Solutions and Concentration
Key Takeaways
- A solution is a homogeneous mixture of a solute dissolved in a solvent; in aqueous solutions water is the solvent and 'like dissolves like' (polar dissolves polar, ionic dissolves in water).
- The Solubility Curves table (legacy Table G) plots solubility (g solute per 100 g water) vs. temperature; points on a curve are saturated, below are unsaturated, and above are supersaturated.
- Most solids become more soluble as temperature rises, but gases become LESS soluble as temperature rises and MORE soluble as pressure increases.
- Molarity (M) from the Mathematical and Computational Models section (legacy Table T) equals moles of solute divided by liters of solution; parts per million equals grams of solute divided by grams of solution times 1,000,000.
- Dissolving rate increases with stirring, heating, and smaller particle size (more surface area); these change how FAST it dissolves, not how MUCH can dissolve.
What a Solution Is
Solutions are tested in the Structures and Properties of Matter core idea and connect to one of the required chemistry Investigations. A solution is a homogeneous mixture: it is uniform throughout and the particles will not settle out. It has two parts:
- Solute: the substance being dissolved (present in the smaller amount), such as salt or sugar.
- Solvent: the substance doing the dissolving (present in the larger amount). When the solvent is water, the mixture is an aqueous solution, written (aq).
The guiding rule is "like dissolves like." Polar solvents like water dissolve polar molecules and ionic compounds; nonpolar solvents dissolve nonpolar substances. That is why ionic NaCl dissolves in water but nonpolar oil does not.
Electrolytes
When an ionic compound dissolves, it dissociates into mobile ions, so the solution conducts electricity. A substance that produces ions in solution is an electrolyte. Molecular sugar dissolves without forming ions, so a sugar solution does not conduct. A frequent cluster item shows a salt solution lighting a bulb and asks for the particle-level reason: free-moving ions carry charge. The full explanation Regents wants is that the ionic solid separates into positive and negative ions that are free to move through the water and carry electric current.
Properties of solutions
Dissolving a solute also changes the physical properties of the solvent. Adding a solute raises the boiling point and lowers the freezing point of water, which is why salt is spread on icy roads (it lowers the freezing point so ice melts at a colder temperature) and why salted water for cooking boils slightly above 100 degrees Celsius. The more dissolved particles present, the larger the effect. These are physical changes; evaporating the water recovers the original solute unchanged.
Saturation and the Solubility Curves table (legacy Table G)
The Solubility Curves table (titled "Solubility Curves at Standard Pressure," the section the legacy edition labeled Table G) plots solubility in grams of solute per 100 grams of water (y-axis) against temperature (x-axis). Reading it lets you classify any solution:
| Solution type | Position vs. the curve | Meaning |
|---|---|---|
| Unsaturated | below the curve | more solute can still dissolve |
| Saturated | exactly on the curve | holds the maximum at that temperature |
| Supersaturated | above the curve | holds more than the maximum; unstable |
A saturated solution is at equilibrium: dissolved and undissolved solute interchange at equal rates. A supersaturated solution is made by saturating at a high temperature and cooling slowly; it will crash out excess solute if disturbed.
Worked Solubility Curves (legacy Table G) example
The Solubility Curves table (legacy Table G) shows 36 g of NaCl dissolves in 100 g of water at 20 degrees Celsius. A beaker holds 30 g of NaCl in 100 g of water at 20 degrees Celsius. Classify it.
Thirty grams is less than the 36 g maximum, so the point lies below the curve and the solution is unsaturated (about 6 g more NaCl could still dissolve).
How Temperature and Pressure Affect Solubility
This is a high-frequency distinction, and solids and gases behave oppositely:
- Most solids: solubility increases as temperature increases (curves on the Solubility Curves table, legacy Table G, slope up).
- Gases in liquid: solubility decreases as temperature increases. Warm soda goes flat because dissolved CO2 escapes.
- Gases in liquid: solubility increases as pressure increases. A sealed soda bottle holds more CO2; opening it drops the pressure and bubbles form.
Pressure changes barely affect the solubility of solids and liquids.
A quick way to remember the gas rules: a warm, low-pressure environment is bad for keeping a gas dissolved. Open soda (pressure drops) and warm soda (temperature rises) both lose their fizz, and these are the exact real-world scenarios Regents uses to test the concept. The Solubility Curves table (legacy Table G) itself only covers solids and one or two gases dissolved in water, and every curve on it is read at standard pressure, so any pressure-effect question is about gases and must be reasoned, not read off the graph.
Concentration: Molarity and Parts Per Million
The Mathematical and Computational Models section (legacy Table T) supplies two concentration formulas the exam expects you to apply.
Molarity (M) is the most common: moles of solute per liter of solution.
M = moles of solute / liters of solution
Example: 2.0 mol NaCl dissolved to make 1.0 L of solution gives 2.0 mol / 1.0 L = 2.0 M. To find moles in a sample, rearrange: moles = M x liters. So 250. mL (0.250 L) of 0.200 M NaCl contains 0.200 x 0.250 = 0.0500 mol.
Parts per million (ppm) measures very dilute solutions:
ppm = (grams of solute / grams of solution) x 1,000,000
Dilutions use M1V1 = M2V2 (concentration times volume is conserved when you add solvent). To make 500. mL of 0.50 M HCl from 2.0 M stock: V1 = (0.50 x 500.) / 2.0 = 125 mL of stock diluted up to 500. mL.
Dissolving Rate vs. Amount Dissolved (Common Trap)
The single most confused pair on this topic is how fast a solute dissolves versus how much can dissolve. Three factors speed up dissolving without changing the maximum solubility:
- Stirring (agitation) brings fresh solvent to the solute.
- Heating gives particles more energy to collide and separate.
- Smaller particle size / crushing increases surface area for contact.
These change the rate. Only changing the temperature (per the Solubility Curves table, legacy Table G) changes the actual maximum amount that dissolves. Confusing "dissolves faster" with "dissolves more" is a guaranteed wrong answer.
A solution holds less dissolved solute than the maximum amount shown on the Solubility Curves table (legacy Table G) curve at that temperature. The solution is best described as
Which change usually increases the rate at which a solid solute dissolves in water WITHOUT changing the maximum amount that can dissolve?
A 250. mL sample of 0.200 M NaCl solution contains how many moles of NaCl?