Navigating this Pathway
[stextbox id = “info”] This pathway poses a question to students: What happens to salt when it is stirred into a glass of water? (Abraham, Grzybowski, Renner, & Marek, 1992; Abraham, Williamson, & Westbrook, 1994; Çalik & Ayas, 2005; Johnson, 1998; Liu & Lesniak, 2006; Nakhleh & Samarapungavan, 1999; Papageorgiou & Johnson, 2005) We recommend having students write the driving question, “What happens when you stir salt into water?” in their science notebook and/or displaying this question prominently in the classroom throughout the pathway. Students begin by making predictions about the weight of salt and water when mixed together (more, less, or the same as when separate). Next they draw pictures of the saltwater using “super strong glasses,” more powerful than the strongest microscope. They then consider whether it is possible to separate the salt from the water, and observe what happens when some of the saltwater is left in the sun. The students then engage with computer simulations representing salt dissolving in water, in order to gain a better understanding of dissolving in relation to the particle model. Finally, the students revisit their super strong glasses drawings and revise them using their emerging understanding of the particle nature of matter.[/stextbox]
Connections to Other Pathways
[stextbox id = “info”] Part of this pathway relies on students’ understanding of evaporation, so it would be helpful to engage students with the “What happens to the water in a puddle? ” pathway before engaging with this one.[/stextbox]
Teacher Content Background
[stextbox id = “info”] All matter is made of particles that are too small to see, that are in constant random motion, and that have empty space between them. “Empty space” is literally empty; that is, nothing at all is between the particles that make up matter. All particles in one state are still present even after matter has changed states; no particles are added or destroyed. In addition, the size and shape of the particles does not change. Therefore matter (and mass, or weight) is conserved (stays the same) during phase change because all of the particles present in one state are present in the other (and no more or less).When dissolving salt in water, the movement of and collision between the salt and water particles causes the particles to break apart. Neither salt nor water particles are destroyed. When the water evaporates from a mixture of salt and water, all of the original salt particles will remain.
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Student Thinking
[stextbox id = “info”] Students have many misconceptions when it comes to dissolving. For example, common student explanations for the phenomenon of dissolving salt in water include:- the salt disappeared—that is, it doesn’t exist anymore
- the salt melted
- the water absorbed the solute
- the salt evaporated
- the salt turned into the solvent
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Student Experiences
[stextbox id = “info”]- Student Experience 1: Stirring Salt into Water Students make predictions about and observe what happens when salt is stirred into a cup of water.
- Student Experience 2: Imagining Dissolving, Part 1 Students draw what they would see in the water before and after salt is added if they were wearing glasses more powerful than the most powerful microscope.
- Student Experience 3: Separating Salt from Saltwater Students make predictions about and observe what happens when water is evaporated from a saltwater solution.
- Student Experience 4: Dissolving Simulations Students observe computer simulations demonstrating dissolving at the particle level.
- Student Experience 5: Imagining Dissolving, Part 2 Students revisit and revise their drawings of water before and after salt is added.