they have stopped growing. You can pull the string out and allow the crystals to dry. You can eat them or keep them!

The best crystals form when the process happens slowly and the water cools down slowly. The cooled solution has a concentration above the saturation point and is said to be supersaturated. Crystal will more easily form when they have a place to start growing, like on the string.

QUESTIONS:

How long did it take for crystals to start forming on the string?

Learner dependent answer.

What are the crystals made of?

Sugar

Why do you think we boiled the water when dissolving the sugar in the solution?

This might be quite difficult for learners to answer but have this question as a class discussion. Boiling the water allows one to dissolve more solute than if the water was cool or at room temperature. This results in a supersatured solution.

An example of crystals in nature

Have you ever visited a cave? Inside, you may have seen crystal formations called stalactites and stalagmites. Stalactites and stalagmites form inside limestone caves. Stalactites hang down like icicles and stalagmites grow from the floor of the cave upwards. Stalactites and stalagmites always occur in pairs. Caves form when water slowly dissolves the limestone underground. The dissolved limestone can crystallise again when the water evaporates. This is also a slow process and it happens when water drips down from the ceiling of the cave over a long period of time. The water drops that land on the floor of the cave evaporate over time, and where they fall on the same spot repeatedly, a stalagmite eventually forms. Over many thousands of years, the stalactite and stalagmite may connect to become a column.

Stalactites and stalagmites forming in a cave.

Cango Caves in Oudtshoorn in South Africa

Insoluble substances

We have a word for substances that do NOT form solutions when they are mixed with water. These substances are called insoluble substances.

Can you remember what substances are called that DO form solutions when they are mixed with water? Write the term below.

Soluble substances

Some substances that are insoluble in water may be soluble in other solvents! Think about this for a moment: Is nail polish soluble in water? No, of course not, or it would be possible to wash it off! What would be a good solvent for nail polish?

Nail polish remover will be a good solvent.

What have we learnt about solutions as special mixtures?

Soluble substances dissolve in water and insoluble substances do not dissolve in water.

Water is not the only solvent. Some substances which are not soluble in water are soluble in other solvents. When no more solute can dissolve in a solution, we say it is a saturated solution. An unsaturated solution is one where it is possible to dissolve more solute in the solvent.

Solutions are special kinds of mixtures. When we want to decide whether a mixture is also a solution, we can use the following questions to decide:

Question about the mixture	The mixture is a solution	The mixture is NOT a solution
Can you see the solute in the solvent?	No	Yes
Does the solute settle out?	No	Yes
Can the mixture be separated using filtration?	No	Yes
Can the mixture be separated by evaporation?	Yes	No

Asolution is a special kind of mixture. Like all mixtures it consists of two (or more) substances mixed together.
A solution is made up of a solvent (such as water) in which one or more solutes have been dissolved.
In a solution, the solute looks as if it disappears into the solvent. This is because the particles of the solute and the solvent become closely mixed.
There are many kinds of solutions, but the most well-known ones are mixtures of a solid and a liquid, such as sugar and water.
Not all substances dissolve in water. Those that dissolve are called soluble substances; those that do not dissolve are called insoluble substances.
Solutions cannot be separated by sieving, filtering, hand sorting, or settling and decanting. This is because solute particles are dispersed between the solvent particles.
Solutions can be separated by heating so that the solvent evaporates. The dry solute will be left behind.
When we have dissolved so much solute in the solvent that no more solute can possibly dissolve, we say that the solution is saturated.

In the activity Soluble or insoluble? we explored some substances, and found that sand is insoluble in water.

In the same activity we found that sugar is soluble in water.

Can you remember how to separate sand and water? (Hint: Look at the activity Mixing a solid and a liquid.) Write it down below.

In the activity 'Mixing a solid and a liquid' we saw that a mixture of sand and water can be separated by passing the mixture through a towel.

In the picture below a mixture of sand and water is poured through a filter. What is this process called?

Filtering/filtration

Why do the sand grains stay behind on the filter paper, but the water passes through it?

The sand grains are too large to pass through the paper. The water particles are very small and can easily pass through the filter paper.

What was the mixture of sugar and water called? (Hint: It was a special kind of mixture called a.. ?)

Solution

What would happen if the mixture of sugar and water is poured through a filter? Would it be possible to separate the water and the sugar?

The sugar and water would pass through the filter and it would not be possible to separate them in this way.

What happens to the sugar when it dissolves in the water?

The sugar crystals separate into individual particles that mix with the particles of the water.

Why is it not possible to separate a solution through a filter?

Because the particles of the solute and the solvent are thoroughly mixed and are of roughly the same size.

Describe how you can get the solid sugar back from the sugar solution.

Learners must describe the process of evaporation or boiling which leaves behind the sugar crystals.

Draw a flow diagram to show how a mixture of salt and sand can be separated. Each step must be clear. Your first step will be to mix the salt and sand with water.

These two substances cannot be separated by sieving as they will mostly have the same grain size. The first step is to mix the the salt and sand with water so that the salt dissolves. You then filter the mixture so that the salt solution passes through the filter paper whereas the sand remains behind. Then to regain the salt crystals, you evaporate or boil off the water to be left with the salt crystals at the end.