Solvents, solutes and solutions: what is a solvent?
What is a solvent? How solvents, solutes and solutions relate, why water dissolves so much, solubility rules, saturated solutions and colloids.
By Zachery Casey · Published · Reviewed · Editorial policy
A solvent is the substance that dissolves another substance, the solute, to make a solution. In salt water, water is the solvent and salt is the solute. Almost every lab job starts with dissolving something, and this guide explains each of the three words, why water dissolves so many things, how polar and non-polar solvents differ, and the everyday words for how strong a solution is.
Terms are defined in the glossary.
What is a solvent?
A solvent is the substance that does the dissolving. It is usually a liquid, and it is the part of the mixture present in the largest amount.
A solute is the substance being dissolved. A solution can contain one solute or many.
A solution is the result: an even mixture, the same all the way through, in which the solute can no longer be seen as separate particles (OpenStax).
| Example | Solvent | Solute | Solution |
|---|---|---|---|
| Salt water | Water | Sodium chloride | A salt solution |
| A buffer | Water | The buffer salts | The buffer solution |
| An HPLC mobile phase | Water and acetonitrile | Trifluoroacetic acid | The mobile phase |
| A peptide stock solution | Water, or water with a little DMSO | The peptide | The stock solution |
A mixture of two liquids can make it hard to say which is the solvent. By convention, it is the one present in larger amount.
Solute vs solvent: what's the difference?
The solute is what gets dissolved; the solvent is what does the dissolving, and there is usually more of it. Solutes can be solids, liquids or gases. Sugar in water is a solid solute. Acetic acid in water is a liquid solute. Carbon dioxide in fizzy water is a gas solute.
When a solute dissolves, its particles spread out evenly among the solvent's molecules. Salts split into charged ions, each surrounded by solvent. Molecules such as sugar stay whole but separate from each other. See ions: cations and anions.
Water: the universal solvent
Water is often called the universal solvent because it dissolves more substances than any other common liquid. The name is an exaggeration: oils, waxes and many plastics don't dissolve in water. But for salts, sugars, acids, bases and many peptides, water is the first choice.
Two features of the water molecule explain this:
- It is polar. The oxygen pulls electrons away from the two hydrogens. The oxygen end carries a slight negative charge and the hydrogen ends a slight positive charge. These ends are attracted to ions and to other polar molecules.
- It forms hydrogen bonds. Water's hydrogens can bond weakly to oxygen and nitrogen atoms on other molecules, which helps it hold polar molecules in solution.
For more on both, see chemical bonds and polarity.
For lab work the water itself needs to be clean: see lab water grades.
Polar and non-polar solvents
Solvents are usually sorted by polarity. The rule of thumb is "like dissolves like": polar solvents dissolve polar and charged substances, and non-polar solvents dissolve non-polar ones.
| Type | Examples | Good at dissolving |
|---|---|---|
| Polar, can hydrogen-bond | Water, methanol, ethanol | Salts, sugars, charged molecules |
| Polar, can't donate hydrogen bonds | Acetonitrile, DMSO, acetone | Many organic molecules, including hydrophobic peptides |
| Non-polar | Hexane, toluene | Oils, fats, waxes |
For a chart of 15 common lab solvents, with which ones mix with water and their polarity, see the solvent miscibility and polarity chart.
Dilute, concentrated and saturated
Chemists use a few everyday words for how much solute a solution holds.
- Dilute: a small amount of solute for the volume of solvent.
- Concentrated: a large amount of solute for the volume.
- Saturated: as much solute as will dissolve at that temperature. Any more stays undissolved.
- Unsaturated: below that limit, so more could dissolve.
- Supersaturated: more solute than the limit, held in solution temporarily. A small disturbance can make the extra crystallise out (OpenStax).
"Dilute" and "concentrated" are relative words. For an exact figure, chemists give a concentration with units. Acids and bases are a common case: see acids and bases for what makes one strong or weak, as opposed to concentrated or dilute. See molarity, mg/mL and dilutions and parts per million.
What is solubility?
Solubility is the most of a substance that will dissolve in a given amount of solvent at a given temperature, often written in g/L or mg/mL. A solution that holds exactly that much is a saturated solution: add more and it stays undissolved, and cool it or let solvent evaporate and some of the solid can come back out.
Solubility and temperature
Solubility changes with temperature. Most solids dissolve better in warm water than cold, though there are exceptions. Gases do the opposite: warm water holds less dissolved gas than cold (OpenStax).
This matters in the lab. A solution made up warm can drop solute out as it cools, and a stock solution stored in a fridge may need checking for crystals before use.
Solubility rules
For simple salts in water, chemists use rules of thumb called solubility rules (OpenStax):
- Salts of sodium, potassium and ammonium dissolve, with no common exceptions.
- Nitrates and acetates dissolve.
- Most chlorides dissolve, except those of silver, lead and mercury(I).
- Most carbonates, phosphates and hydroxides don't dissolve, except with sodium, potassium or ammonium.
Solutions, suspensions and colloids
Not everything that looks mixed is a true solution.
| Particle size | Looks | Settles out? | |
|---|---|---|---|
| True solution | Single molecules or ions | Clear | No |
| Colloid | About 1 nm to 1 µm | Clear to hazy; scatters a light beam | No, or only very slowly |
| Suspension | Larger than about 1 µm | Cloudy | Yes, over time |
A colloid is the tricky case. Its particles are small enough to stay spread through the liquid without settling, so it can look almost clear. Shining a narrow light beam through it shows the difference: a true solution lets the beam pass unseen, while a colloid scatters it into a visible path (the Tyndall effect). Milk and fog are everyday colloids. In the lab, peptide aggregates can form a colloid, which is why a hazy peptide sample should be treated as not fully dissolved; see peptide solubility.
Choosing a solvent for a peptide
Peptides follow the same rules, but each one has many groups along a single chain. Its overall charge and how hydrophobic it is decide which solvent works. See peptide solubility: choosing a solvent for the step-by-step method, and acetic acid as a peptide solvent for one common choice. For the difference between sterile and preserved water as a lab solvent, see bacteriostatic water vs sterile water.
Related guides
For what happens to a solution's freezing and boiling points, see melting, boiling and freezing points. For making up solutions accurately, see pipettes and volumetric glassware. For the solvents used in HPLC, see HPLC and LC-MS solvents. Every lot has its own COA, published on the product page and on that lot's own page. See Quality.
More on solvents and lab solutions
- Bacteriostatic water vs sterile water: laboratory solvent chemistryBacteriostatic water vs sterile water as lab solvents: what sterile water is, what benzyl alcohol adds, how each is stored, and which suits which method.
- Dilute acetic acid as a solvent for basic peptidesWhy dilute 0.6% acetic acid dissolves basic peptides that resist neutral water: net charge, isoelectric point and pKa, and when to choose another solvent.
- Lab water: distilled, deionised, reverse osmosis and ultrapureHow distilled, deionised, reverse osmosis and ultrapure (Type 1) water are made, what each removes, and which grade suits buffers, HPLC and lab work.