Why Does Water Dissolve Sugar?

Distribute M&M’s to students and have them look at the outside candy coating. Then have students break an M&M to look closely at the coating from the inside.

Ask students:

• What do you think the coating of an M&M is made from?
  Students will see the layer of color with a layer of white beneath it and suggest that the coating is made of sugar and coloring. Explain that the coating is mostly sugar.
• Have you ever noticed what happens to the coating of an M&M when it gets wet? 
  The color comes off and if it gets wet enough, the entire coating comes off, leaving the chocolate behind.

Tell students that in this activity, they will see what happens to the sugar and color coating of an M&M when it is placed in water.

Give each student an activity sheet.

• Lesson 5.4 Student Activity Sheet PDF | DOCX | Google Doc
• Lesson 5.4 Activity Sheet Answers PDF | DOCX | Google Doc

Download the student activity sheet, and distribute one per student.

All Downloads

The activity sheet will serve as the “Evaluate” component of each 5-E lesson plan. The activity sheets are formative assessments of student progress and understanding. A more formal summative assessment is included at the end of each chapter.

Students will record their observations and answer questions about the activity on the activity sheet. The Explain It with Atoms and Molecules and Take It Further sections of the activity sheet will either be completed as a class, in groups, or individually depending on your instructions. Look at the teacher version of the activity sheet to find the questions and answers.

Question to Investigate

What happens to the sugar and color coating of an M&M when it is placed in water?

Materials

• Clear plastic cup
• Water
• M&M
• White paper

Ask students:

• What do you notice about the M&M and the water?
  The color comes off and moves through the water in a circular pattern.
• What do you think is happening when the color and sugar come off the M&M? 
  Point out to students that because the water makes the colored coating come off the M&M and mix into the water, the water is dissolving the sugar and color.

Note: There are actually two processes happening in this activity. The color and sugar are dissolving in the water, but they are also diffusing. In order to focus on the amount that dissolves from an M&M, students should look at the amount of coating missing from the M&M, instead of the size of the circle of color in the water.

• Knowing what you do about the polarity of water, why do you think water dissolves sugar?
  Students may think that sugar is made of ionic bonds like salt. Or they might think that sugar has positive and negative areas and this is why water is attracted to it.

Explain to students that sugar is made of large molecules called sucrose. Each sucrose molecule is made of atoms that are covalently bonded.

Ask students:

• The chemical formula for sucrose is C₁₂H₂₂O₁₁. What do these letters and numbers mean?
  Sucrose is made up of 12 carbon atoms, 22 hydrogen atoms, and 11 oxygen atoms.

Project the image Sucrose.

Explain that the first picture is a ball-and-stick model of a single sucrose molecule. The next picture is a space-filling model of a single sucrose molecule. The last picture is two sucrose molecules attract- ed to each other. These two molecules will separate from each other when sugar dissolves. Point out that in the areas on a sucrose molecule where oxygen is bonded to hydrogen (O–H bond), the oxygen is slightly negative, and the hydrogen is slightly positive. This makes sucrose a polar molecule.

Project the image Water Dissolves Sucrose.

Explain that a sugar cube (about a half a teaspoon of sugar) is made up of at least one billion trillion sucrose molecules. When sugar dissolves, these whole sucrose molecules separate from one another. The molecule itself doesn’t come apart: The atoms that make up each molecule stay together as a sucrose molecule.

Project the animation Sucrose.

Explain that sucrose has polar areas caused by the same type of oxygen–hydrogen covalent bonds as in the water molecule. Point out the O–H bonds on the outer edges of the molecule. Point out that the bonding of the oxygen and hydrogen in the sucrose makes parts of the sucrose molecule polar in a similar way as in a water molecule. The area near the hydrogen is positive (blue) and the area near the oxygen is negative (red).

Project the animation Water Dissolves Sucrose.

Tell students that sugar molecules are attracted to each other and held together by the attraction between these polar areas of the molecules.

Help students notice how the positive (blue) area of a water molecule is attracted to the negative (red) area of a sucrose molecule. It also works the other way around. The negative (red) area of a water molecule is attracted to the positive (blue) area of the sucrose molecule.

Explain that the positive and negative areas on water molecules interact with these negative and positive parts of sucrose molecules. When the attraction between water molecules and sucrose molecules overcomes the attraction the sucrose molecules have to other sucrose molecules, they will separate from one another and dissolve.

Point out that one whole sucrose molecule breaks away from another whole sucrose molecule. The molecule itself does not come apart into individual atoms.

Note: Although the focus is on dissolving the polar sugar molecules, the food coloring used to color the M&M is also made from polar molecules. This helps explain why the coloring also dissolves.

Ask students to make a prediction:

• Do you think water, alcohol, or oil would be better at dissolving the sugar and color coating of an M&M?

Discuss with students how to design an experiment to compare how well water, alcohol, and vegetable oil dissolve the color and sugar coating from an M&M. Be sure students identify variables such as:

• Amount of water, alcohol, and oil used
• Temperature of each liquid
• Same color of M&M
• Time and location the M&M’s are placed in each liquid

Question to Investigate

Is water, alcohol, or oil better at dissolving the color and sugar coating from an M&M?

Materials

• 3 M&Ms (same color)
• Water
• Mineral oil
• Isopropyl alcohol (70%)
• 3 clear plastic cups
• White paper

Project the image Polarity of Water, Alcohol and Oil.

Show students the polar areas on a water molecule, isopropyl alcohol molecule, and an oil molecule.

Project the image Citric Acid.

Explain that the projected image is a model of a citric acid molecule. Tell students that citric acid is the substance that gives lemons, limes, grapefruit, and oranges their tangy sour taste. Citric acid is very soluble in water and is dissolved in the water in the fruit.