Solutions and Concentration

By the end of this lesson, you will be able to explain what dissolving actually does, calculate and dilute a molarity, use a solubility curve, and say why salt lowers the freezing point of water.

Dissolving Is a Competition

$$\textbf{solute} + \textbf{solvent} \rightarrow \textbf{solution}$$

The solute is what dissolves, the solvent is what does the dissolving, and the solvent is whichever is present in greater amount.

Dissolving happens when solute-solvent attractions can compete with the attractions already holding each apart.

$$\textbf{like dissolves like}$$

Polar solvents dissolve polar and ionic solutes; nonpolar solvents dissolve nonpolar ones. Water refuses oil not because oil is "repelled" but because water molecules attract each other far more strongly than they attract oil, so they stay together and exclude it.

What happens to an ionic solid in water, step by step:

• Water's negative oxygen ends surround each cation
• Water's positive hydrogen ends surround each anion
• Ions are pulled off the lattice one at a time and carried away
• Each dissolved ion ends up wrapped in a shell of oriented water molecules, which is called hydration

Dissolving is not melting and not a chemical reaction. The $\mathrm{Na^+}$ and $\mathrm{Cl^-}$ ions still exist; they have simply been separated and surrounded. Evaporating the water returns the salt unchanged, which is exactly why it counts as a physical change.

Sort It: Will It Dissolve in Water

Molarity

$$M = \frac{\text{moles of solute}}{\text{liters of solution}}$$

The standard concentration unit in chemistry, because reactions happen by moles and molarity delivers moles directly from a measured volume.

$$n = M \times V$$

Liters of SOLUTION, not liters of solvent. A one-molar solution is made by dissolving one mole and then adding solvent up to the one-liter mark, not by adding one mole to a liter of water. The solute occupies space, and for concentrated solutions the difference is substantial.

Convert milliliters to liters first. Molarity is defined per liter, and using milliliters gives an answer a thousand times too large.

Molarity changes with temperature, slightly, because the solution expands when warmed while the moles stay the same. For precise work molality, moles per kilogram of solvent, is used instead, since mass does not depend on temperature.

Example 1: Preparing a Solution

Here is every step, worked all the way through.

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