How can you tell that mixing made something new? Gas bubbles are one clue that mixing has made something new. Another clue is a precipitate: when two clear liquids are mixed and a solid suddenly appears, that solid is called a precipitate, and it is a sign of a reaction. Rust is evidence of a different kind. The iron in rust has changed into a new kind of matter, one that iron makes with oxygen and water, so the iron has become something else, and a new substance is now there.
Acids and bases react with each other, and the result has a name. In a neutralization reaction, an acid and a base react, and the products are often a salt and water. Acetic acid is the acid in vinegar, and lemon juice and other citrus fruits taste sour because of the acids in them. Baking depends on the same kind of reaction. Baking soda in a cake batter reacts with a sour ingredient such as buttermilk or lemon juice to make gas bubbles, and the batter rises. Baking powder is baking soda packed with a dry acid, so add water and it fizzes.
A solvent cannot take in an unlimited amount of a solute. Solubility means the largest amount of solute that can dissolve in a solvent under given conditions. A saturated solution holds that much, and a supersaturated solution holds more. Temperature is one of the conditions, and it changes how well gases dissolve. Gases usually dissolve less well in warm water than in cold water. Bubbles of air appear in a glass of cold water as it warms up, because the warmer water cannot keep all of the gas that the cold water held.
A fizzy drink shows the idea of supersaturation. A fizzy drink in an open container is supersaturated with carbon dioxide. Over time the extra gas leaves, and the drink goes flat. The bubbles in the drink are carbon dioxide, the same gas that makes baking soda and vinegar fizz. In both cases the gas appears as bubbles, and in the drink the bubbles are the extra gas that the liquid cannot keep. Once enough has left, the drink goes flat, and it has gone flat because that extra gas has left it.
Some mixes are not clear solutions. The particles in a colloid are big enough to scatter light, and this is called the Tyndall effect. A colloid can look cloudy for that reason. In an emulsion, drops of the spread-out liquid join together, so emulsions tend to separate. An emulsifying agent stops the drops from joining and keeps the emulsion stable. Egg yolk contains lecithin. Each lecithin molecule has an end that is attracted to water and an end that is attracted to oil, so droplets of oil can be held in water to make mayonnaise.
Living things need their reactions to go fast, and they have a tool for that. A catalyst is a substance that helps a chemical reaction happen, and enzymes are the special molecules that act as catalysts for the reactions of living things. The speed-up is large. Enzymes can make reactions go 100 million to 10 billion times faster than they would go on their own. A reaction that would crawl without an enzyme can race with one, and that gap is why living things rely on enzymes.
Reactions can give off heat or take it in, and chemists keep track of which. They give an enthalpy change a negative sign for an exothermic reaction, where heat goes out. They give it a positive sign for an endothermic reaction, where heat goes in. So a negative sign means the reaction gives heat out, and a positive sign means the reaction takes heat in. The sign of the enthalpy change tells a chemist at a glance which way the heat goes in a reaction. In this way, the sign gives the direction of the heat. Exothermic and endothermic are opposites: one sends heat out and the other takes heat in.
Browning in the kitchen is chemistry too. Bread gets the smell and taste of its brown crust from the Maillard reaction, which happens as it bakes. Sugar browns by a different route. Caramelization is the name for the first stage of heating sugar, when it turns brown, and it gives caramel apples and caramelized onions a sweet, nutty taste. The brown colour on a crust and on a caramel apple comes with changes in smell and taste that the reactions bring, so the colour is a clue that heat has changed the food.
Words to know
- solubility: how much dissolves
- saturated: full of solute
- colloid: cloudy mix
- emulsifying: keeping drops apart
- caramelization: sugar browning
