# Solubility

*Keywords:*

In this unit students will investigate the solubility of sugar in water. They will design and carry out experiments which involve making accurate measurements of volume, mass, and temperature.

AO elaboration and other teaching resources

- make accurate measurements using a variety of measuring equipment
- design an experiment to investigate a factor effecting solubility of sugar

This unit is designed to give students a practical experience in which they can apply a variety of measuring techniques. Students will measure volume, mass and, temperature using a variety of measuring instruments, and will be asked to consider the accuracy of their measurements and ways to improve accuracy. Hopefully students will see that there is always at least slight inaccuracy when measuring and that in practical experiments such as those in this unit the goal is to reduce the error as much as possible.

**Associated Achievement Objective**

Science, Material World -Properties and changes of matter AO2: compare chemical and physical changes.

accuracy, kilogram, gram, milligram, volume, mass, temperature, comparisons, approximation, scales, calibration, error, prediction, inconsistencies

dissolve, solubility, sodium, ingredients, nutrition, experiment design, hypothesis, factors

#### Getting Started (Session 1)

In this session dissolving and solubility are discussed and students make estimates of how much sugar can be dissolved in 100ml of tap water.

- Present students with a bottle of soft drink. Ask them what they think are the main ingredients.
*What is there most of?*(Students should be able to identify that there is primarily water.)*What is in the water to make it taste the way it does?*(As well as flavours, hopefully sugar will be mentioned.) - Investigate by reading the ingredients and nutritional information. You will find that soft drinks have around 10g of sugar per 100ml, and very little of anything else. A discussion of the difference between mg and g may be called for so that students understand how small the amounts of other ingredients (eg sodium) are. A good way to illustrate just how big the difference is, is to compare a 1g mass with a 1kg mass. There are 1000 grams in a kilogram, similarly there are 1000 milligrams in a gram.
- Discuss why the amounts are given per 100ml. (A standard volume so comparisons can be made between products.)
- Ask students to describe how much 100ml is. (A standard metric cup is 250ml, so just less than half a cup is a reasonable approximation.)
- Ask students to describe how much 10g is. (Two teaspoons.) A 10 g mass might be a useful resource to give students a real feel for the mass.
- Show students a sachet of Raro (or similar). Ask them what they think it is made from.
- Investigate by reading the ingredients and nutritional information. Powdered drink sachets are almost pure sugar.
- Discuss how much sugar there would be in the drink if it were made up following the instructions on the packet.
*Is this more or less than in the soft drink?*(The total volumes will be different so conversion will be required.) - Ask students to estimate how much sugar they think is the most that could be dissolved in 100ml of water. Discuss and record estimates to refer back to later.
- Investigate by allowing groups to measure 100ml of water using a measuring jug and then add sugar one measured teaspoon (5g) at a time, stirring until it dissolves. (You may find results vary from around 50g to 65g depending on room temperature.) Ensure that students keep the measuring jug level when they are measuring and that the teaspoons of sugar are level teaspoons, otherwise their measurements, and hence their results will be incorrect.
- Record each group’s results (using correct units – g/l, or gl
^{-1}) and discuss whether the estimates made were accurate.*Are you surprised at how much dissolves?* - As an at home task this week students are to investigate the amounts of sugar in various drinks that they have in their fridge. See who can find the drink with the most sugar in it.

#### Exploring (Sessions 2-4)

In these sessions we elaborate on the work of the previous session by making our measurements more accurate and by investigating different types of sugar (White sugar/brown sugar/icing sugar/castor sugar), different temperatures, and using salt instead of sugar.

- Remind students of the results of the previous day’s experiment. Ask:
*What are some problems with the way we carried out the experiment?*

How could we make the experiment more accurate?

Hopefully your students will realise that the measuring equipment used was not very accurate and that this could be improved on. - Introduce the equipment that will be available to students for measuring. If possible have a range of types of scales including an accurate electronic set (you should be able to borrow one from a local high school or students may be able to bring one from home), and a range of types of measuring cylinders and cups. Ensure that students are familiar with how to use each, for example, some types of scales need to be ‘zeroed’, or calibrated to a start point, before measuring.
- Discuss how accurate each piece of equipment is:
*Why are some more accurate than others?*

What kind of thing would you be likely to measure with each?

Are any of these measurements exact?

How close are they?

Some measuring equipment gives a level of accuracy, otherwise it is normally assumed that you can measure to within half of the smallest unit on the scale. So, for example if the electronic scales say 0.125kg you can say that the mass is more than 124.5g and less than 125.5g. - Repeat the previous experiment with a higher degree of accuracy. Students should use a measuring cylinder to measure the water, and if possible use an electronic scale to measure the sugar. You will also avoid inaccuracy by making fewer measurements, so measure a larger amount of sugar (but one that you know will dissolve) first, before adding 5g amounts until no more will dissolve. As students work, circulate, ensuring that they are using the measuring equipment accurately.
- Compare the results obtained with those from the previous session. Discuss any differences.
- Discuss the accuracy of the new results.
*Are there any sources of error?*

Is there anything that could be affecting the results?

Could we have controlled them?

Students should realise that there will always be small errors in this kind of experiment, but that it is important to think about ways to reduce them as far as possible. - Discuss what else might influence the amount that dissolves.
*What might affect the amount that dissolves?*(Students might suggest type of sugar, or temperature, if not, you should suggest them.)*Would using salt instead of sugar affect the result?**Could we test to see if these factors make a difference?*(Students must realise that to carry out an experiment like this, everything must be kept constant, except the factor being tested.)

How? - Students should work in groups to investigate either whether using different types of sugar affects the amount that dissolves, or whether changing the temperature makes a difference to the amount that dissolves.
- Students should first write a hypothesis (prediction), and then write a detailed design of the method they will use to investigate their hypothesis.
- The teacher should check and, where necessary, amend the students’ experiment designs. Ensure that they consider how to control as many factors as possible in their experiment, and that all methods of measurement are as kept accurate as possible.

Notes on experimental design:
1. Effect of type of sugar
2. Effect of temperature
3. Using salt instead of sugar |

- As students carry out their investigations, the teacher should circulate, checking and correcting students’ measuring technique as required.
- Any groups exploring the effect of temperature will undoubtedly notice that significantly more sugar dissolves at higher temperatures. Possibly a group could work with the teacher to see how much sugar will dissolve at even higher temperatures. With supervision students could try dissolving sugar in boiling water.
*How much will dissolve now?*(Water lost as steam/sugar melting rather than dissolving.)

Are there extra factors to consider in this experiment?*How can we avoid/compensate for the water loss?*

How can we tell whether the sugar has melted or dissolved?

Will the thermometer you are using still be accurate at these temperatures?

__EXTENSION 1__

Students could be asked to investigate whether dissolving some **salt** in the water affects the amount of **sugar** that can be dissolved. They could try dissolving different amounts of salt in water and then testing how much more or less sugar will dissolve.

__EXTENSION __

More able students could be challenged to calculate how much sugar (or salt) could be dissolved in larger volumes of water, such as the school swimming pool (dimensions could be measured and volume calculated), or even the world’s oceans. Some estimates of the volume of water in the world’s oceans can be found at http://hypertextbook.com/facts/2001/SyedQadri.shtml .

#### Reflection (Session 5)

In this session students are given time to write up their results, and report back to the class.

- Students should be given time here to write up their results for presentation, either in a science/theme book, or as a poster for display on the classroom wall.
- Results should include; hypothesis, experimental design, results, and conclusion.
- Students should be encouraged to think about what they could have done better or what they might have done differently knowing what they do now:
- How could measurements have been more accurate?
- How could other inconsistencies have been avoided?
- What else could they have considered/tested?

- Groups should present their reports to the class.

#### Dear Parents and Whānau,

In maths and science this week we are investigating how much sugar can be dissolved in water.

We'd like your child to investigate drinks in your family fridge to see how much sugar they have in them. Maybe you can help them find the information on the label. If you are going to the supermarket, your child may want to come along to investigate a wider range of samples.

Thank you for your help.