A saturating science project from Science Buddies
Key concepts Chemistry Property of matter Solutions Solubility Introduction Background When you dissolve a soluble chemical in water, you are making a solution. In a solution the chemical you add is called the solute and the liquid that it dissolves into is called the solvent. Whether a compound is soluble or not depends on its physical and chemical properties. To be able to dissolve, the chemical has to have the capability to interact with the solvent. During the process of chemical dissolution, the bonds that hold the solute together need to be broken and new bonds between the solute and solvent have to be formed. When adding sugar to water, for example, the water (solvent) molecules are attracted to the sugar (solute) molecules. Once the attraction becomes large enough the water is able to pull individual sugar molecules from the bulk sugar crystals into the solution. Usually the amount of energy it takes to break and form these bonds determines if a compound is soluble or not. Generally, the amount of a chemical you can dissolve in a specific solvent is limited. At some point the solution becomes saturated. This means that if you add more of the compound, it will not dissolve anymore and will remain solid instead. This amount is dependent on molecular interactions between the solute and the solvent. In this activity you will investigate how much of various compounds you can dissolve in water. How do you think sugar and salt compare? Materials
Observations and results This is because each of these compounds has different chemical and physical properties based on their different molecular structures. They are all made of different chemical elements and have been formed by different types of bonds. Depending on this structure it is more or less difficult for the water molecules to break these bonds and form new ones with the solute molecules in order to dissolve them into a solution. Cleanup More to explore This activity brought to you in partnership with Science Buddies Discover world-changing science. Explore our digital archive back to 1845, including articles by more than 150 Nobel Prize winners. Subscribe Now!
Solubility Why Do Some Solids Dissolve in Water? The sugar we use to sweeten coffee or tea is a molecular solid, in which the individual molecules are held together by relatively weak intermolecular forces. When sugar dissolves in water, the weak bonds between the individual sucrose molecules are broken, and these C12H22O11 molecules are released into solution. It takes energy to break the bonds between the C12H22O11 molecules in sucrose. It also takes energy to break the hydrogen bonds in water that must be disrupted to insert one of these sucrose molecules into solution. Sugar dissolves in water because energy is given off when the slightly polar sucrose molecules form intermolecular bonds with the polar water molecules. The weak bonds that form between the solute and the solvent compensate for the energy needed to disrupt the structure of both the pure solute and the solvent. In the case of sugar and water, this process works so well that up to 1800 grams of sucrose can dissolve in a liter of water. Ionic solids (or salts) contain positive and negative ions, which are held together by the strong force of attraction between particles with opposite charges. When one of these solids dissolves in water, the ions that form the solid are released into solution, where they become associated with the polar solvent molecules.
We can generally assume that salts dissociate into their ions when they dissolve in water. Ionic compounds dissolve in water if the energy given off when the ions interact with water molecules compensates for the energy needed to break the ionic bonds in the solid and the energy required to separate the water molecules so that the ions can be inserted into solution. Solubility Equilibria Discussions of solubility equilibria are based on the following assumption: When solids dissolve in water, they dissociate to give the elementary particles from which they are formed. Thus, molecular solids dissociate to give individual molecules
and ionic solids dissociate to give solutions of the positive and negative ions they contain.
When the salt is first added, it dissolves and dissociates rapidly. The conductivity of the solution therefore increases rapidly at first.
The concentrations of these ions soon become large enough that the reverse reaction starts to compete with the forward reaction, which leads to a decrease in the rate at which Na+ and Cl- ions enter the solution.
Eventually, the Na+ and Cl- ion concentrations become large enough that the rate at which precipitation occurs exactly balances the rate at which NaCl dissolves. Once that happens, there is no change in the concentration of these ions with time and the reaction is at equilibrium. When this system reaches equilibrium it is called a saturated solution, because it contains the maximum concentration of ions that can exist in equilibrium with the solid salt. The amount of salt that must be added to a given volume of solvent to form a saturated solution is called the solubility of the salt. Solubility Rules There are a number of patterns in the data obtained from measuring the solubility of different salts. These patterns form the basis for the rules outlined in the table below, which can guide predictions of whether a given salt will dissolve in water. These rules are based on the following definitions of the terms soluble, insoluble, and slightly soluble.
Solubility Rules for Ionic Compounds in Water
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