Elements and compounds

Splitting water

Heating or cooling water only changes its state; the particles stay water. Passing electricity through it is different. In Activity 8.3, two water-filled test tubes are placed over the terminals of a 9 V battery in water with a few drops of dilute sulfuric acid. Bubbles rise at both terminals, and one tube collects about twice as much gas as the other. A burning candle held near the first tube gives a pop: the gas is hydrogen. Near the second, the flame glows brighter: the gas is oxygen. So water is made of two different substances, and splitting it is a chemical change.

Elements, atoms and molecules

Hydrogen and oxygen cannot be broken down any further. Such pure substances are elements, the building blocks of all matter; gold, silver, sulfur and carbon are others. Each element is made of identical particles called atoms, different from the atoms of any other element. Most atoms do not exist alone: two or more join to form a stable molecule, so a hydrogen molecule is two hydrogen atoms and an oxygen molecule two oxygen atoms.

How many elements are there?

Scientists know 118 elements, and most of them are solids. Eleven are gases at room temperature, all non-metals, such as oxygen, nitrogen and helium. Only two are liquids: mercury, a metal, and bromine, a non-metal; gallium and caesium are solids that melt at about 30 °C. Elements are grouped as metals (gold, silver, magnesium, iron, aluminium), non-metals (carbon, sulfur, hydrogen, oxygen) and metalloids, such as silicon and boron, whose properties lie in between. A single mobile phone uses more than 45 elements.

Compounds: a fixed ratio and new properties

In water, hydrogen and oxygen particles are held so tightly that no physical method can pull them apart. Water is a compound: elements joined chemically in a fixed ratio, here two hydrogen atoms for every oxygen atom. A compound’s properties are its own. Hydrogen is a fuel and oxygen helps things burn, yet water puts out fire. Sodium, a soft metal, and chlorine, a dangerous gas, join one to one as sodium chloride, the common salt that flavours food. Dissolved salt can be recovered from water by evaporation, but no physical process splits salt itself into sodium and chlorine.

Is sugar an element?

Heat a teaspoon of sugar gently in a boiling tube. It turns brown and then chars black, and droplets of water appear near the open end. The tube is being heated, so the water cannot come from the air; it comes from the sugar. Black carbon, or charcoal, is left behind. Since sugar breaks down into carbon and water, and water is hydrogen and oxygen, sugar is a compound of carbon, hydrogen and oxygen.

Elements and compounds in rocks

Most rocks are mixtures of minerals. A few minerals, called native minerals, are pure elements: metals like gold, silver and copper, and non-metals like sulfur and carbon. That is why gold counts as both a mineral and a metal. Most minerals, however, are compounds. Compare mixtures with compounds in the difference between mixtures and compounds, revise mixtures and pure substances, or read the Chapter 8 notes and NCERT solutions.