As you watch salt or sugar particles dissolve in water, they seem to disappear. You know that they must be in the water somewhere because you can taste them and also see them if you let the water evaporate. But what really happened to make them dissolve? And why do some substances dissolve easily, some not at all, and some in between?
Dissolving is as much about the substance doing the dissolving (the solvent) as it is about the substance being dissolved (the solute). Dissolving depends on the interaction between the molecules of solvents and solutes. Let’s look at the molecular structure of water, the universal solvent.
Water, H2O, is made up of two hydrogen atoms bonded to one oxygen atom. The bond between the atoms is called a covalent bond, which means that electrons from each atom are shared among the other atoms. At any given time, the electron from one of the hydrogen atoms might be near the oxygen atom while an electron from the oxygen might be near one of the hydrogen atoms.
The key to the wondrous properties of water is that the sharing is not equal. Oxygen has a greater attraction for electrons than hydrogen does. So, on average, the shared electrons spend more time near the oxygen. Because electrons have a negative charge, this gives the area around the oxygen a slight negative charge and the area near the hydrogens a slight positive charge. The overall charge on the water molecule as a whole is still neutral. Electrons have not been gained or lost but are unevenly distributed in the molecule. When a molecule is neutral but has this type of charge separation, it is called a polar molecule. The polar nature of the water molecule contributes to its dissolving prowess.
The Power of Polarity
Because opposite charges attract, the positive part of one water molecule is attracted to the negative part of another water molecule. These positive and negative areas on water molecules are also attracted to the positive and negative areas of the molecules or ions of other substances. This is the key to water’s great ability to dissolve other substances. When the attractions that water molecules have for a molecule or ion of a solute become stronger than the attractions that particle has for its neighboring solute particles, the substance begins to dissolve.
The amount of attraction that the water molecules have for the particles of a particular substance compared to the attraction these particles have for each other is an important factor in determining whether and how quickly water can dissolve the substance. Therefore solubility, or the extent to which a substance dissolves, is a characteristic property of a substance. In the activity below, students try to identify an unknown crystal by comparing its solubility to the solubility of three known crystals.
James Kessler and Patti Galvan are developers of science teaching resources in the Education Division at the American Chemical Society (ACS) in Washington, D.C.
National Research Council (NRC). 1996. National science education standards. Washington, DC: National Academy Press.
Dissolving Is the Solution!
Supply one set of the following
materials for each student group:
- 4g Table salt in cup
- 4g Epsom salt in cup
- 4g MSG (Accent) in cup
- 4g Kosher salt in cup
- 4 clear plastic cups
- A teaspoon
- Hot tap water
In this activity, appropriate for students in grades 3 to 8, students conduct a dissolving test on an “unknown” crystal. If the unknown crystal dissolves most like one of three known crystals, students will have some evidence about the possible identity of the unknown crystal. The activity is adapted from the ACS book Inquiry in Action—Investigating Matter through Inquiry, available at www.chemistry.org/education/inquiryinaction.html.
To design a fair test and begin to develop the understanding that solubility is a characteristic property of a substance.
Students are presented with a sample of table salt, Epsom salt, MSG, and an unknown (Kosher salt) that looks different from each of the known crystals but is chemically the same as one of them (table salt). Ask students for their ideas for tests that might show differences between the crystals. If students don’t suggest a dissolving test, demonstrate dissolving equal amounts of table salt and sugar in water. Because these dissolve differently, students should realize that a dissolving test could help them distinguish between the crystals. Help students design a fair test by discussing some of the variables that might affect dissolving. Ask students: How can we set up the experiment so that it’s fair? Students should mention using: equal amounts of crystal, equal amounts of water, the same temperature water, and mixing the crystals in the same way for the same length of time. The following procedure is one approach that you and your students could follow.
Place 1 teaspoon (5 ml) of hot tap water into four empty cups. Match up each pair of cups so that each cup of crystal is near its corresponding cup of water. At the same time (with the help of your lab partners), pour the entire amount of each crystal into its cup of water. With the help of your lab partners, swirl each cup at the same time and in the same way for about 20 seconds and observe. Swirl again for 20 seconds and observe, and then, for 20 more seconds and make your final observations. Slowly swirl and carefully pour the solution from each clear plastic cup back into its empty cup, trying not to let any undissolved crystal go into the small cup. Compare the amount of crystal remaining in each cup.
- What did you observe about the amount of undissolved crytal left in each cup? What did you observe about the solutions?
- Based on your observations, which substance is probably not the unknown? (Students are likely to conclude that Epsom salt is not the unknown.)
- What do you think is the identity of the unknown substance?
(Students will probably conclude that the unknown is salt or MSG. A follow-up test will determine the identity conclusively.)
- What evidence do you have to support your conclusion?
- Why was it important to keep all the variables the same?
Students can conduct another test by allowing the solutions of dissolved crystal to evaporate and the crystals to reform. A recrystallization procedure and more activities can be found at http://chemistry.org/education/science_children.html.