Solutions: When Substances Combine
The Basic Components of a Solution
Every solution is formed by the combination of at least two components. Understanding these components is the first step to understanding solutions.
- Solute: This is the substance that gets dissolved. It is typically present in a smaller amount. For example, in a glass of lemonade, the sugar and lemon juice are the solutes.
- Solvent: This is the substance that does the dissolving. It is usually present in a larger amount. In the lemonade example, water is the solvent.
The most common solvent on Earth is water, and solutions where water is the solvent are called aqueous solutions. However, not all solutions involve water. Air is a solution of gases (oxygen, carbon dioxide, etc.) dissolved in nitrogen gas. Brass is a solid solution where zinc is dissolved in copper.
One simple way to express the concentration of a solution is by mass/volume percent. It tells you the mass of solute dissolved in a given volume of solution.
$ \text{Concentration (\% m/v)} = \frac{\text{Mass of Solute (g)}}{\text{Volume of Solution (mL)}} \times 100\% $
Example: If 10 g of salt is dissolved in enough water to make 200 mL of solution, the concentration is (10 g / 200 mL) * 100% = 5% m/v.
How Dissolving Happens: A Molecular View
Dissolving might seem like magic, but it's a physical process governed by the behavior of tiny particles. Imagine adding a teaspoon of table salt (sodium chloride, NaCl) to a glass of water. The water molecules, which have a positive and a negative end (they are polar), surround the individual sodium (Na^+) and chloride (Cl^-) ions that make up the salt crystal. The positive ends of water molecules are attracted to the chloride ions, and the negative ends are attracted to the sodium ions. This attraction pulls the ions away from the crystal and into the solution, where they move around freely. This process is called solvation; when water is the solvent, it's specifically called hydration.
Solubility: How Much is Too Much?
Not every substance is infinitely soluble. Solubility is the maximum amount of a solute that can dissolve in a given amount of solvent at a specific temperature. When you add sugar to iced tea and it all disappears, you have an unsaturated solution—more solute can still dissolve. If you keep adding sugar until no more will dissolve and some sugar sits at the bottom of the glass, you have a saturated solution. The solution is holding the maximum amount of solute possible. It is possible to create a supersaturated solution, which contains more dissolved solute than a saturated solution under the same conditions. These are unstable and can crystallize suddenly.
Several factors affect solubility:
- Temperature: For most solid solutes in liquid solvents, solubility increases as temperature increases. This is why it's easier to dissolve sugar in hot tea than in iced tea.
- Nature of Solute and Solvent: The rule of thumb is "like dissolves like." Polar solvents (like water) dissolve polar solutes (like salt and sugar) and ionic compounds. Nonpolar solvents (like oil) dissolve nonpolar solutes (like grease). This is why oil and water don't mix—their molecules are not alike.
- Pressure: Pressure has a significant effect on the solubility of gases in liquids. Higher pressure forces more gas molecules into the liquid. This is how carbon dioxide is dissolved in soda. When you open the bottle, pressure decreases, and the gas bubbles out.
| Solute Phase | Solvent Phase | Resulting Solution Phase | Example |
|---|---|---|---|
| Gas | Gas | Gas | Air (oxygen in nitrogen) |
| Gas | Liquid | Liquid | Carbonated water (CO$_2$ in water) |
| Liquid | Liquid | Liquid | Vinegar (acetic acid in water) |
| Solid | Liquid | Liquid | Seawater (salt in water) |
| Solid | Solid | Solid | Brass (zinc in copper) |
Expressing Concentration: Beyond Strong and Weak
In everyday language, we say a solution is "strong" or "weak," but in science, we need precise ways to describe how much solute is present. This is called concentration. We already saw the mass/volume percent. Other common ways include:
- Molarity (M): This is the number of moles of solute per liter of solution. It is one of the most important units in chemistry. $ \text{Molarity (M)} = \frac{\text{moles of solute}}{\text{liters of solution}} $
- Parts per million (ppm): Used for very dilute solutions. It is the ratio of parts of solute to one million parts of solution. For example, if a water sample has 1 ppm of lead, it means there is one gram of lead per million grams of water.
A dilute solution has a relatively small amount of dissolved solute. A concentrated solution has a large amount. These are relative terms, unlike the precise measurements of molarity or percent concentration.
Solutions in Action: From the Kitchen to the Clinic
Solutions are everywhere, and their properties are harnessed in countless ways.
Cooking and Food Preparation: Cooking is essentially applied chemistry with solutions. Making soup, brine for pickling, or simple syrup for cocktails all involve creating solutions. The dissolution of salts, sugars, and acids enhances flavor.
Medicine and Healthcare: Many medicines are administered as solutions. Saline solution (0.9% NaCl in water) is used for intravenous drips because it has the same solute concentration as blood plasma, making it isotonic1. Antiseptics like hydrogen peroxide and rubbing alcohol (isopropyl alcohol solution) are used to clean wounds.
Environmental Science: The health of aquatic ecosystems depends on the solubility of gases like oxygen in water. When water is polluted, harmful substances can dissolve and spread, affecting plants and animals. Scientists measure concentrations of pollutants in ppm or even parts per billion (ppb).
Industrial Applications: Solutions are vital in manufacturing. Electroplating uses solutions of metal salts to deposit a thin layer of metal onto an object. Cleaning products often use solvents to dissolve grease and grime.
Common Mistakes and Important Questions
No, this is a common misconception. While liquid solutions are the most familiar, solutions can exist in any state of matter. Air is a gaseous solution, and alloys like brass and steel are solid solutions. The key defining feature is that the mixture is homogeneous, not its physical state.
A solution is homogeneous and stable; the solute particles are molecular or ionic in size and do not settle out. A suspension is a heterogeneous mixture where the particles are larger (like sand in water). These particles will settle over time and can be separated by filtration. If you shine a light through a solution, it will not scatter the light (mostly true). In a suspension, the light beam will be visible because the large particles scatter the light (the Tyndall effect2).
This goes back to the principle of "like dissolves like." Water molecules are polar, meaning they have a slight positive charge on one end and a slight negative charge on the other. Oil molecules are nonpolar; their charges are evenly distributed. The strong attraction between water molecules "squeezes out" the nonpolar oil molecules, which are more attracted to each other. Therefore, they separate into distinct phases.
Solutions are fundamental to both the natural world and human technology. From the air we breathe to the blood flowing in our veins, we are surrounded by and depend on these homogeneous mixtures. Understanding the simple concepts of solute, solvent, solubility, and concentration provides a powerful lens through which to view everyday phenomena and complex scientific processes alike. By grasping how substances combine on a molecular level, we can better appreciate the chemistry of life itself and innovate in fields ranging from medicine to environmental protection.
Footnote
1 Isotonic: A solution that has the same osmotic pressure as another solution, typically a bodily fluid like blood. When two isotonic solutions are separated by a membrane, there is no net movement of water across the membrane.
2 Tyndall Effect: The scattering of a beam of light by particles in a colloid or a fine suspension. It is what makes a light beam visible when it passes through fog or dusty air.