## Dissolving Sugar: Making Solutions

The American Chemical Society has developed a set of very useful online lessons about Chemistry, one of which is particularly relevant to the work of this lab. When answering the questions about explaining your results using events at the molecular level the visualizations and animations found at this site are particularly valuable.

Here is a spreadsheet with a sample data table for the second objective. It can help students to see this so they can get organized about their data collection.

Sample Graph (xls)
Note to Teachers: This lab is an inquiry-based lab. The idea is for students to come up with their own procedure. They must decide what will be held constant, what will be varied, and how to make their measurements. The notion is to have students learn science by doing science the way scientists do it. Students may either write narrative answers to the questions as posed in the objective section (complete with graphs) or write a full lab report. I expect that it will require anywhere from 1 to 3 full hours of classroom time, depending on the level of complexity you require from the students. Students may extend their investigations at home in order to get a better grade. If you try it, please get in touch (my email address is on the home page) and let me know how it worked for you.

### Objective

The process of forming a solution can be affected by a number of variables. Larger pieces of material versus smaller pieces may dissolve in a shorter or longer period of time. The temperature of the solvent may affect the rate at which a material dissolves. Finally, the amount that the solvent is stirred may also change the rate at which a solution forms. In this lab you will design experiments to explore the effects of these variables and answer the following objective questions.

Be prepared to make graphs for all of these objectives. In each case the rate of dissolution is your dependent variable and belongs on the y-axis. The variable you are testing is your independent variable and belongs on the x-axis.

1. How does the size of the pieces of material you start with affect the rate at which it dissolves? Can you formulate a general rule for rate of dissolution as a function of the size of a chunk?
2. How does the amount of agitation affect the rate at which a material dissolves? Can you formulate a general rule for rate of dissolution as a function of amount of agitation? Measure the rate the same way. Measure the amount of agitation in a logical, repeatable, and quantifiable way. (Creativity may be required).
3. How does the temperature affect the rate at which a material dissolves? Can you formulate a general rule for rate of dissolution as a function of temperature? Measure the rate as before and measure the temperature with a thermometer. Try to gather data for several different temperatures.

### Materials

• crystal sugar tablets
(Domino® brand)
1.7 × 2.4 × 1.0 cm, ~4.7 g each
• several 250-mL beakers
• glass stirring rods
• metal scoops
• tap water
• beaker tongs
• bunsen burner
• ring stand
• mortar and pestle
• thermometer that reads in °C
• graph paper
• stopwatch or clock with a second hand
• lab balance
• weighing boats

### Background

A solution is a homogeneous mixture of two or more substances. These are most often liquids in which a solvent (e.g., water) dissolves a solid solute (e.g., kool-aid powder). Certain substances will dissolve readily in another and it is said that “like dissolves like.” This saying refers to polarity. Water is a polar substance and will easily dissolve other polar substances such as salt and sugar. Water will not dissolve a non-polar substance like vegetable oil—no matter how much you stir, the oil will remain in suspension. Would you expect oil to dissolve salt or sugar?

A molecule or substance is said to be polar when atoms form a covalent bond in which the electrons are not shared equally. This causes one end of the molecule to be slightly positive, and the other to be slightly negative. We can think of this molecule as a mini-magnet with two poles. When water dissolves another polar substance, opposites attract—the negative side of the water molecule is attracted to the positive side of the solute molecule or the positive side of the water molecule is attracted to the negative side of the solute molecule. As water molecules come into contact with a substance the charged poles of the H2O molecules will pull the poles of other molecules until all the two substances are evenly mixed and dispersed into a solution. The more often water molecules strike the molecules in the solid and the harder they strike them the faster the solid dissolves.

This process can happen at various speeds depending on the conditions. The purpose of this lab is to explore how and why the rate of dissolution can be affected by variables such as the size of a chunk, agitation (or stirring) and temperature.

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### Procedure

The procedure for this lab is mostly up to you. For each objective you must design and carry out experiments to answer the objective question(s). Keep the following in mind as you plan your work:

• What experiments are you going to perform?
• Before you set up the experiments try to decide what you think is going to happen. Write this down and refer back to it once you are done.
• In your experiments, what are you going to hold constant?
• What are you going to change?
• Set up data tables to before doing your experiments.
• How are you going to decide whether you have answered the questions?

The lab is broken up into sections on the following pages. Complete each section and check in with your teacher to get his/her initials.

#### Tips and Pointers

Here are a few things you need to know how to do in order to be successful.

• Make a plan before you start working.
• Be methodical. Use techniques that are repeatable.
• Experiments are a given setup or set of conditions. Trials are when you repeat experimental setups. Perform multiple trials for each experimental setup so you can gauge how repeatable your methods are.
• Do not spend too much time on answering only one of the questions: be efficient!
• Share labor. Delegate.
• Keep tabs on your partners’ progress.
• In each trial keep the amount of water constant.
• Designing a way to stir that is repeatable is a big challenge. Think carefully about this issue before diving into that set of experiments.
• You will be using a 250-mL beaker to heat water. Do not put more than 200 mL of water in it.
• Do not heat water above about 75°C.
• Stir water carefully while heating after moving the burner away from the beaker. Measure temperature after stirring to ensure that all the water is the same temperature.
• Measure temperature by immersing about 7 cm (3 in) of the thermometer in the liquid. Always hold the thermometer upright and do not leave it in the beaker at any time. This often leads to the beaker being tipped over, causing beaker and thermometer to break.

### Safety

This lab generally presents few hazards except when heating water with the burner. Nevertheless, please follow the safety precautions below:

• Always wear safety glasses in the lab
• If anything goes wrong when using the burner immediately close the lab bench fuel valve
• If you cannot get the burner to light within about 15 sec then immediately close the lab bench fuel valve and wait for assistance
• Never ever leave the burner unattended!
• Keep flammable substances far away from the burner
• Be careful not to touch objects recently heated by the burner: hot glass looks just like cold glass!
• Flames and hot objects can cause fires and burns: use caution at all times
• Never leave thermometers in beakers while heating or while standing on the lab bench. They are a hazard that can lead to spills of hot liquids and broken glass and thermometers.

A formal lab report is required. See the last page of this lab packet for guiding questions for the Analysis section of your report.

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### Objective One

#### Effect of not Stirring

Even substances very soluble in water, such as sugar, will not dissolve very much when not stirred. In order to answer the question about whether chunk size (sugar tablet, granulated, ground-up) affects the rate of dissolving simply do experiments for part two on all three sizes. If chunk size affects dissolving rate you should see that in your data comparing stirring rates. For a comparison between stirred water and un-stirred water measure out an amount of sugar and add it to water you have allowed to come to rest in a beaker and do not stir it. Once this is done move directly to Objective Two. Record your observation of this beaker at the end of the lab period and be sure to refer to it in your report. Try placing about 15 g granulated sugar into a beaker containing about 200 mL of water. Let the water come to rest before adding the sugar and do not stir it. Write what you see has happened in this beaker at the end of the lab period.

### Objective Two

#### Effect of Agitation

Stirring the water has a measurable effect on the amount of time it takes for the sugar to dissolve. Shaking counts as agitation, too, but may be difficult to do with the lab equipment available. Design a quantifiable way to stir the water: controlling how many times around per minute may be a way to start. Don’t forget to control for chunk size as well. Will it help to stir when you drop the whole tablet in? Does it help proportionally more when the chunk size is smaller? Use at least three different stirring speeds and three different chunk sizes (tablet, granulated and ground up).

Write down what you are holding constant in the space below. Specify amounts.

Write down the steps of your experiment below and show them to your teacher before proceeding.

#### Check-in ______

Create a data table in the space below. Beneath your data table record your observations and notes about your procedure. Use a separate sheet of graph paper to make a graph of your data.

#### Check-in ______

When you have completed your work in this section check in with your teacher. This is a required part of the lab and your teacher’s initials are required before you can move on to the next objective. Initials will be given for good data, observations and procedural notes.

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### Objective Three

#### Effect of Temperature

Sugar may dissolve at different rates in cool, warm or hot water. Is there a quantifiable relationship between temperature and rate of dissolution? Try at least three different temperatures (iced water, room temperature, and hot). Do stir your mixtures but be sure to do so in the same way for each one. If there is time to do so then try different sizes of chunk to see whether size or temperature is more important.

Write down what you are holding constant in the space below. Specify amounts.

Write down the steps of your experiment below and show them to your teacher before proceeding.

#### Check-in ______

Create a data table in the space below. Beneath your data table record your observations and notes about your procedure. Use a separate sheet of graph paper to make a graph of your data.

#### Check-in ______

When you have completed your work in this section check in with your teacher. This is a required part of the lab and your teacher’s initials are required before you can move on to the next objective. Initials will be given for good data, observations and procedural notes.

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### Lab Questions

Analysis:
1. What is happening at the molecular level when a sugar cube dissolves?
2. How is dissolving different from melting?
3. What effect does the size of the chunks of sugar have on the rate at which it dissolves? What evidence do you have to support the effect you report? Use evidence from all three basic experiments, not just the first one.
4. Why does the size of a chunk of sugar have an effect on the rate of dissolution? Explain what’s happening at the molecular level.
5. What effect does the amount of agitation of the water have on the rate at which the sugar dissolves? What evidence do you have to support the effect you report?
6. Why does agitation have an effect on the rate of dissolution? Explain what’s happening at the molecular level.
7. What effect does the temperature of the water have on the rate at which the sugar dissolves? What evidence do you have to support the effect you report?
8. Why does temperature have an effect on the rate of dissolution? Explain what’s happening at the molecular level.
Conclusion:
1. If you have room temperature water, ice cubes, and powdered lemonade mix in what order would you mix them to dissolve the powder the quickest to make a cold drink? Why?
2. When washing your hands or the dishes why is it more effective to use hot water rather than cold water?
Introduction written by Nathan Guerin. Other text and lab design by Aaron Keller with input from Mr. Guerin.
Sample Data Table for Collecting Data (xls)
Sample Graph (xls)
Last updated: Jul 22, 2014 Home