In chemistry, a solute is a substance that dissolves in a solvent to form a solution. For example, when salt dissolves in water, salt is the solute and water is the solvent. A solution can contain one solute or several solutes.
The solute is often the component present in the smaller amount, while the solvent is present in the larger amount. However, amount alone does not always define which substance is the solute or solvent. The solvent is generally the component that determines the physical state of the solution or serves as the dissolving medium.
The process in which solute particles become surrounded by solvent particles is called solvation. When water is the solvent, the process is specifically called hydration. Concentration describes the amount of solute present in a given amount of solution or solvent. For example, a 0.1 M HCl solution contains 0.1 mole of HCl per liter of solution.
The word “solute” comes from the Latin solvere, meaning “to loosen” or “dissolve.” The words “solvent” and “dissolve” have related origins.
Although solutes are most familiar in liquid solutions, solutions can also be gases or solids. So, a solute does not have to be a solid, and a solvent does not have to be a liquid.
Key Takeaways: Solute Definition
- A solute is a substance that dissolves in a solvent to form a solution.
- The solute is often, but not always, the component present in the smaller amount.
- Solutes may be solids, liquids, or gases, and solutions may occur in any phase.
- A solution can contain more than one solute.
- Salt in salt water, sugar in sugar water, and carbon dioxide in soda are familiar examples of solutes.
- Solubility describes how much solute dissolves under specified conditions.
Solute Properties
A solute in a true solution has several characteristic features:
- Solute particles are distributed uniformly throughout a homogeneous solution.
- Individual solute particles are atoms, molecules, or ions and are too small to see with the unaided eye.
- Dissolved solute particles do not settle out when the solution stands.
- Ordinary filtration does not separate a dissolved solute from its solvent.
- A true solution does not show the Tyndall effect because its dissolved particles are too small to scatter visible light appreciably.
- A solution may contain one solute or multiple solutes.
- The solute takes part in determining properties of the solution, including concentration, density, boiling point, freezing point, and electrical conductivity.
How to Identify the Solute and Solvent
In many solutions, identifying the solute and solvent is straightforward:
- Identify the substance doing the dissolving. This is the solvent.
- Identify the substance dissolved in it. This is the solute.
- If both components are in the same phase, the component present in the greater amount is usually considered the solvent.
For example, in salt water, water is the solvent and salt is the solute. In a sugar solution, water is the solvent and sugar is the solute.
The “smaller amount” rule is useful, but it is not absolute. The identities of the solute and solvent may also depend on how the solution is prepared and which component is treated as the dissolving medium.
In chemical equations, the state symbol (aq) means “aqueous” and indicates that a chemical species is dissolved in water. For example, Na⁺(aq) represents sodium ions dissolved in water.
Solute vs Solvent
| Property | Solute | Solvent |
|---|---|---|
| Definition | Substance dissolved in a solution | Substance that dissolves the solute |
| Relative amount | Often the smaller amount | Often the larger amount |
| Physical state | Solid, liquid, or gas | Solid, liquid, or gas |
| Role | Substance being dissolved | Dissolving medium |
| Salt water example | Salt | Water |
| Sugar water example | Sugar | Water |
| Air example | Oxygen and other minor gases | Nitrogen |
Solute Examples
Solutes can be solids, liquids, or gases. The solvent also does not have to be a liquid.
| Solute | Solution | Solvent | Solution Type |
|---|---|---|---|
| Sodium chloride | Salt water | Water | Solid in liquid |
| Sugar | Sugar water | Water | Solid in liquid |
| Carbon dioxide | Carbonated water | Water | Gas in liquid |
| Ethanol | Alcohol-water solution | Water* | Liquid in liquid |
| Acetic acid | Vinegar | Water | Liquid in liquid |
| Oxygen, argon, carbon dioxide, and other gases | Air | Nitrogen | Gas in gas |
| Zinc | Brass | Copper | Solid in solid |
| Sodium hydroxide | NaOH(aq) | Water | Solid in liquid |
*In a mixture containing more water than ethanol, water is conventionally treated as the solvent.
A substance can be a solute in one solution and a solvent in another. Its role depends on the composition and context of the particular solution.
Predicting Whether a Solute Will Dissolve
Whether a solute dissolves depends on the interactions between the solute and solvent. A useful introductory rule is “like dissolves like.” Polar solvents tend to dissolve ionic and polar substances, while nonpolar solvents tend to dissolve nonpolar substances.
For example, ionic sodium chloride dissolves readily in polar water but does not dissolve well in nonpolar oil. Nonpolar substances such as many hydrocarbons dissolve more readily in nonpolar solvents.
However, “like dissolves like” is a guideline rather than a complete rule. Solubility depends on intermolecular forces, temperature, pressure, and the identities of both the solute and solvent. Pressure is particularly important for gases. Temperature often strongly affects the solubility of solids and gases, although the direction and magnitude of the effect depend on the substance.
Solubility is the maximum amount of solute that dissolves in a specified amount of solvent under specified conditions.
Solute, Solubility, and Concentration
These related terms describe different aspects of a solution:
- Solute: The substance dissolved in the solvent.
- Solvent: The substance that dissolves the solute.
- Solution: A homogeneous mixture of solute and solvent.
- Solubility: The maximum amount of a solute that can dissolve under specified conditions.
- Concentration: The amount of solute present in a specified amount of solution or solvent.
For example, sugar is a solute when dissolved in water. Its solubility describes how much sugar can dissolve, while the concentration describes how much sugar is actually present in a particular solution.
Frequently Asked Questions
Is water always the solvent?
No. Water is a common solvent, but solvents may be other liquids, gases, or solids. For example, nitrogen is considered the solvent in air because it is the major component.
Is a solute always a solid?
No. A solute can be a solid, liquid, or gas. Carbon dioxide dissolved in water is a gas solute, while ethanol dissolved in water is a liquid solute.
Is the solute always present in the smaller amount?
Usually, but not necessarily. For components in the same phase, the substance present in the larger amount is generally called the solvent. However, the roles of solute and solvent can also depend on convention and how the solution is described or prepared.
Can a solution have more than one solute?
Yes. Many solutions contain multiple solutes. Seawater, for example, contains many dissolved ions and gases in water.
References
- Clugston, M.; Fleming, R. (2000). Advanced Chemistry (1st ed.). Oxford: Oxford Publishing.
- Hefter, G.T.; Tomkins, R.P.T (ed.) (2003). The Experimental Determination of Solubilities. Wiley-Blackwell. ISBN 978-0-471-49708-0.
- Houk, C.; Post, R. (eds.) (1997). Chemistry, Concept and Problems. John Wiley & Sons. ISBN 978-0-471-12120-6.
- IUPAC (1997). “Solute.” Compendium of Chemical Terminology (2nd ed.) (the “Gold Book”). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications. Online version (2019-) created by S. J. Chalk. ISBN 0-9678550-9-8. doi:10.1351/goldbook