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We just wrapped up A9: Dissolving, Solutions, and Crystallization. We started our week with some reading, to get an idea of what happens when various substances are put into water. We recently added Chemistry Pre-level I from Real Science-4-Kids as a reference book, and Chapter 6 (Mixtures) was a great introduction to what we covered this week.
Part 1: Some things dissolve: Solutions and Mixtures
We covered a few basic concepts first.
What is a mixture? When we combine difference substances, we have a mixture. We talked about a mixture of diffferent types of fruit in a bowl, and various toys in a box. Then, we talked about making a mixture by putting a solid into a liquid, like when we combine sugar and water.
For a demonstration, we made up a sugar solution as a demo. After stirring our sugar, we watched the granules slowly get smaller and finally disappear. Our solid dissolves, which means the substance, in this case sugar, comes apart into its basic particles and interact with the particles of water. This makes a special kind of mixture called a solution.
We compared our sugar water mixture to a mixture of flour in water. After stirring a small amount of flour into a glass of water, we observed our mixture remain cloudy – our flour doesn’t come apart to interact with the water molecules as a solution but instead remains just a mixture.
Part 2: Soluble and Insoluble
After comparing sugar-in-water and flour-in-water, we then explored a variety of materials to see what was soluble and insoluble. While we did some basic kitchen items, like vinegar and baking soda, we also found some fun objects like a matchbox car, a small block of wood, and a plastic toy. The kids enjoyed stirring these to see whether they would dissolve.
Why don’t certain objects dissolve? This was a great opportunity to review our earlier lesson on solids, liquids and gases, where we learned how objects’ particles are either very close together (in solids), interacting but not closely packed (as in liquids) or not in contact with each other (in gases). Our solid objects, with particles closely packed together, were not able to break apart and interact with the water particles. We prepared a chart to record our observations.
Part 3: Crystallization
We dissolved salt in water to observe not only the process of salt dissolving and forming a solution, but also of salt particles forming crystals. We recently learned about about evaporation, when the water molecules leave the liquid state and go into the gas state. Any solids that are dissolved in the water are left behind and reformed crystals
To help with our observation, we placed a couple of teaspoons of our salt solution on a dark plate, and left out to evaporate. We later observed a crusty film where our salt solution was before evaporating.
We also made a straw by twisting up a piece of paper and securing with tape. This straw was then placed in a jar of salt solution. Our liquid wicked up the paper straw and after the liquid evaporated, a salt crust (crystals) was observed on the surface of the straw.
This lesson helped reinforce our earlier lessons on the particle nature of matter, and the states of matter. Understanding this particle nature is essential for upcoming lessons on atomic and molecular motion.
Michelle Home Scholar 