Chemistry

What chemistry studies

Chemistry is the study of matter: anything that has mass and takes up space. Air, water, rock, food, and your own body are all matter.

All matter is made of tiny particles called atoms. There are about ninety kinds of atoms found in nature, and each kind is called an element. Oxygen, carbon, iron, and gold are elements. The periodic table is the chart that lists every element in order.

Each atom has a center, the nucleus, made of protons and neutrons. Smaller particles called electrons move around the nucleus. The number of protons decides which element an atom is: every atom with six protons is carbon, and every atom with eight is oxygen.

Molecules and compounds

Atoms join together, or bond, to make molecules. A molecule of water is two hydrogen atoms bonded to one oxygen atom. Its chemical formula is H₂O: the small 2 means two hydrogen atoms, and the O with no number means one oxygen atom.

When two or more different elements bond, the result is a compound. A compound can behave nothing like the elements that make it. Sodium is a soft metal that reacts violently with water, and chlorine is a poison gas, but bonded together they make table salt.

Solids, liquids, and gases

Matter usually comes in one of three states. In a solid, the particles are packed close and hold their places, so a solid keeps its shape. In a liquid, the particles are still close but slide past each other, so a liquid takes the shape of its container. In a gas, the particles are far apart and move freely, so a gas spreads to fill any space.

The particles in matter are always moving. Heat makes them move faster. Temperature is a measure of how fast, on average, they are moving. Add heat to ice and it melts into water; add more and the water boils into steam. Take heat away and the steps run backward: steam condenses into water, and water freezes into ice.

Physical and chemical changes

The test often asks you to tell two kinds of change apart. In a physical change, a substance changes its form but stays the same substance. Melting ice, tearing paper, and dissolving sugar in water are physical changes. The ice is still water; the sugar is still sugar.

In a chemical change, also called a chemical reaction, the atoms rearrange and new substances form. Burning wood, rusting iron, and baking a cake are chemical changes. Signs that a chemical change has happened include heat or light given off, a gas bubbling out, a new color, or a new smell. A chemical change is usually hard to undo: you cannot turn ash back into wood.

Mixtures and solutions

A mixture is two or more substances mixed together but not bonded. Each keeps its own properties, and they can be separated again. Sand and iron filings are a mixture; a magnet pulls the iron out.

A solution is a mixture in which one substance dissolves evenly into another, like salt in water. The substance that dissolves (the salt) is the solute. The substance it dissolves into (the water) is the solvent. The more solute in a given amount of solvent, the more concentrated the solution.

Nothing is lost: conservation of mass

In a chemical reaction, atoms are not created or destroyed. They only rearrange. So the total mass before a reaction equals the total mass after it. This rule is called the conservation of mass.

When wood burns, it seems to disappear. But if you could catch and weigh the ash, the smoke, and the gases that rise from the fire, they would weigh as much as the wood and the oxygen it burned with. When iron rusts, the rust weighs more than the iron did, because the iron has combined with oxygen from the air.

A balanced chemical equation shows this on paper. Hydrogen and oxygen react to make water:

2H₂ + O₂ → 2H₂O

Count the atoms on each side. On the left, there are four hydrogen atoms and two oxygen atoms. On the right, there are also four hydrogen atoms and two oxygen atoms. The equation is balanced because nothing was lost.

Energy in reactions

Some reactions give off heat. These are called exothermic reactions. Burning gas on a stove and the hand warmers people carry in winter are exothermic.

Other reactions take in heat from their surroundings, so the surroundings get colder. These are called endothermic reactions. The instant cold pack in a first-aid kit is endothermic.