What Is a Solvent in Chemistry?
A solvent in chemistry is the substance present in the greatest amount in a solution, and it is the one that does the dissolving. The substance that gets dissolved is the solute, and the two together make a solution. In a glass of saltwater, water is the solvent and salt is the solute. Water is by far the most common solvent in chemistry, which is why so many reactions are written as aqueous, but ethanol, acetone, and hexane dissolve things water cannot touch.
Key takeaways
- A solvent is the component of a solution present in the greatest amount. The solute is the smaller component that dissolves into it.
- Water is nicknamed the universal solvent because it dissolves more substances than any other common liquid, thanks to its bent, polar shape.
- Like dissolves like: polar solvents dissolve polar molecules and many ionic compounds, while nonpolar solvents dissolve fats, oils, waxes, and greases.
- Stirring, heating, and crushing the solute make dissolving faster. Only temperature, and pressure for gases, changes how much will dissolve.
- The state symbol (aq) after a formula means the substance is dissolved in water, as in NaCl(aq).
What is a solvent in chemistry?
A solvent is the dissolving medium in a solution. Its particles surround the solute, pull it apart, and carry the pieces until they are spread evenly through the container. The result is a homogeneous mixture, so a sip from the top of a sugar solution is exactly as sweet as a sip from the bottom.
The usual test for spotting the solvent is simple arithmetic. Stir 5 g of copper(II) sulfate into 95 g of water and you hold 100 g of solution, so water is the solvent. A second rule helps when the components start out in different states: the solvent is the one whose state matches the finished solution. Carbon dioxide dissolved in liquid water gives a liquid, so water is the solvent there too.
Why is water called the universal solvent?
Water earns the nickname because it dissolves more substances than any other common liquid, not because it dissolves everything. The reason is its shape. A water molecule is bent, with an H–O–H angle of about 104.5°, and oxygen attracts the shared electrons far more strongly than hydrogen does. That unequal sharing makes each O–H link a polar bond, leaving a partial negative charge on the oxygen and partial positive charges on the hydrogens. Because the molecule is bent rather than straight, those charges do not cancel, so the whole molecule is polar.
When sodium chloride meets water, the negative oxygen ends crowd around each Na⁺ ion while the positive hydrogen ends crowd around each Cl⁻ ion. Those attractions peel ions off the crystal one at a time, and every freed ion ends up wrapped in a cage of water molecules, a process called hydration. Water also weakens the pull between separated ions better than almost any other liquid: its dielectric constant is about 80, against roughly 2 for hexane.
The limits are real. Water will not dissolve oils, fats, waxes, or most plastics, and ionic compounds such as silver chloride and calcium carbonate barely dissolve at all. Universal here means versatile, not unlimited.
What does "like dissolves like" mean?
Like dissolves like is the rule of thumb that a solvent dissolves solutes held together by the same kind of attraction the solvent uses itself. Polar solvents dissolve polar molecules and many ionic compounds. Nonpolar solvents dissolve nonpolar substances such as fats, oils, and greases.
The reason is energy bookkeeping. Separating solute particles and pushing solvent particles apart both cost energy, and that cost is only repaid if the new solute-solvent attractions are about as strong as the ones broken.
Hexane and cooking oil are both held together by weak dispersion forces, so they mix freely. Water and oil do not, because water molecules would have to trade strong hydrogen bonds with each other for weak attractions to an oil molecule that cannot hydrogen bond at all. The oil gets squeezed out and floats.
Treat the rule as a guide, not a law. Ethanol has a polar –OH end and a nonpolar –C₂H₅ end, so it mixes with water in any proportion and still dissolves many nonpolar substances. Longer alcohols behave less and less like water as the carbon chain grows.
Which solvents turn up in the lab and at home?
Six solvents cover most of what you meet in a school lab or under the kitchen sink. Chemists sort them into three groups. Polar protic solvents contain an O–H or N–H group and can donate hydrogen bonds. Polar aprotic solvents are polar but have no such hydrogen to give. Nonpolar solvents have almost no charge separation at all. That single label predicts what a solvent will dissolve better than anything else about it.
| Solvent | Formula | Polarity | Typical use |
|---|---|---|---|
| Water | H₂O | Polar, protic | Dissolving salts, sugars, and acids; nearly all biological chemistry |
| Ethanol | C₂H₅OH | Polar, protic | Flavor extracts, tinctures, hand sanitizer, disinfectants |
| Acetone | (CH₃)₂CO | Polar, aprotic | Nail polish remover, drying glassware, dissolving resins and some plastics |
| Dichloromethane | CH₂Cl₂ | Weakly polar | Paint stripper, extracting caffeine from coffee beans |
| Hexane | C₆H₁₄ | Nonpolar | Extracting cooking oil from seeds, degreasing metal parts |
| Toluene | C₇H₈ | Nonpolar | Paints, adhesives, and thinners |
Every organic solvent on that list is flammable, and the vapors belong in a fume hood. Water is the outlier: cheap, safe, and effective across a huge range of solutes.
How do solvents affect dissolving rate and solubility?
Two questions hide inside that one: how fast does the solute dissolve, and how much of it will dissolve at all? Stirring, heating, and crushing the solute all speed things up, because each puts more solute surface in contact with fresh solvent. None changes the final amount.
Solubility is that final amount: the maximum mass of solute a given mass of solvent can hold at a stated temperature. About 36 g of table salt dissolves in 100 g of water at 25 °C. Heating helps less than you might expect, since only about 39 g dissolves at 100 °C. Potassium nitrate is the opposite extreme, rising from about 13 g at 0 °C to more than 240 g at 100 °C.
Gases run backwards. Warm water holds less dissolved gas than cold water, which is why a soda left out goes flat. Gas solubility also rises with pressure, which is what keeps carbon dioxide dissolved in a sealed bottle until you crack the cap.
Solubility sets the ceiling. Concentration tells you how much solute is actually in the solution you are holding, anywhere from a trace up to that ceiling.
What does (aq) mean in a chemical equation?
The symbol (aq) stands for aqueous, and it means the substance is dissolved in water. Equations use four state symbols: (s) for solid, (l) for liquid, (g) for gas, and (aq) for anything dissolved in water. Writing NaCl(aq) says something genuinely different from NaCl(s), because dissolved sodium chloride exists as separate hydrated ions rather than as a crystal lattice.
NaCl(s) → Na⁺(aq) + Cl⁻(aq)
The distinction matters in reactions. In AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq), the symbols tell you the silver chloride drops out of the solution as a solid precipitate while everything else stays dissolved. Note that (aq) is reserved for water alone. A solution in any other solvent is described in words instead.
Frequently asked questions
Is water always the solvent?
No. The solvent is whichever component is present in the greater amount, so in a mixture of 95 g of ethanol and 5 g of water, ethanol is the solvent. That said, chemists often treat water as the solvent by convention whenever it is present, because so much chemistry is aqueous.
What is the difference between a solvent and a solute?
The solvent does the dissolving and the solute is what gets dissolved. The solvent is present in the greater amount and usually sets the physical state of the solution. In saltwater, water is the solvent and salt is the solute.
Can a solvent be a solid or a gas?
Yes. In a solid solution such as brass, copper is the solvent and zinc is the solute, and both stay solid throughout. In air, nitrogen is the solvent for oxygen, argon, and other gases. Liquid solvents are simply the most common.
Why does oil not dissolve in water?
Oil molecules are nonpolar and cannot form hydrogen bonds, while water molecules cling to each other with strong hydrogen bonds. Mixing the two would force water to trade strong attractions for weak ones, so the oil is pushed out and floats on top. A nonpolar solvent such as hexane dissolves oil easily.
Need a precise definition of solvent, solute, or any term around them? Search the word in the A to Z Chemistry Dictionary.