Water is one of the essential items for the life of living beings. No life can survive without it. It has unique properties that make it different from other compounds. On adding salt, many changes occur in the physical and chemical properties. In this article, I will discuss in detail the question of whether saltwater evaporates or not.
So, does saltwater evaporate? Yes, saltwater evaporates leaving behind salt crystals. And the addition of salt in pure water decreases the rate of evaporation of the salt solution. This is because of the decrease in the saturation vapor pressure of the salt solution. Evaporation is directly proportional to the difference in vapor pressure of air in contact with the salt solution and the air at some altitude.
Natural water bodies contain varieties of salts (sodium chloride, magnesium chloride, potassium chloride) in it. According to the experiments, the effect of evaporation of saltwater depends on the salt present in it.
In simple words, evaporation can be defined as the process in which a liquid state substance changes into a gaseous state by the effect of temperature/pressure.
The molecules of a substance present in a liquid state lose their molecules in the form of vapors at some rate. The rate of evaporation depends on some external environmental factors such as pressure, temperature, humidity,etc..
Similarly, salt water is a solution in a liquid state and with its vapor pressure and temperature, the saltwater also loses its molecules in the form of vapors.
In salt solution, the salt particles are surrounded by water molecules that keep the salt particles apart and avoid recrystallization of the salt.
On evaporation, the water molecules evaporate in the form of vapor and lesser water molecules remain to keep the salt particles apart. And finally, when the whole water evaporates, the salt particles remain and get recrystallized.
Salt Water Experiment
During the practicals in high schools, we perform the experiment of salt crystallization in the chemistry laboratory. The aim of this is to extract salt crystals from the salt solution.
The approach to get salt crystals is by opting for the method of evaporation of saltwater. To perform this experiment below are the steps.
- Take a pan of water and pour some amount of salt into it such that the salt dissolves completely in the water.
- Place the black paper on the baking sheet. The black color of the sheet absorbs all the frequencies of the light that helps to energize the water molecules to evaporate faster.
- Put the pan containing the salt solution on the baking sheet somewhere in a warm place like outside in the sun, or near the window.
- After a few hours, the water level decreases because the water molecules lose the solution in the form of vapors due to evaporation.
- Put it for a few hours and record the observation, you will find that all the water evaporates leaving behind the salt residue in the form of crystals.
Similarly, the salt is prepared from the seawater. The salt extraction from the seawater is done on a larger scale. However, the concept is the same.
How does salt affect the evaporation of water?
Under the standard conditions of temperature, pressure, and other factors, the rate of evaporation of saline water is lesser than that of pure water.
This is because of the following reasons:
On the addition of salt like NaCl in the water, the vapor pressure of the water lowers down. It means that fewer water molecules can escape out of the solution from its surface. Therefore, the higher the concentration of salt present, the lesser is the evaporation.
It is important to understand that the basic phenomenon of this is the evaporation from the surface. It means that the only molecules can evaporate that is on the surface of the solution.
In NaCl aqueous solution, Na ions and Cl ions also occupy space on the surface. This reduces the molecules of H2O present on the surface (liquid-air boundary). Therefore the rate of evaporation of water decreases on the addition of salt into it.
Does seawater evaporate?
The water cycle of the earth planet is mainly because of natural water bodies. Oceans cover a large part of the earth’s surface. Around 71% of the earth’s surface is covered by the oceans.
They provide 97% of the earth’s water. Whereas the remaining 2 to 3 percent of water is contributed by the ice caps and glaciers. Freshwater contains less than 1% of the total water of planet earth.
The seawater contains different types of salt. The main six abundant salts ions are sulfate (SO24−), chloride (Cl−), potassium (K+), sodium (Na+), calcium (Ca2+), and magnesium (Mg2+). The content of salt present in the seawater is denoted by the term salinity (S).
In seawater, the salt is not chemically bonded to water. It is merely dissolved in seawater.
It should be clearly understood that the seawater evaporates slowly as compared to the freshwater keeping the external factors constant. Salts present in seawater lower its vapor pressure due to which the rate of evaporation decreases.
Only the pure water molecules of seawater evaporate in the form of vapors leaving behind minute loads of salt as residue.
The air blowing around the sea contains salt particles that can be analyzed by checking the rusting of automobiles of nearby residents.
Do saltwater pools evaporate faster?
It is important to understand that the salinity of pool water lowers the rate of evaporation. The salt particles lower the vapor pressure of saltwater in the pool and also occupy space at the surface of saltwater (air-liquid boundary).
And it should be kept in mind that in a saltwater solution, only the pure water molecules at the surface of saltwater evaporate/escape.
Therefore, a lesser number of molecules of H2O are present on the surface and therefore lesser molecules of H2O can escape out in the form of vapors because some space on the surface is covered by the salt particles.
Therefore, saltwater pools evaporate slower than the freshwater pool. The salinity of the water always decreases the rate of evaporation.