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The pH Scale
Acidity is a chemical property of a substance. It is measured using the pH scale, which is based on the ratio of hydrogen-based ions produced when the substance is in solution with water. Acids produce a higher percentage of hydronium ions (H3O+) in solution, while bases (alkaline substances) produce a higher percentage of hydroxide ions (OH-) in solution. The pH scale assigns a pH value to substances on a scale from 0-14, with a pH of 7 indicating a neutral substance (neither acidic or basic). Any substance with a pH value less than 7 is considered an acid, with stronger acids having lower pH values; any substance with a pH value greater than 7 is considered a base, with stronger bases having a higher pH.
Characteristics of Acids and Bases
Acids tend to have a sour taste and react with metals and carbonates. They can be classified using blue litmus paper, which turns red in the presence of an acid. Common acids include citrus fruit, vinegar, and tea. Bases tend to have a bitter taste and a slippery feel. They can be classified using red litmus paper, which turns blue in the presence of a base. Bases are often used as cleaning products, including soap, toothpaste, and bleach. Both acids and bases, if they are strong enough, can cause serious chemical burns to human tissue (skin/eyes).
Neutralization Reactions
When an acid is mixed with a base, it often forms a type of chemical reaction called a neutralization reaction. Despite the name, neutralization reactions do not always result in a neutral product. For example, if a strong acid reacts with a weak base, the product won’t necessarily have a pH of 7; it may be a weaker acid. Neutralization reactions are often exothermic, meaning they give off heat. Frequently, the products of a neutralization reaction will be water and some type of salt. For example, if hydrochloric acid (acid) reacts with sodium hydroxide (base), the products are H2O (water) and NaCl (table salt). Not all salts are the type of salt you are familiar with because you put it on your food, however. “Salt”, in chemistry, is a general term for many ionic compounds. Salts can occur in a variety of colors. They can even burn a wide variety of bright colors and are sometimes used to make colored fireworks. Strangely, salts can also have a variety of tastes beyond “salty”, including sweet, sour, and bitter. Finally, while salts themselves are electrically neutral, they can be good conductors of electricity when dissolved in water.
