Chem – Solutions and Solubility Definitions

What is a solution?

Unfortunately there are many definitions you have to learn in the solution lesson before you can move on any further. If you do not learn them first they will be handicapped the rest of the lesson. First we will focus on the definition of a solution. A solution is when two or more substances are mixed together on a molecular level. Different parts of a solution are not able to be separated simply by waiting for them to fall out. You have to change some kind of condition to separate them, like temperature or pressure. We also want to know what a solution is in terms of a mathematical perspective. A solution is A SOLUTE + A SOLVENT. Now that may seem confusing at first but really a solute is just one substance like salt and a solvent is another substance like water. It actually does not matter which one you call the solute and which one you call the solvent, but most of the time the teacher will either name which one is which or they will tend to call the solid (like salt) the solute and the liquid (like water) the solvent. In fact, in most classes you will only talk about the solute as a solid and the solvent as a liquid.


What is solubility?

Next we have to make a clear definitions for soluble and insoluble. Soluble is when two or more substances are able to mix together their individual molecules. For example, if you mix sugar in your tea then the sugar breaks up and blends in with the tea. You can say that the sugar is soluble in the tea. Insoluble is when two or more substances are not able to mix together their individual molecules. For example, if you mix a granite rock with water. The rock will not break up and blend in with the water so the granite rock is insoluble in water. How well different substances mix together we refer to as solubility. We can say a substance has high or low solubility in another. For example, butter has high solubility in olive oil but butter has low solubility in water. Solubility usually refers to a situation of a solid chemical mixing in a liquid chemical but the state of each chemical really does not matter. You can have two gases mix and still call them soluble in each other. For example oxygen gas (O2) and has high solubility in nitrogen gas (N2).


What is Miscible?

As if we did not have enough definitions already, we can now come to miscible versus immiscible. They are the exact same definition as soluble versus insoluble. That is miscible is the same thing as soluble while immiscible is the same thing as insoluble. Why chemistry has two definitions for the same thing may not make a whole lot of sense but miscible and immiscible usually refer to two liquids mixing. For example, olive oil is immiscible in water.


What kind of things are mixtures but not solutions?

Finally we come to the mixtures of things that do not interact with each other on the individual molecule scale but instead as large chunks of thousands of the same molecules that are stuck together. These are known as colloids or suspensions. The easiest way to tell colloids and suspensions apart from solutions is that colloids or suspensions if left to sit in a container (while at the same temperature) will separate into different substances after hours or days and will therefore form layers in the container. However, solutions will never separate out just by allowing them to sit in a container (while at the same temperature). Colloids are any clumps of molecules that are up to 1 micrometer (um) in size. Suspensions are clumps of molecules that are lager than 1 micrometer (um) in size. A classic suspension example is soil (dirt) in water. When you first mix the soil and water together they looked like they will stay mixed forever but as soon as you stop mixing you will see the clumps of soil start to float to the bottom and separate out.


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