Composition of Solutions
Solutions are homogeneous mixtures of two or more substances. "Homogeneous" means the mixture looks uniform throughout; you can't see where one substance ends and another begins. Understanding how solutions form and what affects them is foundational for the rest of this unit on acids, bases, and chemical interactions in everyday life.
Components of a Solution
Every solution has two main parts:
- Solute: the substance that gets dissolved. It's typically present in smaller quantity.
- Solvent: the substance doing the dissolving, usually present in larger quantity. In most chemistry problems, the solvent is water, which makes it an aqueous solution.
Solubility is the maximum amount of solute that can dissolve in a given amount of solvent at a specific temperature. Think of it as a capacity limit. For example, you can dissolve about 36 g of table salt () in 100 mL of water at room temperature. After that, no more will dissolve.
Factors Affecting Solubility
Four main factors determine how well a solute dissolves:
- Temperature: For most solid solutes, raising the temperature increases solubility (hot water dissolves more sugar than cold water). For gases, it's the opposite: gases dissolve better in colder liquids. That's why a warm soda goes flat faster than a cold one.
- Pressure: This mainly matters for gases. Higher pressure forces more gas into solution. This relationship is described by Henry's Law, and it's why a sealed soda bottle holds its carbonation until you open it and release the pressure. For solids and liquids, pressure changes have almost no effect on solubility.
- Nature of solute and solvent: The general rule is "like dissolves like." Polar solutes (like salt) dissolve well in polar solvents (like water). Nonpolar solutes (like grease) dissolve well in nonpolar solvents (like paint thinner or hexane). This comes down to intermolecular forces: molecules with similar types of attractions interact well with each other.
- Particle size: Smaller particles dissolve faster because they have more surface area exposed to the solvent. This affects the rate of dissolving, not how much can ultimately dissolve. Crushing a sugar cube speeds things up, but the same total amount of sugar still dissolves either way.
Types of Solutions
Saturation Levels
Solutions are classified by how much solute they contain relative to the solubility limit:
- Unsaturated: The solution can still dissolve more solute. If you add more solute, it dissolves completely.
- Saturated: The solution holds the maximum amount of dissolved solute at that temperature and pressure. If you add more solute, it won't dissolve. You'll often see undissolved crystals sitting at the bottom.
- Supersaturated: The solution actually contains more dissolved solute than a saturated solution normally holds. This can happen when you dissolve solute in hot solvent and then carefully cool it without disturbing it. Supersaturated solutions are unstable: a small disturbance, like dropping in a single seed crystal, can trigger rapid crystallization as the excess solute crashes out of solution.
In a saturated solution, a dynamic equilibrium exists. Solute particles are constantly dissolving and coming back out of solution at equal rates, so the overall concentration stays the same even though activity is happening at the molecular level.
Expressing Concentration
The concentration of a solution describes how much solute is dissolved in a given amount of solution or solvent. Here are the common ways to express it:
- Molarity (M): moles of solute per liter of solution. If you dissolve 1 mole of in enough water to make 1 L of solution, that's a 1 M solution.
- Molality (m): moles of solute per kilogram of solvent. Notice the difference: molarity uses volume of solution, while molality uses mass of solvent.
- Mass percent: mass of solute divided by total mass of solution, multiplied by 100. A solution with 5 g of salt in 100 g of total solution has a mass percent of 5%.
You'll use these units frequently when working with acids, bases, and reactions later in this unit.
Solution Properties
The Dissolution Process
Dissolving isn't just "disappearing." Here's what actually happens at the molecular level:
- Solute particles separate from each other. This requires energy to overcome the attractions holding them together.
- Solvent molecules spread apart slightly to make room for the solute particles. This also requires energy.
- Solvent molecules surround and interact with the solute particles. This step releases energy and is called solvation, or hydration when the solvent is specifically water.
The overall energy change depends on the balance of these three steps. If the energy released during solvation (step 3) is greater than the energy needed in steps 1 and 2, the process is exothermic and the solution warms up. If more energy is required than released, it's endothermic and the solution cools down. A cold pack is a common example of endothermic dissolving: ammonium nitrate () dissolving in water absorbs heat from its surroundings.
Miscibility and Immiscibility
When both the solute and solvent are liquids, we use different terms:
- Miscible: The two liquids mix completely in any proportion. Ethanol and water are miscible; they blend uniformly no matter the ratio.
- Immiscible: The two liquids don't mix and form visible separate layers. Oil and water are the classic example. This happens because oil is nonpolar and water is polar, so the intermolecular forces between them are too weak to keep them mixed.
- Partially miscible: The liquids mix to a limited extent, then separate. Ether and water behave this way.
Miscibility comes back to the same principle: "like dissolves like." The strength of intermolecular forces between the two liquids determines whether they'll mix.