A solution is a homogenous (fine) mixture of a substance (called solute) dissolved in another substance (called solvent). Although the term solution is used for the liquid state of matter, solutions of gases and solids are also possible. For example, steel is a solution of two solids (iron is the solvent and carbon is the solute). Air is an example of a gaseous solution containing various gases, mainly nitrogen and oxygen. Our lungs add oxygen to the blood that is carried to various cells and tissues. This is an example of a gas in a liquid solution.
Various medicines are available as solutions of the active compound (drug) in a carrier solvent. The amount of the drug (solute) in the solvent is important because it determines what amount of drug will go into your body when you consume a particular dosage. Thus, knowing the amount of solute dissolved in a particular amount of the solvent is a crucial parameter and is called the concentration of the solution. Suppose you have two bottles, A and B, carrying the same drug C. If bottle A has 5 units of drug C in every 10 ml of the solution and bottle B has 2 units of the drug C in every 10 ml of the solution, the solution in bottle A will be called more concentrated than that in bottle B. This means to get the same amount of drug C, you will have to consume a lower amount of the solution from bottle A than bottle B. Solution A will be called more “concentrated” than B whereas solution B will be called “dilute” when compared to solution A. If you ever had an experience where you received an injection of powdered medication, you must have seen the nurse add some liquid (generally distilled water) to dilute the concentrated drug. To understand more about the concentration of the solution, let us see some of the ways concentrations are expressed.
- Mass percent: This is a common way to express the concentration of a solution in which the solute is a solid substance. Mass percent refers to the weight of the solute in grams, per 100 g of the solution (and not the solvent). Thus, if 20 grams of table salt is dissolved in 80 grams of water, the concentration of this solution in mass percent will be: 20 divided by 100 (80+20) and then multiplied by 100 (to express as a percentage), which is 20%. So,
- Mass percent=(mass of solute/mass of solution)×100%
- Mass percent is also written as percentage weight/weight or w/w.
- Volume percent: When the concentration of the solute is expressed as a percentage per unit volume of the solution, it is called volume percent. If both solute and solvent are liquids, the volume of the solute is divided by the volume of the solution and expressed as volume percent (also called v/v). Thus, 10 ml of alcohol in 90 ml of water will have a volume percent of 10% (volume/volume or v/v). If the solute is a solid, volume percent may be expressed as the weight of the solute in grams per 100 ml of the solution (weight by volume or w/v). Thus, if 25 ml of sugar is dissolved in water to make 50 ml of the solution, the volume percent is 50% (w/v).
Other ways to express the concentration are:
- Parts per million (ppm): this means how many parts of the solute there are in 1 million parts of the solvent.
- Parts per billion (ppb): This means how many parts of the solute are there in 1 billion parts of the solvent.
- Molarity (M): This refers to the number of moles (weight in grams divided by the molar mass) per liter of the solution.
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Chemistry for Allied Health