 You may be familiar with the term acid to describe a substance that has a pH lower than 7, turns blue litmus paper red, and turns universal indicator solution, or paper, red, or orange, all depending on its strength. An acid is simply a substance that dissociates to release a hydrogen ion or proton in water. Let's start by thinking about citrus fruits, oranges, lemons, and limes. Many citrus fruits have a very defining sour taste to them. This sour taste is due to the presence of a certain weak acid, citric acid. It is a common misconception that citric acid is vitamin C. In fact, vitamin C is ascorbic acid, and although it is found in citrus fruits, kiwi fruits and red peppers have a much higher ascorbic acid concentration. Another food item that you may be familiar with is vinegar. Vinegar is a very dilute solution of ethanoic acid, also known as acetic acid. Like citrus fruits, vinegar has a very distinctive sour taste. Rhubarb leaves are toxic because of a high concentration of oxalic acid. Oxalic acid, however, is still found in low concentrations in spinach, parsley, and chives. Let's open a cold soft drink can or bottle in our minds. Most likely, you imagine many bubbles rising from the drink. This happens because carbonic acid is formed when carbon dioxide is fully dissolved in soft drinks during the manufacturing process. Here is a challenge for you. You know that carbon dioxide can dissolve in water to give carbonic acid. Can you write down the equation for this reaction? Pause the lesson and continue when ready. The reaction equation is carbon dioxide and water equilibrates to give carbonic acid. Notice the reaction arrow that shows you that this is a reversible reaction. This carbonic acid can dissociate to give hydrogen carbonate and a proton. In fact, an acidic solution is made because of the presence of these protons. The extent of the dissociation of carbonic acid is critical in maintaining the optimal pH. Our blood has a very specific pH 7.41. If the concentration of protons is too high, it will increase the acidity of blood. Even at 7.35, the medical condition is known as acidosis. If the concentration of protons is too low, it will decrease the acidity of blood. Even at 7.45, then the condition is known as alkalosis. If you exercised for an extended period of time, your muscles will begin to cramp. This cramping feeling is due to the presence of lactic acid produced by your muscle cells when there is insufficient oxygen. Milk contains lactose and certain types of bacteria can ferment lactose into lactic acid. The lactic acid then reacts with a certain milk protein called chazen. The combination of lactic acid and curdled chazen gives yogurt and sour cream their texture and taste. So to summarize, there are many kinds of acids, not just hydrochloric acid and sulfuric acid. A lot of these acids can be found in everyday food items, inside our body and in many other real-life applications.