Most students experience scientific phenomena each day, although they rarely consider the science behind these experiences as they occur. As teachers, we can take advantage of everyday experience in order to introduce and explain science concepts. Something with which all students have familiarity is the idea of dissolving. Anyone who has ever added sugar to a bowl of cereal has probably noticed that it seems to disappear after pouring milk on it. Although it looks like it has mysteriously vanished, it has actually just dissolved in the milk and become part of a solution. A solute is the substance being dissolved. A solvent is the substance in which the solute is dissolved. In our sugar and milk example, the sugar is the solute and the milk is the solvent.
To introduce the idea of dissolving, give each child a chocolate kiss. They should place it in their mouths without chewing it, or moving it in any way (yes, this requires willpower!). Time how long it takes for the candy to dissolve, or fully disappear. Do the same thing with another piece of candy, this time swirling it around. Finally, ask students to try chewing a third piece of candy. Dissolving time in this case should be the fastest because chewing increases the surface area of the candy, thereby exposing more of it to saliva in the mouth and speeding up the process. To demonstrate the role of temperature in dissolving, place a candy in a container of water, and compare the dissolving time to that of the first candy in which students kept it stationary in their mouths. Increased temperatures greatly influence the time it takes a solute to dissolve, and this should be evident when looking at the data.
In the above investigation, the chocolate is the solute, and saliva is the solvent. Water is often called the “Universal Solvent” because of its ability to dissolve many different substances. Challenge your students to discover solids that do and do not dissolve in water, such as salt, baking soda, flour, powered sugar, corn starch, sand, etc . . . Caution them not to use too much solute, or the solution may become saturated and sink to the bottom (something many students will also have experienced when spooning too much sugar on their cereal!).
For some great lesson ideas involving solutions and dissolving, check out the following.
Solute and Solvent Activities and Lessons:
Students compare the temperature in two containers of water. They measure the temperature of boiling water. Then they add a tablespoon of sugar to the boiling water and count how many seconds it takes before it dissolves completely. They also do experiments with ice water and water at room temperature.
In this activity students learn about solutions, and solutes. They find out how stirring, shaking, and crushing affects the rate at which solutes dissolve.
Students learn about a variety of processes including evaporation, diffusion, osmosis, and how solutes dissolve in liquid. They discuss what happens to molecules as they go through these different processes.