How Evaporation Affects Molarity: A Simple Explanation

Evaporation is a common phenomenon that occurs when a liquid changes into a gas. You may have observed evaporation when you see water droplets disappear from a wet surface, or when you smell perfume after spraying it on your skin. But did you know that evaporation can also affect the concentration of solutes in a solution? In this article, we will explain how evaporation is related to molarity, which is a measure of the amount of solute in a solvent.

What is Molarity?

Molarity is one of the ways to express the concentration of a solution. It is defined as the ratio of the number of moles of solute to the volume of solution in liters. A mole is a unit that represents a fixed number of particles, such as atoms, molecules, or ions. The symbol for molarity is M, and the unit is mol/L.

For example, if you dissolve 1 mole of sodium chloride (NaCl) in 1 liter of water, you will get a 1 M solution of NaCl. This means that there are 1 mole of NaCl particles in every liter of solution.

How Evaporation Changes Molarity?

Evaporation occurs when liquid molecules gain enough energy to escape from the surface and enter the gas phase. The rate of evaporation depends on several factors, such as temperature, pressure, surface area, and the presence of solutes.

When a solution evaporates, the solvent molecules leave the solution and become gas. However, the solute particles remain in the solution. This means that the amount of solute stays constant, but the volume of solution decreases. As a result, the molarity of the solution increases.

For example, if you have 1 liter of 1 M NaCl solution and half of it evaporates, you will be left with 0.5 liter of 2 M NaCl solution. The number of moles of NaCl is still 1, but the volume of solution is now half. Therefore, the molarity is doubled.

Why Molarity Matters?

Molarity is an important concept in chemistry because it affects many properties and reactions of solutions. For instance, molarity affects the boiling point, freezing point, osmotic pressure, and electrical conductivity of solutions. Molarity also determines how fast and how far a chemical reaction can proceed in a solution.

Therefore, it is important to control and monitor the molarity of solutions in various applications, such as laboratory experiments, industrial processes, and biological systems. Evaporation can alter the molarity of solutions and cause unwanted effects or errors. For example, evaporation can make a solution more acidic or basic, change its color or odor, or affect its ability to dissolve or precipitate other substances.

How to Prevent or Minimize Evaporation?

There are several ways to prevent or minimize evaporation and maintain the desired molarity of solutions. Some common methods are:

  • Covering the container with a lid or a plastic wrap to reduce the surface area exposed to air.
  • Storing the container in a cool and dry place to lower the temperature and humidity.
  • Adding an anti-evaporation agent, such as glycerol or ethanol, to reduce the vapor pressure of the solvent.
  • Using a volumetric flask instead of a beaker or a graduated cylinder to measure the volume of solution accurately.


Evaporation is related to molarity because it changes the concentration of solutes in a solution by reducing the volume of solvent. Molarity is an important factor that affects many properties and reactions of solutions. Therefore, it is essential to prevent or minimize evaporation and keep the molarity constant in various situations.

I hope this article has helped you understand how evaporation affects molarity and why it matters. If you have any questions or comments, please feel free to share them below. Thank you for reading! 😊

Doms Desk

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