Why does sodium chloride dissolve?

I recently came across a question on Twitter about sodium chloride (salt) dissolving in water. The question was effectively: “If the bonds between the Na+ and Cl ions are so strong, why is water able to break these bonds?” 

There are probably several ways to answer this question and the most satisfying will probably involve mathematics that is way above my pay grade. However, I will attempt to offer an answer here that might help…

First, it might be useful to look at the particulate nature of matter. When you add a clump of sodium chloride into water, the water molecules will be constantly moving and random directions, and thus will constantly collide (I prefer the term interact but let’s stick with collide in this explanation as it is more visual) with the sodium chloride lattice. This means that there is every chance (although small) that a highly energetic water molecule will hit one of the ions and transfer enough energy to it so that it overcomes the attraction to the other ions. Once this ion is “free” from the lattice, it collides with many water molecules which can form an attraction to it and so “surround” it. This is a relatively energetically stable state. 

Secondly, it might be useful to think of bonds in degrees of transience. They are not permanent, since electron clouds are constantly fluctuating. This means that at any one point, there is a chance that a single ionic bond on the edge of the lattice is weakened. This means that less energy is needed to overcome that bond, and so if an energetic water molecule hits that ion at the right time, then it will be “released” from the lattice. Then the water molecules can interact freely and “surround” it, forming the more energetically stable hydrated ion. 

These two ideas are highly dependent on probability. If you leave a clump of sodium chloride in a glass of water and do nothing, it will dissolve relatively slowly. This is because the likelihood of the above happening at room temperature is relatively low. However, once it does happen, the likelihood of the hydrated ions returning to the lattice is even lower. Hence, gradually, the salt will dissolve. In actual fact, we very rarely leave the clump of salt to dissolve naturally. We will either heat the water (thus increased the average kinetic energy of the ions in the lattice and the water molecules) making the above to occurences more likely or we will stir the water, making it much more likely that the water collides with the lattice, again increasing the chances of the above happening. 

I hope that explanation is useful. As ever, I would love the hear the thoughts of other teachers who might be able to pick holes in this explanation (or indeed tell me that it’s completely wrong!). 

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