 A solution is a homogenous mixture of two more substances, a solution has two parts, the solutes and a solvent. The solvent is defined as the component that is the majority of the solution, and the solute is the small component that dissolves in the solvent. A lot of people think that a solution is always a liquid, but that's not necessarily true. Technically, a solution can be of any phase, gas, liquid or solute, as long as the overall mixture is homogenous. For example, a mixture of oil and water is not a solution, even though oil and water are both liquid, this is because the oil and water are not evenly distributed in the system. So you put some sodium chloride salt in a cup of water, now when the salt hasn't dissolved yet, this is strictly still not a solution, it's still only a mixture of sodium chloride and water, but when you give it some time or give it a stir, the salt had dissolved, the mixture is now a solution, with sodium chloride being the solute and water being the solvent. You can also have solid solutions like metal alloys, for example sterling silver, it is a homogenous solute that is mainly made of silver, but it also has some nickels and copper, which are the solutes. And because they are homogeneously distributed in alloy, it is a solution. Now, a big part of chemistry is being able to monitor the amount of substances in your solutions or constituents, the easiest way is through concentration. Concentrations are defined as an amount of a constituent per an amount of the mixture, it describes the abundance of a constituent of a mixture, the concept of concentration is necessary because it allows you to compare and monitor the amount of constituent in your mixture, say you have a solution of sugar and water. So the water is the solvent because there is so much of it, and the sugar is the solute. So you have three sugar molecules in this solution, then ten sugar molecules in the next one, then fifteen, then twenty sugar molecules. As you increase the amount of sugar in the solution, you say that the solution gets more concentrated. Now concentration is the amount of constituent per an amount of volume in that mixture. So if the total volume of each of these solutions is one litre, then what's the concentration of sugar in these solutions? So the first one has three sugar molecules in one litre of solution, so the concentration is three per litre. The next one has ten sugar molecules in one litre of solution, so this concentration is ten per litre. And the next two are first, fifteen per litre and twenty per litre. So you can clearly see that the more concentrated solution will have a higher concentration value. So these concentration values are actually called number concentrations. There are many kinds of concentrations that you will see. So number of concentration is defined as the number of entities of a constituent in a mixture divided by the volume of the mixture.