Whether you realize it or not, you eat or use weak acids on an everyday basis. In this lesson, you’ll learn about the definition and properties of weak acids and become familiar with some examples.

Weak Acids

Chances are, you interact with weak acids on daily basis. They may be in the food you eat, the vitamins you take, or in the cleaning supplies you use. Weak acids also appear in our lives in less desirable ways. Have you ever had the misfortune of tasting spoiled milk? That sour, sticky flavor you taste is from a weak acid. Have you ever exercised so hard your muscles cramped up or got tired? Your exhausted muscles are dealing with the build up of a weak acid.

According to both the Arrhenius and Bronsted-Lowry definitions of an acid, an acid is a substance that produces hydrogen ions (H+) when it is in an aqueous solution, also referred to as a watery solution. The pH scale is used to measure the acidity of a solution. Acids have pH values of 0 – 7. A weak acid is an acid that does not produce many hydrogen ions (H+) when it is in an aqueous solution. By contrast, a strong acid is an acid that produces many hydrogen ions.

Properties

Weak acids taste sour, feel sticky and often burn the nostrils when smelled. Weak acids have low pH values (2-7), but not as low as strong acids (0-2). If in high concentrations, weak acids can be corrosive and even dangerous. Most of the time, however, weak acids are encountered in low concentrations.

Weak acids react with bases to produce neutral solutions. A base is a substance that is the chemical opposite of an acid; bases have high pH values and contain hydroxide ions (OH-). When strong bases need to be cleaned up, weak acids are used to neutralize them so that they will no longer be dangerous. When a weak acid reacts with a base, the hydrogen ions from the acid react with the hydroxide ions from the base to make water, a neutral and safe product.

Examples

Need to clean the coffee pot? Try running some white vinegar through it! Vinegar is a weak acid often referred to as acetic acid. Vinegar is often used as a cleaning agent and is very useful for cooking. Hot sauces, mustards, and salad dressings contain vinegar, and many foods like pickles are preserved in vinegar.

Lactic acid is a weak acid that is responsible for muscle cramps after strenuous exercise. Lactic acid builds up in the muscle cells, making it harder for the cells do their normal jobs, causing pain. Believe it or not, this is the same weak acid responsible for the sour taste of spoiled milk.

Fruits that are sour or citrus-like contain the weak acid known as citric acid. Limes, lemons, grapefruits, oranges, and even tomatoes contain citric acid. Many of these fruits also contain ascorbic acid, a weak acid also known as vitamin C.

Check out the label of a soda and you’ll see at least one weak acid listed among the ingredients: phosphoric acid. Phosphoric acid is added as a flavor enhancer. Often times, sodas contain citric acid as well.

Lesson Summary

Weak acids are common substances in everyday life. They are common in foods, in our bodies, and used as cleaning agents. An acid is a substance that produces hydrogen ions (H+) in an aqueous solution. A weak acid is an acid that doesn’t produce many hydrogen ions when in aqueous solution. Weak acids have relatively low pH values and are used to neutralize strong bases. Examples of weak acids include: acetic acid (vinegar), lactic acid, citric acid, and phosphoric acid.