Have you ever prepared a solution of salt and water to use either as a gargle, nasal rinse or simply as a beverage? Ever wandered how the salt dissolves to form this homogenous clear solution?
If we go back to our lesson on the structure of salt (What is the chemistry of salt) we recall that salt comprises an Ionic bond with Na being positive (Cation) and Cl being negative (Anion). Referring to our Periodic table of Elements we know that water is a compound formed between Hydrogen (H) and Oxygen (O) and is referenced by the formula H20.
It is interesting to note that when these formulas are written, the Cation element is listed first. So, in these examples, Na is positive and H is positive.
Let us now combine salt and water
What happens, is that the electric charges (polarity) go to work, which results in the positive Sodium being attracted to the negative Oxygen and vice versa, the negative Chloride being attracted to the positive Hydrogen.
Watch this short video for a clear visualization of what happens.
Assuming we used the measurements of 1 teaspoon of salt and 1 glass (250ml) of water, then you would have plenty of H2O molecules surrounding the NaCl crystals. Several Oxygen molecules would attach themselves to the Sodium molecules and with the energy being created in the solution, combined with the Chlorine/Hydrogen energy paring, the water particles are able to enter the spaces between the Sodium and Chlorine (NaCl) bonds and thus eventually break apart and disintegrate the NaCl crystals. Once all the Na and Cl ions have been separated from each other and surrounded by water molecules, they are said to be hydrated.
At higher temperatures, salt will dissolve into solution quicker because the additional heat creates more vibration (energy) and thus the various molecules will collide with each other more aggressively, weakening bonds and allowing the polarity of Hydrogen and Oxygen to create space within the salt crystals to eventually engulf them.
Salinity and Saturation
In our discussions thus far, we have referred to using only a teaspoon (5g) of salt into a glass of water (250ml).
What would happen if we added more salt into that same glass of water. Here is where we learn some further interesting terminology.
- Salinity: this term refers to the amount of salt dissolved in a given volume of water and is often expressed as grams (g) of salt per kilogram/litre of water or something called Parts per Thousand (ppt).
When we visit a hospital we often see saline solution being administered to patients. This is a salty solution that is drip fed into the patients bloodstream to support a variety of needs essential to maintaining healthy functioning of cells, organs and muscle.
Typically saline solution is administered as a 0.9% solution, or 9g of salt per litre of water.
Did you know? The salinity of the ocean is about 35g per litre, or 3.5%.
- Saturation : this is the point at which the maximum concentration of a substance has been reached, such that any additional solid placed in the solution, will remain as a solid. The temperature of the solution will have an effect on the concentration, with higher temperatures typically allowing for higher concentration.
The interesting facts about salt (sodium chloride), is that at a temperature of 20°C, one liter of water can dissolve approximately 357 grams of salt, before it is said to be saturated.
