3.5 Solutions and Mixtures

Types of mixtures

A mixture is a physical combination of substances. Homogeneous mixtures (solutions) have uniform composition throughout, like saltwater — the solute dissolves in the solvent. Heterogeneous mixtures have non-uniform composition, including suspensions (particles settle) and colloids (particles stay dispersed but scatter light, showing the Tyndall effect).

Solubility and "like dissolves like"

Solubility depends on intermolecular forces between solute and solvent. Polar solutes dissolve in polar solvents (e.g., ionic salts in water via ion-dipole forces); nonpolar solutes dissolve in nonpolar solvents (via London dispersion forces). If forces are mismatched, substances don't mix well, like oil and water.

Concentration units

Molarity (M) = moles solute / liters solution is the most common concentration unit. Other measures include mass percent, molality, and mole fraction. Concentration can be experimentally determined using spectroscopy (Beer's Law), since absorbance is proportional to concentration for colored/UV-absorbing solutions.

Key terms

Solution

A homogeneous mixture with uniform composition, consisting of a solute dissolved in a solvent.

Colloid

A heterogeneous mixture with dispersed particles that don't settle and scatter light (Tyndall effect).

Suspension

A heterogeneous mixture with particles large enough to settle out over time.

Like dissolves like

Principle that solutes dissolve best in solvents with similar polarity/intermolecular forces.

Molarity (M)

Concentration unit defined as moles of solute per liter of solution.

Try an AP-style question

What is the molarity of 0.50 mol NaCl dissolved in 250 mL of solution?

1. 0.125 M
2. 0.50 M
3. 2.0 M
4. 125 M