Other salts dissolve in water, too, but some of them dissolve more easily than others. You could easily dissolve about g of table salt in a liter of water, but the solubility of calcium carbonate is only about 0. The higher the charges on the ions, the stronger their electrostatic attraction for each other, and the harder it is for the water to pull them apart.
It's a good thing, too; if the calcium carbonate in marble were to dissolve too easily, think of how many buildings and statues would dissolve in the rain. The factors that control the solubility of ions can be complex, though. For example, the charges on the ions also affect the strength of the interaction with the water molecules. Sodium carbonate breaks apart completely into ions that are incorporated throughout the water, forming a solution.
The sodium and carbonate ions will not settle to the bottom and cannot be filtered out of the water. But calcium carbonate does not break up into its ions. Instead it is just mixed in with the water. If given enough time, the calcium carbonate will settle to the bottom or can be filtered out of the water.
Sodium carbonate dissolved in water is a good example of a solution, and undissolved calcium carbonate is a mixture, not a solution. These types of ions, called polyatomic ions, are made up of a group of covalently bonded atoms that act as a unit. They commonly gain or lose one or more electrons and act as an ion. You can decide if you would like to introduce students to these two common polyatomic ions. The American Chemical Society is dedicated to improving lives through Chemistry.
Skip Navigation. Lesson 5. Engage Make a model of a salt crystal. Ask students: What is it about water molecules and the ions in salt that might make water able to dissolve salt? The positive and negative polar ends of a water molecule are attracted to the negative chloride ions and positive sodium ions in the salt.
Give each student an activity sheet. Question to Investigate How does salt dissolve in water? Materials Activity sheet with sodium and chloride ions and water molecules Construction paper, any color Scissors Tape or glue Procedure Make a model of a salt crystal Cut out the ions and water molecules. Arrange the ions on a piece of construction paper to represent a 2-D salt crystal. Do not tape these pieces down yet. Project an image and have students model what happens when salt dissolves in water.
Show students a series of four pictures to help explain the process of water dissolving salt. Model how water dissolves salt Look at the pictures showing how water molecules dissolve salt.
Move the water molecules and sodium and chloride ions to model how water dissolves salt. Tape the molecules and ions to the paper to represent water dissolving salt. Explore Have students conduct an experiment to find out whether water or isopropyl alcohol would be better at dissolving salt. Ask students to make a prediction: Think about the polarity of water molecules and alcohol molecules. Do you think alcohol would be just as good, better, or worse than water at dissolving salt?
If you add salt to the mixture, however, the salt wants to dissolve in the water and competes with the alcohol for the water molecules. Because there are fewer water molecules available to make hydrogen bonds with the alcohol molecules, the alcohol becomes less soluble in the water—alcohol mixture, eventually forming a separate layer on top of the water. Both layers should have a different color, with the water mostly clear and the alcohol more colored. This occurs because the marker ink is more soluble in the rubbing alcohol.
Cleanup Flush all your mixtures down the sink with plenty of cold water. Wash your hands and clean your work area. This activity brought to you in partnership with Science Buddies. Already a subscriber? Sign in. Thanks for reading Scientific American. Create your free account or Sign in to continue.
See Subscription Options. Go Paperless with Digital. Key concepts Chemistry Solutions Miscibility Polarity Solubility Introduction You probably know some liquids, such as oil and water, do not mix together. Materials Four transparent mini cups two ounces with lids Permanent marker Tap water Rubbing alcohol 70 percent isopropyl alcohol Table salt Set of measuring spoons Work area that can tolerate spills Ethanol or acetone can be found in hardware stores optional Salt substitute such as potassium chloride or Epsom salt optional Preparation With the permanent marker label the mini cups 1, 2, 3 and 4.
Add one and a half tablespoons of water to cups 1 and 3. Add one and a half tablespoons of rubbing alcohol to cups 2 and 4. Procedure Add one teaspoon of salt to the water in cup 1. What happens to the salt? Does it dissolve in the water? Put on the lid and shake the cup for about 20 to 30 seconds. What does the mixture look like?
Repeat the previous two steps using cup 2 with rubbing alcohol. What happens to the salt this time? The positively-charged side of the water molecules are attracted to the negatively-charged chloride ions and the negatively-charged side of the water molecules are attracted to the positively-charged sodium ions. Essentially, a tug-of-war ensues with the water molecules winning the match.
Water molecules pull the sodium and chloride ions apart, breaking the ionic bond that held them together. After the salt compounds are pulled apart, the sodium and chloride atoms are surrounded by water molecules, as this diagram shows. Once this happens, the salt is dissolved, resulting in a homogeneous solution.
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