Courtesy of FEMA. In many areas, ice on the streets and sidewalks represent a serious walking and driving hazard. One common approach to melting the ice is to put some form of deicing salt on the surface. Materials such as sodium chloride or calcium chloride are frequently employed for this purpose. In order to be effective, the solid material must first dissolve and break up into the ions that make up the compound.
An ionic crystal lattice breaks apart when it is dissolved in water. Dissociation is the separation of ions that occurs when a solid ionic compound dissolves. It is important to be able to write dissociation equations. Simply undo the crisscross method that you learned when writing chemical formulas of ionic compounds.
The subscripts for the ions in the chemical formulas become the coefficients of the respective ions on the product side of the equation. The formula unit of sodium chloride dissociates into one sodium ion and one chloride ion. The calcium nitrate formula unit dissociates into one calcium ion and two nitrate ions.
The ammonium phosphate formula unit dissociates into three ammonium ions and one phosphate ion. 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.
Biochemical Properties of Water - Advanced. Skip to main content. Search Search. Point out that the water molecules are attracted to the sodium and chloride ions of the salt crystal. Explain that the positive area of a water molecule is attracted to a negative chloride ion. The negative area water of a water molecule is attracted to a positive sodium ion.
Dissolving happens when the attractions between the water molecules and the sodium and chloride ions overcome the attractions of the ions to each other. This causes the ions to separate from one another and become thoroughly mixed into the water. Tell students that the amount of a substance that can dissolve in a liquid at a particular temperature is called solubility.
Point out the similarity in the words dissolve and solubility. Also tell them that the substance that is dissolved is called the solute. The substance that does the dissolving is called the solvent. Discuss how to set up a test to compare how water and alcohol dissolve salt. Be sure students identify variables such as:. There will be less undissolved salt in the cup with the water than the alcohol.
This means that more salt dissolved in the water than in the alcohol. Read more about polarity in the teacher background section. Remind students that isopropyl alcohol has an oxygen atom bonded to a hydrogen atom so it does have some polarity but not as much as water. Since water is more polar than alcohol, it attracts the positive sodium and negative chloride ions better than alcohol. This is why water dissolves more salt than alcohol does.
Another way of saying this is that the solubility of salt is greater in water than in alcohol. The sodium carbonate will dissolve, but the calcium carbonate will not. Explain that not all ionically bonded solids dissolve in water. Explain that on the molecular level, the ions that make up calcium carbonate are attracted so strongly to each other that the attraction by water molecules cannot pull them apart.
That is a good thing because calcium carbonate is the material that sea shells and bird eggs are made of. Calcium phosphate is another ionic solid that does not dissolve in water. This is also good because it is the material that bones and teeth are made of.
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?
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