Brine: The Salt-Water Solution
What Exactly is Brine?
At its simplest, brine is a mixture where one substance, the solute, is dissolved in another, the solvent. In the case of brine, the solute is common table salt—sodium chloride (NaCl)[1]—and the solvent is water ($H_2O$). A solution becomes "concentrated" when a large amount of solute is dissolved in a given amount of solvent. While seawater is a natural, weak salt solution (about 3.5% salt), brine is much stronger, often containing 20% to 26% salt by mass.
Picture making a cup of hot chocolate. When you add a little cocoa powder, it dissolves easily, making a light mixture. This is like seawater, a dilute solution. But if you keep adding spoonfuls, you eventually reach a point where no more powder will dissolve; the hot water is saturated. If you then heat the mixture, you can dissolve even more cocoa. This is similar to how brine is made: by adding salt to hot water until it can't hold any more, or by letting water evaporate from seawater, leaving the salt behind in a higher concentration.
The concentration of a solution can be expressed as mass percent:
$Mass \ Percent = (\frac{Mass \ of \ Solute}{Mass \ of \ Solution}) \times 100\%$
For example, if 35 grams of NaCl is dissolved in 100 grams of water, the total solution mass is 135 g. The mass percent is $(\frac{35}{135}) \times 100\% = 25.9\%$.
The Science of Salty Water: Properties and Behavior
Brine is more than just salty water. The presence of a high concentration of dissolved ions (Na+ and Cl-) fundamentally changes the physical properties of the water. These changes are called colligative properties, which depend only on the number of dissolved particles, not their identity.
First, brine freezes at a lower temperature than pure water. Pure water freezes at 0°C (32°F). A concentrated brine solution might not freeze until -21°C (-6°F)! The salt ions disrupt the formation of the orderly ice crystal lattice, requiring more cooling to solidify. This is why we use brine (as rock salt or a pre-wetting liquid) to melt ice on roads in winter.
Second, brine boils at a higher temperature than pure water. While pure water boils at 100°C (212°F) at sea level, a strong brine solution might boil at over 102°C. This property is cleverly used in some power plants and industrial processes.
Finally, brine has a higher density than fresh water. Adding salt makes the water heavier. A saturated brine can have a density of about 1.2 g/mL, while pure water is 1.0 g/mL. This is why it's easier to float in the Dead Sea (which is essentially a natural brine) than in a swimming pool. Objects that sink in fresh water may float in brine.
| Property | Pure Water | Concentrated Brine (~25% NaCl) | Scientific Principle |
|---|---|---|---|
| Freezing Point | 0°C (32°F) | Approx. -21°C (-6°F) | Freezing Point Depression |
| Boiling Point | 100°C (212°F) | Approx. 102°C (216°F) | Boiling Point Elevation |
| Density | 1.00 g/mL | Approx. 1.20 g/mL | Increased Mass per Volume |
| Electrical Conductivity | Very Low | Very High | Presence of Mobile Ions (Na+, Cl-) |
From the Sea to the Shaker: How Brine is Made and Used
Humans have been making and using brine for thousands of years. The most traditional method is solar evaporation. Seawater is channeled into large, shallow ponds. The sun and wind evaporate the water over weeks or months, leaving behind increasingly concentrated brine and, finally, crystals of salt. This salt can then be harvested and purified.
In a modern factory or lab, brine is often made by simply dissolving mined or purified rock salt in hot water while stirring, because the solubility of NaCl increases slightly with temperature. The solubility limit of NaCl in water at 20°C is about 36 grams per 100 mL of water. This means you can't make a solution stronger than about 26.5% at room temperature—any extra salt just sits at the bottom as solid crystals.
You can find the saturation point at home. Take a clear glass of warm water. Keep adding table salt one teaspoon at a time, stirring until it completely disappears. Eventually, you'll see salt collecting at the bottom that won't dissolve no matter how much you stir. You've just made a saturated brine solution!
Brine at Work: Practical Applications in Our World
The unique properties of brine make it incredibly useful across many fields.
Food Preservation: This is one of the oldest uses. Soaking foods like pickles, olives, and certain meats in brine draws water out of harmful bacteria through a process called osmosis[2]. The bacteria dehydrate and die, preventing spoilage. The salt also adds flavor. When you eat a pickle, you are tasting the result of osmosis!
De-icing Roads and Walkways: In winter, trucks spread brine (either as a liquid spray before a storm or via rock salt that turns into brine on contact with ice). The brine's freezing point depression causes the ice to melt even when the air temperature is below 0°C. The resulting saltwater has a lower freezing point, preventing new ice from forming as quickly.
Industrial and Chemical Processes: Brine is a key raw material for the Chlor-alkali process[3], one of the most important industrial chemistry reactions. When electricity is passed through brine, it breaks down into chlorine gas ($Cl_2$), hydrogen gas ($H_2$), and sodium hydroxide ($NaOH$), which is a strong cleaner (lye). These three products are used to make countless items, from plastics and paper to soaps and pharmaceuticals.
Heat Transfer: Because of its high boiling point, brine is sometimes used as a coolant in refrigeration systems and in some types of solar power plants. It can absorb a lot of heat before boiling, making it efficient for moving thermal energy.
Important Questions About Brine
The salt itself doesn't directly melt the ice. When salt grains come into contact with the thin layer of water that often exists on the surface of ice, they dissolve to form a brine. This brine has a lower freezing point than 0°C. When this liquid brine touches the solid ice, it causes the ice to melt (because the brine's freezing point is lower than the ice's temperature). This creates more liquid water, which dissolves more salt, creating more brine, and the process continues, melting the ice.
It can be, which is why its use must be managed. When brine from road de-icing or industrial processes runs off into soil and freshwater streams, it increases the salinity (saltiness) of those environments. Most plants and freshwater animals are not adapted to high salt levels and can be harmed or killed. Excess sodium and chloride ions can also corrode metal structures and degrade concrete. Scientists and engineers are working on better application methods and more eco-friendly alternatives.
Chemically, it's the same substance: sodium chloride (NaCl). The difference is their form. Table salt is the solid, crystalline form of NaCl. Brine is the form where NaCl is dissolved and broken apart into its individual sodium ions (Na+) and chloride ions (Cl-) within water. When you evaporate all the water from brine, you are left behind solid table salt.
Footnote
[1] NaCl: Sodium Chloride. The chemical compound formed from the elements sodium (Na) and chlorine (Cl), commonly known as table salt or rock salt.
[2] Osmosis: The movement of water molecules through a semipermeable membrane from an area of lower solute concentration (more water) to an area of higher solute concentration (less water).
[3] Chlor-alkali Process: An industrial electrochemical process that uses an electric current to decompose brine (NaCl solution) into chlorine gas (Cl2), hydrogen gas (H2), and sodium hydroxide (NaOH).