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Why does salt crystals dissolve in the water?

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3 Answers

Natalee's answer is very good. I only wish to add a few things to it. Depending on the level of biology or chemistry course, these other factors might be part of your answer.

The dissolution of NaCl in liquid water is what we call a spontaneous process. It occurs without the addition of any more energy. Spontaneous processes occur because they lower the free energy of a system. In general, breaking bonds takes energy and making bonds releases that energy. Free energy can take the form of bonds (enthalpy, S) or degree of disorder (entropy, H). The most well known equation for free energy changes is:

ΔG = ΔH – TΔS

(ΔG is the change in free energy. If ΔG < 0, the process is spontaneous. ΔH is the change in enthalpy, T is the absolute temperature, and ΔS is the change in entropy.)

When NaCl dissolves, the Na+:Cl- ionic bonds are lost. Some of this increased free bond energy is offset by the formation of new bonds, the transient interactions between the polar water molecules and the dissolved ions. However, most of the increased free bond energy is made up for by the increased entropy. The number of ways you can make a salt crystal is very small. NaCl crystals are highly organized, with very low entropy. The number of states the same Na+ and Cl- ions can occupy when surrounded by water molecules is relatively much, much greater. Thus the entropy of the solution is much greater. There are many ways to organize the Na+ and Cl- ions within a see of H20 where the Na+ and Cl- are not adjacent to each other. This is a more entropic situation.

The increase in entropy is a decrease in free energy. This is the main driver in the dissolution of water.

Another consideration is the kinetics of the dissolution. While the change in free energy tells you which direction the process goes (towards dissolution), it doesn’t tell you how fast it occurs. It turns out that the energy present at room temperature, in liquid water, is strong enough to break those Na+:Cl- ionic bonds, so the process happens at a fairly reasonable rate (minutes, not years).

Again, all of this might be too advanced, depending on the biology class. Use your judgment.

Good question Samantha. Let’s first take a look at salt and water at the molecular level. Salt, also written out as NaCl (Sodium Chloride), is made up of positive sodium ions (Na+) along with negative chloride ions (Cl-). Because we have a positive and negative, the two are attracted to each other forming an ionic bond, which results in NaCl. When many sodium and chloride ions attract to one another, they form a salt crystal. When this formation occurs, you get a sodium ion that is surrounded by 6 chloride ions and a chloride ion surrounded by 6 sodium ions to form a cube.

Now let’s look at water (H20) on the molecular level. A water molecule is made up of two hydrogen atoms bonded to one oxygen atom. The oxygen molecule has a great attraction for the electrons that are found on the hydrogen. As a result, the electrons spend most of their time near the oxygen than the hydrogens. This then makes a slight negative charge around the area of the oxygen and slight positive charge around the hydrogen.

So, when salt crystals are placed in water, the positive end of the water molecule attracts the negatively charged chloride ions and the negative ends of the water molecules attract the positive sodium ions. As a result, the salt will dissolve once the attraction between the water and the ions overcomes the attraction the ions have for one another. Once dissolved, the positive sodium ions and negative chloride ions will associate with the oppositely charged polar ends of many water molecules.

Hope this helps!

I think these are both good answers. However, I wish to point out that the term "salt" may refer to any of a tremendous number of ionic compounds. The term "ionic" refers to the property that each of these compounds will dissolve in water, splitting into the "ions" that carry the positive (cation) or negative (anion) charge.

Comments

That is a good point, David.

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