Source: Solutions Summary, General Chemistry (University of Minnesota Twin Cities)
Difficulty: Introductory | Prerequisites: Basic understanding of atoms, molecules, and the mole concept.
Solutions are one of the foundational topics in general chemistry, sitting at the intersection of matter classification, intermolecular forces, and quantitative analysis. Nearly every reaction you study from this point forward takes place in solution, so the vocabulary and concentration units here become tools you will use constantly. You should already be comfortable with moles, molar mass, and the idea that matter exists in different phases. If those feel shaky, revisit them first.
A solution is a homogeneous mixture of a solute (the substance dissolved) and a solvent (the substance doing the dissolving). Concentration can be expressed as molarity, molality, or percent by mass. Solubility depends on the nature of the solute and solvent, temperature, and pressure, and special categories like electrolyte and colloidal solutions come up often in both chemistry and biology.
Solution
A homogeneous mixture of two or more substances in which the components are evenly distributed at the molecular level.
In simple terms, this means everything looks and behaves the same throughout, no matter where you sample it.
Solute
The substance that is dissolved in a solution. It is typically present in a smaller amount than the solvent.
Think of it as the ingredient that disappears into the mixture, like salt dissolving into water.
Solvent
The substance in which the solute is dissolved. It is typically present in the greater amount.
Think of it as the host, the medium that does the dissolving. Water is the most common solvent in chemistry.
Molarity (M)
The concentration of a solution expressed as the number of moles of solute per litre of solution.
In simple terms, this tells you how many moles of solute are floating around in each litre of your mixture.
Molality (m)
The concentration of a solution expressed as the number of moles of solute per kilogram of solvent.
This is similar to molarity, but uses mass of solvent instead of volume of solution. It does not change with temperature because mass does not expand or contract the way volume does.
Percent by Mass (% mass)
The concentration of a solution expressed as the mass of solute per hundred grams of solvent.
Think of it as "what percentage of the total weight comes from the solute."
Solubility
The maximum amount of a substance that can be dissolved in a given amount of solvent at a given temperature.
In simple terms, it is the point at which the solvent simply cannot take on any more solute. Add more and it just sits there undissolved.
Electrolyte Solution
A solution that contains ions and is capable of conducting electricity. Examples include saltwater and blood.
Think of it as a solution with charged particles swimming around, which means it can carry an electrical current.
Colloidal Solution (Colloid)
A mixture in which solute particles are dispersed throughout the solvent but are too large to form a true solution. Examples include blood, fog, and smoke.
Think of it as being halfway between a true solution and a suspension. The particles are small enough to stay dispersed but large enough to scatter light (the Tyndall effect).
Solid solutions – two or more solids evenly distributed at the atomic or molecular level.
Example: steel (iron + carbon), bronze (copper + tin).
Liquid solutions – a solute dissolved in a liquid solvent. The most common type you will encounter.
Example: saltwater (NaCl dissolved in H₂O), vinegar (acetic acid in water).
Gaseous solutions – a gas solute mixed with a gas solvent.
Example: air (oxygen, argon, CO₂, and trace gases dissolved in nitrogen).
Molarity (M) = moles of solute / litres of solution
The go-to unit in most general chemistry calculations.
Volume-dependent, so it changes slightly with temperature.
Molality (m) = moles of solute / kilograms of solvent
Temperature-independent because it relies on mass, not volume.
Preferred for colligative property calculations (boiling-point elevation, freezing-point depression).
Percent by mass = (mass of solute / mass of solution) × 100
Useful when you are working with mass measurements rather than moles.
Nature of solute and solvent – "like dissolves like." Polar solutes dissolve in polar solvents; nonpolar solutes dissolve in nonpolar solvents.
Temperature – for most solid solutes, solubility in water increases as temperature rises. For gases, solubility in water decreases as temperature rises.
Pressure – primarily affects gases. Henry's Law: the solubility of a gas is directly proportional to the partial pressure of that gas above the solution.
Electrolyte solutions contain dissolved ions and conduct electricity.
Strong electrolytes dissociate completely (e.g., NaCl, HCl).
Weak electrolytes dissociate partially (e.g., acetic acid, NH₃).
Non-electrolytes do not produce ions at all (e.g., glucose, ethanol).
Colloidal solutions have dispersed particles larger than those in a true solution (roughly 1–1000 nm).
They scatter light (Tyndall effect), which is how you can distinguish them from true solutions.
Examples: milk, fog, smoke, gelatin.
M = \frac{\text{moles of solute}}{\text{litres of solution}}m = \frac{\text{moles of solute}}{\text{kilograms of solvent}}\% \text{ by mass} = \frac{\text{mass of solute}}{\text{mass of solution}} \times 100Note the distinction: molarity uses volume of the entire solution in the denominator, while molality uses mass of the solvent alone. This catches people out on exams.
Saline drips in hospitals are carefully prepared electrolyte solutions, their concentration measured in molarity so that the solution is isotonic with blood. Get the concentration wrong and red blood cells either swell and burst or shrink and crumple.
Alloys like steel and bronze are solid solutions engineered at precise compositions. The ratio of iron to carbon in steel determines whether you get a soft, ductile metal or a hard, brittle one, and that ratio is described using the same solute-in-solvent language covered here.
Students often confuse molarity and molality. Molarity divides by litres of solution (solute + solvent together); molality divides by kilograms of solvent alone. They are not interchangeable.
Students sometimes think "like dissolves like" means identical substances. It refers to polarity: polar dissolves polar, nonpolar dissolves nonpolar.
Colloidal solutions and true solutions are frequently mixed up. If you can see a beam of light passing through the mixture (Tyndall effect), it is a colloid, not a true solution.
Students often assume that increasing temperature always increases solubility. This holds for most solids in water, but gases become less soluble in water as temperature rises.
⚠️ Expect calculation questions that require converting between molarity, molality, and percent by mass. You need to be fluent in all three.
⚠️ "Like dissolves like" is a perennial short-answer and multiple-choice favourite. Be ready to explain why oil does not dissolve in water.
⚠️ Distinguishing electrolyte solutions from non-electrolyte solutions (and strong from weak electrolytes) is a common exam topic. Know the examples and the reasoning.
⚠️ Solubility curves (graphs of solubility vs temperature) appear frequently. Know how to read them and predict whether a solution is saturated, unsaturated, or supersaturated at a given temperature.
True or false: Molarity and molality always give the same numerical value for a given solution.
Fill in the blank: The maximum amount of solute that can dissolve in a solvent at a given temperature is called the ______ of that substance.
True or false: Air is an example of a gaseous solution.
Fill in the blank: A colloidal solution scatters light, a phenomenon known as the ______ effect.
True or false: Increasing the temperature of water always increases the solubility of gases dissolved in it.
Answers: 1. False (different denominators). 2. Solubility. 3. True. 4. Tyndall. 5. False (gas solubility in water decreases with rising temperature).
Q: Define molarity and explain how it differs from molality.
A: Molarity (M) is moles of solute per litre of solution. Molality (m) is moles of solute per kilogram of solvent. The key difference is the denominator: molarity uses the total volume of the solution, while molality uses only the mass of the solvent. Because volume changes with temperature but mass does not, molality is temperature-independent.
Q: A solution is prepared by dissolving 5.85 g of NaCl (molar mass 58.5 g/mol) in enough water to make 500 mL of solution. What is the molarity?
A: Moles of NaCl = 5.85 / 58.5 = 0.100 mol. Volume = 0.500 L. Molarity = 0.100 / 0.500 = 0.200 M.
Q: Explain why oil does not dissolve in water using the "like dissolves like" principle.
A: Oil is nonpolar and water is highly polar. Polar solvents dissolve polar solutes, and nonpolar solvents dissolve nonpolar solutes. Because oil and water sit at opposite ends of the polarity spectrum, they do not form a solution.
Q: What distinguishes a colloidal solution from a true solution?
A: In a true solution, solute particles are individual molecules or ions (less than 1 nm). In a colloid, dispersed particles are larger (roughly 1 to 1000 nm), large enough to scatter light (Tyndall effect) but small enough to remain suspended rather than settling out.
Q: How does temperature generally affect the solubility of a solid in water versus the solubility of a gas in water?
A: For most solids, solubility in water increases as temperature rises. For gases, solubility in water decreases as temperature rises, because higher kinetic energy allows gas molecules to escape the liquid phase more readily.
This material connects directly to colligative properties (boiling-point elevation, freezing-point depression, osmotic pressure), which all depend on the concentration of solute particles in solution. You will need molality especially for those calculations.
Electrolyte behaviour ties into acid-base chemistry and electrochemistry later in the course. Understanding strong vs weak electrolytes now will make dissociation equilibria and cell potentials far more intuitive.
The "like dissolves like" principle is rooted in intermolecular forces (dipole-dipole, hydrogen bonding, London dispersion), so revisiting that chapter alongside this one is worthwhile.
Solutions, solute, solvent, homogeneous mixture, molarity, molality, percent by mass, concentration, solubility, saturated solution, unsaturated solution, supersaturated solution, electrolyte, strong electrolyte, weak electrolyte, non-electrolyte, colloidal solution, colloid, Tyndall effect, like dissolves like, Henry's Law, dissolution, miscible, immiscible, aqueous solution, alloy, solid solution, gaseous solution, solution chemistry, general chemistry, CHEM 1061, CHEM 1065