Start with two things from a kitchen cupboard. Vinegar contains an acid, while baking soda is a base, and the two react with each other. The acid that makes vinegar sour is acetic acid, and another name for baking soda is sodium bicarbonate. Citrus fruits, lemon juice included, get their sour taste from the acids that are inside them. Put the two together and they bubble up, and the gas in those bubbles is carbon dioxide. A stream of gas is one hint that mixing has produced something new. The baking soda and an acid first form carbonic acid, which breaks down, and the gas it gives off forms the bubbles.
Chemists have defined the words acid and base in more than one way. In 1884 Svante Arrhenius called a compound an acid if it gives hydrogen ions when dissolved in water, and a base if it gives hydroxide ions. A broader definition followed in 1923. By that one, an acid is a compound that gives a proton, which is a hydrogen ion, to another compound, and a base is a compound that takes a proton. That is a broader idea than Arrhenius's. Neutralization is the name for an acid reacting with a base, and it often yields a salt and water. Chemists also label whole solutions. A solution is neutral when its hydronium and hydroxide ions are present in equal amounts, acidic when hydronium ions outnumber hydroxide ions, and basic when they are outnumbered.
Next comes a number that sums up acidity. The idea of pH dates to 1909, when the Danish biochemist S. P. L. Sørensen introduced it. In the name, the p refers to the power of ten of the hydrogen ion concentration, and the H is the symbol for hydrogen. The numbers run from zero up to 14, and seven, in the middle, is neutral: anything lower is acidic and anything higher is basic. Every step along the scale is a tenfold change, which means that a sample at pH 5 has ten times the acidity of one at pH 6. Neither vegetable oil nor alcohol has a pH, since only water-based solutions do.
Red cabbage juice is purple to begin with, and it changes color after something is mixed into it, passing through six colors in all: purple, pink, red, blue, green and yellow. Acids push it first to pink and finally to red, while bases push it to blue, then green, ending at yellow. Since its color in an acid differs from its color in a base, it works as a natural indicator, and berries and beetroot are among the other plants that can do the same. Blood is a buffered solution. A buffer resists changes in pH, and in blood the main partners are the bicarbonate ion and carbonic acid, which soak up any added acid or base and keep the blood steady.
Not every mixture is a reaction. Salt dissolves in water, and a solution is uniform throughout, with its parts mixed evenly. Oil and vinegar behave differently, because they separate into two layers with the oil floating on top. Shaken oil and water stay mixed for only a short while before they part again, although egg yolk can hold oil and water together, as it does in mayonnaise, and milk is an emulsion too, like many cosmetics. Wax melting is a physical change that makes no new substance, whereas a chemical change does. When two clear liquids are combined and a solid abruptly forms, that solid is a precipitate and signals a reaction, because a precipitation reaction turns dissolved substances into a solid product.
Gas bubbles make cakes and bread rise, and gas trapped as bubbles inside them is what lifts both. In cake batter, a sour ingredient like buttermilk or lemon juice reacts with the baking soda and gives off gas, so the batter swells. Baking powder contains baking soda along with a dry acid, and wetting it makes it fizz. In bread dough, yeast gives off carbon dioxide as it ferments, and that gas stretches the dough's tiny air bubbles. Heat changes an egg too, since cooking transforms food. At home, one rule covers every kind of mixing: cleaning products must never be mixed together, because the fumes they can make are poisonous and hard to breathe.
Corrosion is metal being worn away by an electrochemical process, and iron rust, silver tarnish and the green-blue patina on copper are all cases of it. Iron rusts after sitting in air and water, and the rust is no longer iron but a new kind of matter formed from iron together with oxygen and water. A layer of paint, grease or plastic stops rust by keeping air and water away from the iron. Heating or cooling can also be a clue, since some reactions give off energy while others take energy in. Antoine Lavoisier, a French chemist who lived from 1743 to 1794, demonstrated through careful weighing that the substances coming out of a reaction weigh as much as those that went in. Heating metals in sealed vessels and weighing everything, he found that the metal took on mass equal to what the air gave up. Whatever the reaction, the total quantity of matter stays constant, and nothing disappears.
Words to know
- acid: gives away protons
- proton: hydrogen ion
- buffer: resists pH change
- precipitate: solid from liquids
- indicator: color clue
