How Substances Dissolve Chapter 7.2 How Substances Dissolve

Objectives Explain how the polarity of water enables it to dissolve many different substances. Relate the ability of a solvent to dissolve a solute to the relative strength of forces between molecules. Describe three ways to increase the rate at which a solute dissolves in a solvent. Explain how a solute affects the freezing point and boiling point of a solution.

Water: A Common Solvent Water is often referred to as the universal solvent because it dissolves so many different substances. Water can dissolve most ionic compounds because of its polar structure. Water has a positive and negative pole which makes it natural at dissolving ionic compounds. Polar compound – a molecule that has an uneven distribution of electrons. Water molecules attract both the positive and negative ions of most ionic compounds, eventually pulling the ionic solid apart. The force of attraction between the water and the ions is greater than the attraction between the positive and negative ions (at least in most cases.)

Water: A Common Solvent Not all ionic compounds will dissolve in water; some ions have a greater attraction than the dissolving force of the water. In those cases, the ionic compound will not dissolve. Dissolves  water attraction > ion attraction Does not dissolve  water attraction < ion attraction

Water: A Common Solvent Water can also dissolve many molecular compounds. The hydrogen bonds that are found in water contribute to the ability of water to dissolve these molecular substances. Examine Figure 13 on page 234. Essentially, water forms hydrogen bonds with parts of the molecular compound, which eventually pull the compound away from the solid – causing it to dissolve.

“Like Dissolves Like” A general rule of thumb in chemistry: Polar substances will dissolve polar substances. Non-polar substances will dissolve non-polar substances. Generally speaking, non-polar compounds will not dissolve in water because it is a polar compound.

The Dissolving Process The Kinetic Theory contributes to the dissolving process – remember all matter is made of tiny particles that are constantly in motion. These particles constantly collide and transfer energy in the process – aiding the dissolving process. There are several factors that increase the rate of dissolve: Increasing the surface area – break the solute into several pieces. Stirring or shaking – increases the number of collisions. Increase the temperature of the solvent – speeds up particle collisions.

Solutes & Physical Properties Physical properties, such as, boiling point and freezing point can be greatly affected by adding a solute. Many solutes increase the boiling point of a pure solvent, and also decrease the freezing point of that solvent. For example, pure water normally boils at 100 degrees Celsius; however, when you add sodium chloride to it, the boiling point increases to 102 degrees Celsius. Water typically freezes at 0 degrees Celsius, but after adding sodium chloride, the freezing point becomes -7 degrees Celsius. That’s why we spread salt on ice, it lowers the freezing point, which makes it harder to freeze. Thus, it melts.