How solutions work: what dissolves, how much, and why cells swell or shrink
TEAS 7 outline: Properties of solutions
Also covers HESI A2 Chemistry (Solutions)
- A solution is a homogeneous mixture: a solute dissolved in a solvent.
- Like dissolves like: water dissolves polar and ionic solutes.
- Heat usually helps solids dissolve but drives gases out.
- In osmosis, water moves toward the side with more solute.
- Cells swell in hypotonic, shrink in hypertonic, and keep size in isotonic.
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A semipermeable membrane lets water molecules cross in both directions but blocks the dissolved solute particles.
Key facts
- A solution is a homogeneous mixture; the solvent is the component present in the greater amount and the solute is the component present in the smaller amount.
- Concentration is the relative amount of solute; molarity is moles of solute per liter of solution, and physiological saline is 0.9% m/v (0.9 g per 100 mL).
- Unsaturated solutions hold less solute than the solubility limit, saturated solutions hold exactly that much, and supersaturated solutions hold more and are not at equilibrium, so the extra can suddenly crystallize.
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- Raising temperature generally increases the solubility of solids but decreases the solubility of gases; raising gas pressure increases gas solubility.
- "Like dissolves like": polar solvents such as water dissolve polar and ionic solutes, while nonpolar substances such as oil do not mix with water.
- In osmosis, water crosses a semipermeable membrane toward the side with the higher solute concentration.
- A red blood cell swells and may burst in a hypotonic solution, shrinks (crenates) in a hypertonic solution, and keeps its size in an isotonic solution, such as the 0.9% physiological saline used to prepare IV fluids.
Quick check1 question · 2 more at the end
A clear sodium acetate solution contains more dissolved solute than its solubility at room temperature. When a student drops in one small sodium acetate crystal, solid quickly forms throughout the liquid. Which term describes the solution before the crystal was added?
Correct answer: C.
Solubility is the maximum concentration of solute that can dissolve under given conditions. A saturated solution is exactly at that limit, an unsaturated one is below it, and a supersaturated one holds more than the limit. A supersaturated solution is not at equilibrium, so a seed crystal or shaking makes the extra solute crystallize out quickly.
A A saturated solution holds exactly as much solute as its solubility allows, so it does not contain extra solute ready to crystallize.
B An unsaturated solution is below its solubility limit, so an added crystal would tend to dissolve rather than trigger crystallization.
C Correct: a solution holding more solute than its solubility is supersaturated, and a seed crystal makes the excess crystallize.
D Dilute only means a relatively low concentration; a solution above its solubility limit is not dilute.
Full lesson
Each part starts with its main idea. Open a part when you want the details.
The short answer: A solution is a homogeneous mixture: a solute dissolved evenly in a solvent, the component present in the larger amount. Water, a polar solvent, dissolves polar and ionic solutes ("like dissolves like"). Solubility is the most solute that can dissolve; heat usually helps solids dissolve but drives gases out. In osmosis, water moves toward more solute.
1 What exactly is a solution?A solution is a homogeneous mixture of two or more substances. 1 min
A solution is a homogeneous mixture of two or more substances. Homogeneous means it has the same makeup at every point. A spoonful from the top of a well-stirred glass of sugar water is just as sweet as one from the bottom.
Every solution has two roles. The solvent is the component present in the greater amount, and the solute is the component present in the smaller amount. When water is the solvent, the solution is called aqueous, written (aq).
Solutions are not only liquids. Air is a gas solution (oxygen dissolved in nitrogen), and brass is a solid solution (zinc dissolved in copper). Rubbing alcohol is a liquid dissolved in a liquid.
2 How do you describe how strong a solution is?Concentration is the relative amount of solute in a solution. 1 min
Concentration is the relative amount of solute in a solution. In everyday words, a dilute solution has relatively little solute and a concentrated solution has a lot.
Chemists put numbers on it in a few ways:
- Molarity (M) = moles of solute ÷ liters of solution.
- Mass-volume percent (m/v) = grams of solute per 100 mL of solution. Physiological saline is 0.9% m/v, which means 0.9 g of solute in every 100 mL.
- Volume percent (v/v): rubbing alcohol is commonly sold as a 70% by volume solution.
Dilution means adding more solvent. The concentration drops, but the amount of solute does not change; it is just spread through a bigger volume.
3 What decides whether something dissolves?" Polar solutes dissolve in polar solvents, and nonpolar solutes dissolve in nonpolar solvents. 1 min
The rule of thumb is "like dissolves like." Polar solutes dissolve in polar solvents, and nonpolar solutes dissolve in nonpolar solvents.
Water is polar: its oxygen end carries a slight negative charge and its hydrogen ends carry slight positive charges. Those partial charges attract ions and other polar molecules.
When table salt dissolves, water molecules surround each ion in a hydration shell, with oxygen ends facing the sodium ions and hydrogen ends facing the chloride ions. Substances that mix well with water are called hydrophilic; nonpolar substances that do not are hydrophobic.
Two liquids that mix in any proportion, like ethanol and water, are miscible. Liquids that stay in separate layers, like oil and water, are immiscible.
4 What do saturated, unsaturated and supersaturated mean?Solubility is the maximum concentration of a solute that can dissolve in a solvent under given conditions. 1 min
Solubility is the maximum concentration of a solute that can dissolve in a solvent under given conditions. Compare any solution to that limit:
| Type | Amount of dissolved solute | What happens if you add more solute |
|---|---|---|
| Unsaturated | Below the solubility limit | It dissolves |
| Saturated | Exactly at the limit | The extra stays undissolved |
| Supersaturated | Above the limit (not at equilibrium) | Extra solute crystallizes out |
A supersaturated solution is usually made by dissolving a lot of solute in a hot solvent and then cooling it carefully. It can sit unchanged for a while, but it is not at equilibrium: adding a tiny seed crystal or shaking it can make the excess solute crystallize suddenly. Reusable sodium acetate hand warmers work this way.
5 What changes how much can dissolve?For most solids, solubility goes up as temperature rises (there are a few exceptions). 1 min
Temperature. For most solids, solubility goes up as temperature rises (there are a few exceptions). For gases it is the opposite: warmer liquid holds less dissolved gas. That is why warming a river lowers its dissolved oxygen, which can harm fish.
Pressure matters for gases. The more pressure a gas puts on a liquid, the more of that gas dissolves (Henry's law). Soda is bottled under high carbon dioxide pressure. When you open it, the pressure drops and the extra gas escapes as bubbles.
Stirring is different: it makes a solid dissolve faster, but it does not raise the solubility limit.
6 How do solutions explain cells swelling or shrinking?Osmosis is the movement of water across a semipermeable membrane, a barrier that lets water through but not the solute. 1 min
Osmosis is the movement of water across a semipermeable membrane, a barrier that lets water through but not the solute. Water moves toward the side with the higher solute concentration (the side with less free water).
Tonicity describes how a solution outside a cell changes the cell's volume:
- Hypotonic outside (less solute than the cell): water moves in, and a red blood cell swells and may burst (lyse).
- Hypertonic outside (more solute than the cell): water moves out, and the cell shrinks (crenates).
- Isotonic outside (same solute concentration): no net water movement, so the cell keeps its size.
This is why many IV fluids are made isotonic with blood. Physiological saline (0.9% m/v), used to prepare IV fluids, is the classic example.
Common mistakes
The traps students fall into most. Read these before you practice.
Watch out: Water moves toward the higher solute concentration in osmosis, not away from it; the solute itself does not cross the membrane.
Watch out: Hypertonic describes the solution outside the cell having more solute than the cell, so the cell shrinks, not swells.
Watch out: Hot water holds less dissolved gas, not more; warming helps solids dissolve but drives gases out.
Watch out: Diluting a solution adds solvent and lowers concentration, but the amount of solute stays the same.
Watch out: A supersaturated solution holds more solute than its solubility, not just a lot of solute; a single seed crystal makes the extra solute crystallize out.
How to remember it
Memory hook: "Water follows solute." Wherever there is more dissolved stuff, water heads that way. And "hyper = higher outside, cell shrivels"; hypo sounds like "hippo," the cell swells up big.
Self-check:
- In salt water, name the solute and the solvent, and say what kind of solution forms if you add salt until no more dissolves.
- Explain why a warm soda can hold less carbon dioxide than a cold one.
- Predict what happens to a red blood cell dropped into pure water, and why.