Compounds are the substances that can be decomposed by chemical means. A compound contains two or more elements bonded together in a fixed proportion, and those bonds can be broken through chemical reactions to release the simpler substances inside. Elements, by contrast, cannot be broken down any further by chemical processes. This single distinction is one of the most fundamental ideas in chemistry.
Why Compounds Can Be Decomposed
A compound exists because atoms of different elements have formed chemical bonds with each other. Water is a compound of hydrogen and oxygen. Table salt is a compound of sodium and chlorine. Sugar is a compound of carbon, hydrogen, and oxygen. In every case, energy can break those bonds apart and separate the compound into simpler substances.
An element, on the other hand, is made of only one type of atom. Gold, oxygen, carbon, and iron are all elements. No chemical reaction can turn gold into something simpler because there’s nothing simpler for it to become. That’s essentially the definition of an element: a substance that can’t be broken down into anything simpler by a chemical reaction.
Chemical Decomposition vs. Physical Change
It’s worth clarifying what “by chemical means” actually requires. A chemical change alters the composition of a substance. Bonds break and new ones form, producing entirely different materials. When you decompose water, you don’t just separate it into droplets. You break the bonds holding hydrogen and oxygen together, and you end up with two gases that behave nothing like liquid water.
A physical change only alters appearance, shape, or state without changing composition. Boiling water turns it into steam, but it’s still water molecules. Crushing salt into a fine powder doesn’t change it into sodium and chlorine. Filtering sand out of a mixture doesn’t alter any chemical bonds. These physical processes can separate mixtures, but they cannot decompose compounds.
Three Ways Compounds Are Decomposed
Heat (Thermal Decomposition)
Many compounds break apart when heated to high enough temperatures. Calcium carbonate, the main ingredient in limestone and chalk, decomposes into calcium oxide (lime) and carbon dioxide gas when heated beyond about 750°C. Mercury oxide, a red powder, decomposes into liquid mercury and oxygen gas when heated. This is actually one of the earliest decomposition reactions ever studied: in the 1770s, Joseph Priestley heated mercury oxide and isolated the oxygen it released.
Baking relies on thermal decomposition too. Sodium bicarbonate (baking soda) breaks down when heated, releasing carbon dioxide gas. Those gas bubbles are what make cakes and bread rise in the oven.
Electricity (Electrolysis)
Passing an electric current through certain compounds splits them into their elements. The classic example is water: electricity decomposes two molecules of water into two volumes of hydrogen gas and one volume of oxygen gas. Potassium chlorate can similarly be decomposed by electrolysis into potassium chloride and oxygen gas. This method is especially useful for compounds that are too stable to break apart with heat alone.
Light (Photolysis)
Some compounds are sensitive to light and decompose when exposed to it. Silver chloride, used in traditional photography, breaks down under sunlight into silver metal and chlorine. Hydrogen peroxide gradually decomposes when exposed to light, which is why it’s sold in dark brown bottles. The light provides enough energy to break the chemical bonds holding these compounds together.
Common Examples at a Glance
- Water: decomposes into hydrogen and oxygen through electrolysis
- Calcium carbonate (limestone): decomposes into calcium oxide and carbon dioxide when heated
- Hydrogen peroxide: decomposes into water and oxygen, accelerated by heat, light, or catalysts
- Sodium bicarbonate (baking soda): decomposes into sodium carbonate, water, and carbon dioxide when heated
- Mercury oxide: decomposes into mercury and oxygen when heated
- Silver chloride: decomposes into silver and chlorine when exposed to light
What Cannot Be Decomposed
Elements are the substances that cannot be decomposed by any chemical means. The periodic table lists 118 elements, and none of them can be broken into simpler substances through chemical reactions. You can change an element’s physical form (melting iron, freezing nitrogen), but its atomic identity stays the same. Breaking an element down further requires nuclear reactions, not chemical ones, and that’s an entirely different category of process.
Mixtures also don’t undergo chemical decomposition in the usual sense. A mixture like saltwater can be separated by evaporation, but that’s a physical process. The salt and water were never chemically bonded to each other, so no chemical reaction is needed to pull them apart. Only compounds, where different elements are locked together by chemical bonds, require chemical means to decompose.