Organic compounds are built around carbon, usually bonded to hydrogen. Inorganic compounds are nearly everything else, from table salt and rocks to metals and water. The word “organic” is a leftover from a time when people believed only living things could make these substances, and that belief was disproved almost 200 years ago.1Hektoen International. The discovery of urea and the end of vitalism Today the divide is a practical way chemists sort substances rather than a hard line in nature, and some compounds sit right on it.
What Makes a Compound Organic
The most common modern test is whether a compound contains carbon bonded directly to hydrogen, often called a carbon-hydrogen bond.2Wikipedia. Organic compound Carbon can form four bonds and link to other carbon atoms in chains and rings, and that flexibility allows millions of different organic molecules. Sugar, methane in natural gas, alcohol, fats, and plastics are all organic. Table salt, water, baking soda, rust, and the minerals in rocks are inorganic.
A quick screen works on the formula. Look for C and H together, like CH4 for methane or C6H12O6 for glucose. A formula with no carbon, like NaCl or H2O, is inorganic. One with carbon but no hydrogen, like CO2, needs a closer look, which comes in the next section. The screen isn’t perfect, though. Baking soda, NaHCO3, has C and H in its formula, yet the hydrogen is bonded to oxygen, not carbon, so it is inorganic. Hydrogen cyanide and carbonic acid are also usually treated as inorganic. The real test is carbon bonded to hydrogen, so the formula is a first check and the structure settles doubtful cases.
In a drawn structure, that bond appears as a line from a C to an H, or as an H written beside a carbon. Chemists often hide the hydrogens in shorthand drawings, where each corner of a zigzag line is a carbon with enough hydrogens to fill its four bonds. A bare-looking hexagon of corners is benzene, an organic compound, even though no H is written.
Some compounds without the bond still count as organic. Strictly applied, the carbon-hydrogen rule would leave out urea, a main waste compound in urine, and oxalic acid, found in spinach and rhubarb. The international rulebook for naming chemicals lists both as organic anyway.3Wikipedia. Organic compound Carbon tetrachloride, an old cleaning solvent with no hydrogen at all, is traditionally treated as organic too.4Wikipedia. Organic compound
Why Carbon Dioxide and Diamonds Are Inorganic
Carbon dioxide contains carbon but no carbon-hydrogen bond.5ThoughtCo. Why Carbon Dioxide Isn’t an Organic Compound It also forms in nonliving ways, released from volcanoes and minerals, which was the older test for being inorganic.6ThoughtCo. Why Carbon Dioxide Isn’t an Organic Compound So chemists class it as inorganic, along with carbon monoxide. Carbonates such as limestone and baking soda are treated as inorganic minerals and salts. Limestone, chalk, and marble are mostly calcium carbonate, a mineral that formed from nonliving processes and from shells. Baking soda and the scale in a kettle are carbonates too. When vinegar is poured on baking soda, the fizz is carbon dioxide being released from a carbonate.
That makes “contains carbon, so it’s organic” a common mistake. Carbon is necessary but not enough. Pencil graphite and a diamond ring are both pure carbon yet inorganic, because the bonds a carbon atom makes matter more than its presence.
Pure carbon is inorganic as well. Carbon atoms alone can arrange in different ways to make graphite, diamond, and newer forms like carbon nanotubes.7Advanced Anti-Corrosive Materials. Carbon allotropes They are a single element rather than carbon combined with hydrogen.
Graphite and diamond show how much arrangement matters. Graphite has flat sheets that slide over each other, which is why pencils leave marks. Diamond locks every atom in a rigid 3D network, making it extremely hard. Same element, very different material.
How Organic and Inorganic Compounds Behave Differently
Generally, organic compounds have low melting and boiling points, while inorganic compounds have high ones.8Chemistry Hall. What Is the Difference Between Organic and Inorganic Compounds? Many inorganic compounds are held together by ionic bonds, the attraction between charged atoms, which is much stronger than the attractions between organic molecules.9Chemistry Steps. Boiling Point and Melting Point in Organic Chemistry Table salt melts only above about 800 °C, while butter melts in a warm kitchen.
Most organic compounds are made of molecules held to each other by weaker attractions, so little heat is needed to pull them apart. Salt is a lattice of charged atoms locked together, which takes far more energy to break up.
Dissolving differs too. Organic compounds generally dissolve poorly in water but dissolve in other organic liquids, while inorganic compounds generally dissolve in water and less so in organic liquids.10Chemistry Hall. What Is the Difference Between Organic and Inorganic Compounds? Salt vanishes in water while oil floats on top, but oil stains lift with soap or solvents.
Water molecules attract one another strongly, and oily organic molecules lack that kind of pull, so they get pushed together and separate out, floating if they are lighter than water. Salt dissolves because water molecules surround the charged atoms and pull them away from the crystal.
Many organic compounds also burn, which is why wood, gasoline, and wax are fuels, while rocks and salt don’t. Their carbon and hydrogen combine with oxygen and release heat, making carbon dioxide and water. Salt and most rocks can’t burn, since they are already combined with oxygen or have no carbon-hydrogen fuel to give. Burning is only a clue: some organic materials, like certain flame-retardant plastics, resist burning, and some inorganic substances such as hydrogen and some metal powders burn.
A kitchen test shows the difference. Heat a little sugar in a pan and it melts and browns, then chars to black carbon. Heat salt the same way and nothing visible happens. Charring is one sign of an organic compound breaking down, though not every organic compound chars.
These are tendencies, not rules. Sugar is organic but dissolves easily in water, and some inorganic compounds, like sand, don’t dissolve in water at all.
Does Organic Mean It Comes From Living Things
People once believed organic substances needed a special life force to form, an idea called vitalism.11The Quantum Observer. 1828: Friedrich Wöhler Synthesizes Urea, Disproving Vitalism Then in 1828, chemist Friedrich Wöhler made urea from nonliving starting chemicals, the first organic compound made without a living thing.12Hektoen International. The discovery of urea and the end of vitalism Chemists followed with ever more complex organic substances made from inorganic ones, which disproved vitalism.13Hektoen International. The discovery of urea and the end of vitalism Organic compounds follow the same chemical laws as minerals.14The Quantum Observer. 1828: Friedrich Wöhler Synthesizes Urea, Disproving Vitalism
Wöhler was actually trying to make a different salt, and he noticed that the white crystals he made matched urea from urine. Vitalism didn’t vanish overnight, though, and it took many more lab syntheses to convince chemists.
That’s why a molecule is defined by its atoms and their arrangement, not its source. Vitamin C from a factory and from an orange is the same compound. A chemist can call a drug synthetic and organic at once. Most medicines, plastics, and synthetic fabrics are organic compounds made in factories.
Life still runs on organic compounds. Living things are built from four main types of large organic molecules: carbohydrates, fats, proteins, and DNA and RNA.15General Organic And Biological Chemistry Structures Of Life. General Organic And Biological Chemistry Structures Of Life Carbohydrates supply energy, proteins act as enzymes and hormones, and DNA directs how cells divide and build proteins.16Concepts of Biology. 2.3 Biological Molecules Smaller organic molecules such as vitamins and hormones are also made by living things.17Wikipedia. Biomolecule
The word also means something different on a food label, where organic is a legal and farming standard about how crops or animals were raised and which inputs, such as pesticides, were allowed. In chemistry, nearly all food is organic whether or not the farm was certified. Table sugar, vegetable oil, and flour are organic compounds either way, while the salt and water in a certified organic product are still inorganic. A synthetic pesticide can even be an organic compound in the chemistry sense.
Where the Two Groups Overlap
Some compounds join metals directly to carbon. These organometallic compounds contain at least one bond between a metal and a carbon atom of an organic molecule.18Wikipedia. Organometallic chemistry That puts them between organic and inorganic chemistry, and they are studied in both.19Journal of Organic & Inorganic Chemistry. Organometallic Chemistry: Bridging the Intersection of Organic and Inorganic Chemistry A metal salt of an organic acid, where the metal attaches through oxygen, doesn’t count; the metal must be attached directly to a carbon.
Many are used as catalysts, helping reactions go faster without being used up, which is how some plastics and medicines are made. Others are ingredients for making electronic materials.
Hybrid materials mix the two on purpose. Metal-organic frameworks are built from inorganic metal clusters joined by organic linking molecules, so they are called hybrid organic-inorganic materials.20Wikipedia. Metal–organic framework They are full of tiny pores, which makes them useful for trapping gases and speeding up reactions.21Coordination Chemistry Reviews. Review MOFs with bridging or terminal hydroxo ligands: Applications in adsorption, catalysis, and functionalization
Living things need inorganic substances too. Metal ions help body molecules fold into shape, drive metabolism, and carry signals.22PubMed Central. Metals Are Integral to Life as We Know It Many enzymes have a metal such as zinc at their working center.23PubMed Central. Metals Are Integral to Life as We Know It Life is organic at its core but depends on inorganic minerals.
Hemoglobin in blood is a protein holding iron to carry oxygen. Chlorophyll in leaves holds magnesium at its center. Bones combine inorganic calcium minerals with organic protein.
How Carbon Moves Between Inorganic and Organic Forms
Plants use photosynthesis to turn carbon dioxide, an inorganic compound, into organic carbon stored in their bodies and in soil.24Scitable. The carbon cycle Sunlight powers the building of sugars from carbon dioxide and water. When plants and soil life break down, that organic carbon returns to carbon dioxide.25Scitable. The carbon cycle Respiration in plants, animals, and microbes does the same by breaking sugars down to release carbon dioxide and energy. Burning wood or fossil fuels also turns organic carbon back into carbon dioxide, only far faster.
Some carbon dioxide reacts with soil minerals to form inorganic carbonates.26Scitable. The carbon cycle In the ocean, tiny living things turn dissolved inorganic carbon into organic matter that sinks to the deep sea.27Wikipedia. Carbon cycle So the same carbon atom can be inorganic in the air and organic in a leaf.
The switch can also make a substance more dangerous. Mercury released into the environment can be converted by microbes from inorganic mercury into methylmercury, an organic form.28PubMed. Global mercury pollution biogeochemical transformations, ecological consequences, human health risks, and remediation strategies Methylmercury is mercury’s most toxic form and builds up in living things.29PubMed. Global mercury pollution biogeochemical transformations, ecological consequences, human health risks, and remediation strategies In lakes and seas, microbes are the main route for this change.30PubMed Central. Overview of Methylation and Demethylation Mechanisms and Influencing Factors of Mercury in Water
Organic forms mix more easily into fats and body tissues. Methylmercury enters tiny organisms at the base of the food chain, and because it is slow to leave the body, each step up the food chain concentrates it further, leaving the most in older, larger fish.
This is why large predatory fish like swordfish and some tuna carry mercury warnings. Check local or national fish advisories, especially if pregnant or feeding young children.