Dissolving: When Substances Disappear into Liquid
The Basic Science: Solute, Solvent, and Solution
At its heart, dissolving is all about particles. Imagine a grain of salt, which is made of sodium (Na+) and chloride (Cl–) ions held together by strong forces. When you drop this grain into water, the water molecules, which have a slightly positive and a slightly negative end, swarm around the ions. The positive ends of the water molecules attract the negative chloride ions, and the negative ends attract the positive sodium ions. This attraction is strong enough to pull the ions away from the crystal surface and into the water. Once separated, the ions are surrounded by water molecules and spread evenly throughout the liquid. The salt grain seems to "disappear," but its particles are still there, just too small to see.
This process creates a solution. The substance that gets dissolved (like salt) is the solute. The substance that does the dissolving (like water) is the solvent. Water is such a good solvent that it is often called the "universal solvent." However, not every substance dissolves in every solvent. Oil, for example, does not dissolve in water because its molecules are not attracted to water molecules; they are more attracted to each other. This is why oil and water separate.
Factors That Affect How Fast Something Dissolves
The speed of dissolving isn't always the same. If you need to make iced tea quickly, you know that using hot water and stirring helps the sugar dissolve faster. This is because several factors control the rate of dissolution.
1. Temperature: Heating a solvent gives its molecules more energy. They move faster and collide with the solid solute more frequently and with greater force. This helps to break apart the solute particles more quickly. For most solid solutes, increasing the temperature increases the rate of dissolving. For gases, the opposite is true; colder water can hold more dissolved gas (like the carbon dioxide in soda) than warm water.
2. Agitation (Stirring or Shaking): Stirring does two important things. First, it brings fresh solvent into contact with the surface of the solute, allowing the dissolution process to continue. Second, it helps to distribute the dissolved solute particles away from the solid, preventing a buildup of solute around it that could slow down further dissolving.
3. Surface Area: If you break a sugar cube into granules, you increase its surface area. More surface area means more solute particles are exposed to the solvent at once. A powdered solute will dissolve much faster than a large chunk of the same substance because the solvent can attack it from all sides simultaneously.
| Factor | Effect on Dissolving Rate | Everyday Example |
|---|---|---|
| Increased Temperature | Increases rate for most solids; decreases rate for gases. | Making hot chocolate vs. iced tea. Sugar dissolves faster in the hot water. |
| Agitation (Stirring) | Increases rate by bringing fresh solvent to the solute. | Stirring a spoonful of juice concentrate into water. |
| Increased Surface Area | Increases rate by exposing more solute to the solvent. | Using granulated sugar instead of a sugar cube. |
Solubility: How Much Can Dissolve?
There is a limit to how much solute a given amount of solvent can hold. This limit is called solubility. When a solution contains the maximum amount of dissolved solute at a particular temperature, it is called a saturated solution. If you add more solute to a saturated solution, it will just sink to the bottom and remain undissolved. An unsaturated solution has less than the maximum amount of solute dissolved. Under special conditions, you can even create a supersaturated solution, which holds more solute than it should under normal circumstances. This is an unstable state; adding a tiny crystal (a "seed") will cause all the excess solute to quickly crystallize out.
Solubility is often measured in grams of solute per 100 g of water. For example, the solubility of table salt (sodium chloride) in water at 20 $^\\circ$C is about 36 g per 100 g of water. This means you can dissolve 36 g of salt in 100 g of water at that temperature before the solution becomes saturated.
Dissolving in Action: From the Kitchen to the Human Body
Dissolving is not just a laboratory concept; it is essential to countless processes in our daily lives.
Cooking and Food Preparation: When you make soup, dissolve broth cubes in hot water. When you bake a cake, you dissolve sugar and salt in water, milk, or eggs. The process of dissolving is what allows flavors to spread evenly throughout food and drink.
Medicine: Many medicines are designed to be dissolved. Effervescent tablets fizz and dissolve in water to create a drinkable solution. Capsules dissolve in your stomach to release the medicine inside. The rate of dissolution is critical for a drug to work effectively and safely.
Biology and the Environment: Your body depends on dissolving. Digestion begins when enzymes in your saliva start to dissolve starches in food. Nutrients from your food dissolve in your blood to be carried to your cells. In nature, gases like oxygen dissolve in water, allowing fish and other aquatic animals to breathe. Carbon dioxide dissolving in ocean water is a key part of the carbon cycle, though it also leads to ocean acidification.
Common Mistakes and Important Questions
A: No, this is a common confusion. Melting is a change of state from a solid to a liquid caused by heating. The substance itself changes (e.g., ice melts into water). Dissolving involves one substance (the solute) mixing into another substance (the solvent) to form a solution. You can dissolve salt in water without melting the salt first.
A: Generally, dissolving is considered a physical change, not a chemical change. This is because no new chemical substances are formed. The salt ions (Na+ and Cl–) are still present. You can recover the salt by evaporating the water, which is evidence of a physical change.
A: Heat provides energy. This energy makes water molecules move faster and vibrate more. When these faster-moving water molecules collide with the solute, they can pull the solute particles apart more effectively. Think of it like using a more powerful tool to break something down.
Footnote
1 Solute: The substance that is dissolved in a solution.
2 Solvent: The substance, usually a liquid, that dissolves the solute to form a solution.
3 Solution: A homogeneous mixture composed of a solute dissolved in a solvent.
4 Solubility: The maximum amount of a solute that can dissolve in a given amount of solvent at a specific temperature.
5 Saturated Solution: A solution that contains the maximum amount of dissolved solute possible under given conditions.