What are acids and bases?
BLUF: Acids are substances that donate hydrogen ions (protons) in water, while bases accept them or release hydroxide ions. Acids taste sour and turn litmus red; bases feel slippery and turn litmus blue. The pH scale measures how acidic or basic a solution is.
This single push-and-pull of protons drives digestion, cleaning, battery chemistry, and much of the reactivity that keeps living cells alive.
The proton exchange
An acid is a substance that gives away a hydrogen ion, which is just a proton, when dissolved in water. A base does the opposite: it takes protons up, often by releasing hydroxide ions that grab them. This proton transfer, spelled out independently by chemists Johannes Bronsted and Thomas Lowry in 1923, is the heart of the reaction. When an acid meets a base, protons move from one to the other, and the two neutralize each other, typically forming water and a salt. Hydrochloric acid in your stomach and the ammonia in cleaning sprays are both defined by this single behavior. Strong acids and bases hand off or grab protons almost completely; weak ones do it only partly, leaving most molecules intact in solution.
Measuring strength with pH
Water itself splits slightly into hydrogen ions and hydroxide ions, and the balance between them defines acidity. The pH scale, introduced by Soren Sorensen in 1909, runs from 0 to 14 and tracks the concentration of hydrogen ions. It is logarithmic: each step of one unit means a tenfold change. A pH of 3 is ten times more acidic than a pH of 4 and a hundred times more acidic than a pH of 5. Pure water sits at 7, the neutral point where hydrogen and hydroxide ions are equal. Below 7 is acidic, above 7 is basic or alkaline. Because the scale compresses enormous ranges into small numbers, it lets chemists compare battery acid, blood, and bleach on one tidy ruler.
Acids and bases at work
Your blood is held near pH 7.4 by buffers that resist change, since even small shifts can be dangerous. Stomach acid, close to pH 2, breaks down food and kills microbes. Farmers test soil pH because crops absorb nutrients only within certain ranges, and gardeners add lime, a base, to sweeten sour soil. Baking works when an acid like buttermilk reacts with baking soda, a base, releasing carbon dioxide that makes batter rise. Car batteries store energy in sulfuric acid. Antacids are bases that neutralize excess stomach acid to ease heartburn. Ocean water is slowly becoming less alkaline as it absorbs carbon dioxide, a shift called ocean acidification that stresses shell-building creatures. From soap to soda, the same proton chemistry is quietly at work.
Common misconceptions
Myth: strong and concentrated mean the same thing. Reality: strength describes how fully an acid releases protons, while concentration describes how much is dissolved; a strong acid can be dilute. Myth: all acids are dangerous and corrosive. Reality: citric acid in lemons and acetic acid in vinegar are mild and edible, while some bases like lye are far more caustic than weak acids. Myth: pH cannot go below 0 or above 14. Reality: those are just the familiar range for water solutions; very concentrated acids and bases can exceed them. Myth: acids and bases are opposites that always cancel out. Reality: neutralization depends on amounts, and mixing them can leave a solution that is still acidic or basic.