Solutions all around you
The saline in a hospital IV drip is a 0.9% sodium chloride solution. Rubbing alcohol, the iodine tincture on a scrape, sweet tea, and soda are all solutions too. Many of the reactions you do in the lab happen in solution as well.
Solutions
A solution溶液 is a uniform and stable mixture formed when one or more substances spread evenly through another.
- The substance that gets dissolved is the solute溶质. It can be a solid, a liquid, or a gas.
- The substance that does the dissolving is the solvent溶剂. Water is the most common solvent, but gasoline and alcohol can be solvents too. In iodine tincture, iodine is the solute and alcohol is the solvent.
Uniform means every part of the solution has the same composition and properties: a glass of sugar water is just as sweet at the top as at the bottom. Stable means that as long as conditions don’t change, the solute doesn’t separate out; a solution doesn’t settle into layers over time.
Mass of solution = mass of solute + mass of solvent. Volumes, however, usually don’t simply add up (remember from lesson 1.2.1 that 50 mL of water and 50 mL of alcohol make less than 100 mL).
Shake cooking oil with water and tiny oil droplets spread through the water, but the mixture isn’t uniform or stable: left alone, it separates into layers. That’s an emulsion乳浊液. Dish soap breaks oil into countless tiny droplets that rinse away with the water; this is called emulsifying乳化. Gasoline removes grease by dissolving it, while soap removes grease by emulsifying it, two different processes.
Temperature changes when dissolving
Dissolving often absorbs or releases heat:
| Substance | Temperature of the solution |
|---|---|
| Sodium hydroxide, | Rises noticeably (heat released) |
| Ammonium nitrate, | Drops noticeably (heat absorbed) |
| Sodium chloride, | Stays about the same |
Instant cold packs for sports injuries work this way: squeezing the pack breaks an inner pouch of water, ammonium nitrate dissolves, and the pack gets cold.
Saturated and unsaturated solutions
At a given temperature, in a given amount of solvent, a solution that can’t dissolve any more of a particular solute is a saturated solution饱和溶液 of that solute. A solution that can still dissolve more is an unsaturated solution不饱和溶液.
So whenever you call a solution saturated, you need to know the temperature, the amount of solvent, and which solute you mean. A glass of saturated salt water can still dissolve sugar.
Add solute to the beaker below, then change the temperature and the amount of water. When does the solution become saturated, and when does it become unsaturated again?
For most solids (potassium nitrate, for example), you can switch between the two like this:
- Unsaturated → saturated: add more solute, evaporate some solvent, or cool it down.
- Saturated → unsaturated: add more solvent or warm it up.
A few solids, such as calcium hydroxide, become less soluble as the temperature rises, so the temperature changes work the other way around for them.
When solute comes out of a solution as crystals, that’s crystallization结晶. Sea salt is made by evaporating the water; potassium nitrate crystals are usually obtained by cooling a hot saturated solution (lesson 1.7.5).
Example 1: Is it saturated?
At 20 °C, 100 g of water can dissolve at most 31.6 g of potassium nitrate. At 20 °C, 40 g of potassium nitrate is stirred into 100 g of water. Is the solution saturated? What is its mass?
- Compare. 40 g is more than the 31.6 g that 100 g of water can dissolve.
- Decide. Only 31.6 g dissolves, leaving 40 − 31.6 = 8.4 g undissolved, so the solution is saturated.
- Find the mass. The undissolved solid isn’t part of the solution: mass of solution = 100 g + 31.6 g = 131.6 g, not 140 g.
Example 2: Making an unsaturated solution saturated
You have a nearly saturated potassium nitrate solution. Without changing its mass, how can you make it saturated? What about a nearly saturated solution of calcium hydroxide (limewater)?
- Options. Adding solute, evaporating solvent, or changing the temperature can all saturate a solution, but the first two change its mass.
- Potassium nitrate. It becomes less soluble as the temperature drops, so cool it.
- Calcium hydroxide. It becomes less soluble as the temperature rises, so warm it.
- Saline (0.9% salt water)
Sodium chloride is dissolved in water, forming a uniform, stable mixture.
- Muddy water
The mud doesn't dissolve and settles out, so it isn't uniform or stable.
- Iodine tincture
Iodine is the solute and alcohol is the solvent.
- Cooking oil shaken with water
That's an emulsion; it separates into layers.
- Distilled water
Distilled water is a pure substance, and a solution must be a mixture.
- Copper(II) sulfate solution
Solutions don't have to be colorless; this one is blue.
- White vinegar
Vinegar is a solution of acetic acid in water.
- Ice cubes in water
Ice and water are the same substance, so this is a pure substance.
Common mistakes
Mistake 1: “Solutions are always colorless.” Solutions are always clear (transparent), but not necessarily colorless: copper(II) sulfate solution is blue, and iron(II) sulfate solution is pale green.
Mistake 2: “Any uniform, stable liquid is a solution.” Water and pure alcohol are uniform and stable too, but they’re pure substances. A solution has to be a mixture.
Mistake 3: “Saturated means concentrated, and unsaturated means dilute.” These are separate ideas. Calcium hydroxide barely dissolves, so even its saturated solution is very dilute, while a nearly saturated potassium nitrate solution can be very concentrated.
Quick check
Which substance makes the water noticeably colder when it dissolves?
Ammonium nitrate absorbs heat as it dissolves. Sodium hydroxide releases heat, and sodium chloride and sugar barely change the temperature.
Which of these will NOT make a nearly saturated potassium nitrate solution saturated?
Potassium nitrate becomes more soluble as the temperature rises, so heating makes the solution even less saturated.
Summary
- A solution is a uniform, stable mixture of a solute in a solvent. Mass of solution = mass of solute + mass of solvent.
- Emulsions aren’t uniform or stable. Soap removes grease by emulsifying it; gasoline does it by dissolving it.
- Sodium hydroxide releases heat as it dissolves, ammonium nitrate absorbs heat, and sodium chloride barely changes the temperature.
- “Saturated” depends on temperature, the amount of solvent, and the solute. Adding solute, evaporating solvent, or (for most solids) cooling makes a solution saturated; adding solvent or warming makes it unsaturated.
On to AP (for later)
AP topic 3.7 describes solution composition with molarity (mol/L). Topic 3.8 asks you to draw particle diagrams of solutions, such as dissolved and ions each surrounded by water molecules, and topic 3.10 explains which substances mix using “like dissolves like.” Why ammonium nitrate dissolves on its own even though it absorbs heat is explained in topic 7.14 with entropy and free energy.