Organic and Inorganic Chemistry

Introduction

On the last page you met the branches of chemistry. Two of the oldest are organic and inorganic chemistry. The dividing line between them is the element carbon. This page explains what makes a compound organic, why carbon is so special, what inorganic chemistry covers, and why a few carbon compounds are treated as inorganic. By the end you should be able to sort everyday substances into the right group and explain your choice.

Core explanation

Organic compounds are built around a framework of carbon atoms. In almost all of them carbon is bonded to hydrogen, and many also contain oxygen, nitrogen, sulfur or halogens. The simplest organic compounds are hydrocarbons, which contain only carbon and hydrogen. Methane (CH₄), the main part of natural gas, is the smallest hydrocarbon. Petrol, candle wax and cooking gas are mixtures of hydrocarbons. Adding other atoms creates new families: alcohols such as ethanol, acids such as the ethanoic acid in vinegar, and the sugars, fats and proteins in our food.

The name "organic" comes from an old belief that these compounds could only be made by living organisms. In 1828 Friedrich Wöhler made urea, a compound found in urine, from inorganic starting materials, helping to show that the same chemical laws apply to living and non-living matter. Today chemists make millions of organic compounds in laboratories and factories.

Inorganic compounds include everything else: salts such as sodium chloride, metal oxides such as rust and quicklime, acids such as hydrochloric and sulfuric acid, bases such as sodium hydroxide, minerals in rocks, and simple molecules like water and ammonia. Inorganic chemistry also covers the elements themselves, from reactive metals such as sodium to unreactive gases such as neon.

The exceptions. A few small carbon-containing substances are traditionally treated as inorganic because they behave like minerals or simple gases rather than like typical organic molecules. These include carbon dioxide (CO₂), carbon monoxide (CO), carbonates such as calcium carbonate (CaCO₃, the main part of limestone and chalk), hydrogencarbonates such as baking soda, and cyanides. The pure forms of carbon — diamond and graphite — are also usually studied in inorganic and materials chemistry.

Organic and inorganic compounds often behave differently. Many organic compounds melt at low temperatures, burn easily and do not dissolve well in water. Many inorganic salts have high melting points, do not burn and dissolve in water to form ions. These are general trends, not strict rules, and there are exceptions on both sides.

Step-by-step reasoning

To classify a compound:

1. Does it contain carbon? If not, it is inorganic. 2. If it contains carbon, is it CO, CO₂, a carbonate, a hydrogencarbonate, a cyanide or pure carbon? If so, it is usually classed as inorganic. 3. Otherwise, if it has a carbon framework bonded to hydrogen, it is organic.

Visual explanation

Substance Contains carbon? Classification Reason --- --- --- --- Methane, CH₄ Yes Organic Carbon bonded to hydrogen Ethanol, C₂H₅OH Yes Organic Carbon chain with an OH group Glucose, C₆H₁₂O₆ Yes Organic Carbon framework, a sugar Carbon dioxide, CO₂ Yes Inorganic Traditional exception Calcium carbonate, CaCO₃ Yes Inorganic Carbonate exception Sodium chloride, NaCl No Inorganic No carbon Water, H₂O No Inorganic No carbon

Real-world analogy

Think of a large family with one very big branch. Organic chemistry is the huge branch descended from one ancestor, carbon, with millions of members. Inorganic chemistry is the rest of the family — fewer members per element but covering more than a hundred elements. A few relatives, like carbon dioxide, share the carbon surname but live with the other side of the family.

Real-world example

A plastic water bottle and the water inside it show both kinds of chemistry. The bottle is made of a long organic polymer built from carbon, hydrogen and oxygen. The water is a simple inorganic compound, and any minerals dissolved in it, such as calcium and magnesium salts, are inorganic too.

Why?

Why do so many organic compounds burn while most inorganic salts do not? Burning is a reaction with oxygen that releases energy. Organic compounds contain carbon and hydrogen that have not yet combined with oxygen, so they can react to form carbon dioxide and water, releasing energy. Salts such as sodium chloride are already in a stable, low-energy state, so reacting with oxygen releases no useful energy.

Common misconception

Some students think any compound containing carbon must be organic. Carbon dioxide and limestone contain carbon but are classed as inorganic. Another misconception is that organic compounds are always natural; plastics, many medicines and synthetic dyes are organic compounds made in factories.

Worked example

Question: Classify (a) propane, C₃H₈, (b) sodium hydrogencarbonate, NaHCO₃, and (c) potassium iodide, KI.

Reasoning: (a) is a hydrocarbon with a carbon chain. (b) contains carbon but is a hydrogencarbonate, an inorganic exception. (c) contains no carbon.

Answer: (a) organic; (b) inorganic; (c) inorganic.

Quick check

1. What is a hydrocarbon? Answer: A compound that contains only carbon and hydrogen.

Exam focus

Remember the list of carbon-containing exceptions (CO, CO₂, carbonates, hydrogencarbonates, cyanides). Examiners like to test whether students can apply the carbon rule carefully instead of mechanically.

Advanced insight

The classification is partly historical, and chemists do not all draw the line in exactly the same place. Organometallic chemistry, which studies compounds with direct carbon–metal bonds, sits right on the border and is studied by both organic and inorganic chemists. You can explore carbon frameworks such as methane, ethene and benzene in the Molecule Builder.

Summary

Organic chemistry studies compounds with a carbon framework, usually bonded to hydrogen, including fuels, plastics, medicines and the molecules of life. Inorganic chemistry studies all other compounds and elements, such as salts, minerals, metal oxides and simple gases. A few carbon compounds — CO, CO₂, carbonates, hydrogencarbonates and cyanides — are traditionally classed as inorganic.

Practice questions

1. What element defines organic chemistry? Answer: Carbon. 2. Why is calcium carbonate classed as inorganic even though it contains carbon? Answer: Carbonates are a traditional exception: they behave like minerals and salts rather than like typical organic compounds. 3. What did Wöhler's synthesis of urea show? Answer: That an organic compound found in living things could be made from inorganic substances, so living and non-living matter follow the same chemical laws. 4. State one general difference between typical organic compounds and typical inorganic salts. Answer: Organic compounds often have low melting points and burn easily, while inorganic salts usually have high melting points, do not burn and often dissolve in water.