Chapter 8

Nature of Matter: Elements, Compounds, and Mixtures

Look around you — the staircase, the air you breathe, the water in your bottle, the food you eat, the clothes you wear: all of these are examples of matter, made up of tiny particles. Most things around us are not made of just one substance; they are made up of two or more substances mixed together.

What Are Mixtures?

Everyday dishes like poha and sprout salad are made by mixing several ingredients — sugar dissolved in water, soups, and lemonade are mixtures too. When two or more substances are mixed, where each substance retains its properties, it is called a mixture. The individual substances that make up a mixture are called its components, and they do not react chemically with each other.

In some mixtures, such as green gram, chickpeas, onion, and tomato in a sprout salad, the components are easy to see with the naked eye or a magnifying device — these are non-uniform mixtures. In others, like sugar dissolved in water, the components cannot be seen separately even with a microscope; these are uniform mixtures, since the components are evenly distributed and cannot be distinguished.

Is Air a Mixture?

Air is a uniform mixture of mainly nitrogen, oxygen, argon, carbon dioxide, and water vapour. Oxygen is required by most living beings and helps combustion; nitrogen, about 78% of the air, does not take part in combustion. When warm, water-vapour-laden air touches a cool surface, the vapour condenses into tiny liquid droplets.

Adding calcium oxide to water forms calcium hydroxide, called lime water. When exposed to air, lime water turns milky because carbon dioxide reacts with calcium hydroxide to form calcium carbonate (insoluble tiny white particles) and water — this confirms that air contains carbon dioxide.

Air also carries suspended dust particles — tiny shining specks visible in a beam of sunlight, or settling on a clean black sheet of paper left near an open window. These particles are not an integral part of the air and are considered pollutants; their nature and number vary with time and place.

Types of Mixtures

In everyday usage, "mixture" refers to any combination of two or more components — even mixtures of mixtures, like poha. In science, however, all the components of a mixture must be pure substances. Mixtures can be classified by the physical states of their components:

• Gas and gas — e.g. air (uniform)
• Gas and liquid — e.g. aerated water, oxygen dissolved in water
• Solid and gas — e.g. carbon particles in air
• Liquid and liquid — e.g. acetic acid in water (vinegar), oil and water
• Solid and liquid — e.g. sand and water, seawater
• Solid and solid — e.g. baking powder (baking soda and tartaric acid), alloys

Mixtures can be separated into their components. In everyday life this is done to obtain the component of interest and discard the rest; in science, the purpose of separation is to obtain pure substances.

What Are Pure Substances?

The word "pure" has different meanings in common usage and in science. In common usage, "pure" means unadulterated — free of cheaper or lower-quality substances illegally added to increase quantity or cut manufacturing cost, which can also make a product hazardous to health.

In science, a pure substance is one that has no other substance present in it — it cannot be separated into other kinds of matter by any physical process, and it consists of the same type of particles throughout. By this definition, even everyday products labelled "pure" can be impure if they are made of more than one substance.

What Are the Types of Pure Substances?

Water can be cooled to ice or heated to vapour and back — through these changes, its particles remain the same. But passing electricity through water is different: filling test tubes with acidified water over a battery's terminals produces gas bubbles at each terminal. Bringing a burning candle near each test tube reveals the gases — a pop sound signals hydrogen, while a brighter flame confirms oxygen. Since these gases don't condense back into water, they cannot be water vapour; water is composed of two different constituents, hydrogen and oxygen.

Elements

Hydrogen and oxygen, formed by splitting water, are pure substances termed elements — substances that cannot be broken down into simpler substances, and the building blocks of all matter. Each element is made up of identical particles called atoms, different from the particles of any other element. Gold, silver, sulfur, and carbon are other examples.

Elements are classified into metals, non-metals, and metalloids:

• Metals — e.g. gold, silver, magnesium, iron, aluminium
• Non-metals — e.g. carbon, sulfur, hydrogen, oxygen
• Metalloids — elements like silicon and boron with intermediate properties between metals and non-metals

Compounds

Hydrogen and oxygen in water are so tightly attached that they generally cannot be separated by physical means — that is why water is a compound. Compounds form when different elements combine in fixed ratios to make something entirely new, with properties different from the elements that formed it. In water, hydrogen and oxygen atoms combine chemically in a fixed 2:1 ratio.

Sodium (a soft metal) and chlorine (a hazardous gas) combine to form sodium chloride, common salt, in a 1:1 ratio — harmless and taste-enhancing, even though dissolved salt can be separated from water by evaporation, its sodium and chlorine cannot be separated by physical processes.

Heating sugar turns it brown, then black (charred), releasing water droplets and leaving behind charcoal (carbon). Since the water must have come from the dry sugar itself, sugar decomposes into carbon and water on heating — and since water itself consists of hydrogen and oxygen, sugar cannot be an element. It is a compound of carbon, hydrogen, and oxygen.

Mixing iron filings and sulfur powder (Sample A) gives a mixture whose black and yellow particles are still visible; a magnet pulls out the iron, showing the two elements can still be separated, and iron in the mixture reacts with dilute hydrochloric acid to give iron chloride and colourless, odourless hydrogen gas that burns with a pop. Heating that same mixture, however, forms a uniform black mass, iron sulfide (Sample B) — a magnet no longer attracts it, and its properties are entirely different from those of iron and sulfur. Iron sulfide reacts with dilute hydrochloric acid to give iron chloride and hydrogen sulfide gas, colourless with a rotten-egg odour. Because its constituents can no longer be separated, Sample B shows that a compound, not a mixture, has formed.

How Do We Use Elements, Compounds, and Mixtures?

Elements, compounds, and mixtures are all around us: the air we breathe is a mixture of gases, water is a compound of hydrogen and oxygen, and elements like iron and aluminium build bridges, buildings, and vehicles.

Understanding these concepts drives innovation — chemists study how elements combine into compounds to invent life-saving medicines, vaccines, and fertilisers that boost crop production for a growing population. Engineers and material scientists design materials with unique properties, developing alloys like stainless steel, stronger and more durable than pure iron. Wood, steel, and concrete — common building materials — are all mixtures too.

What Are Minerals?

Most rocks are mixtures of minerals, visible with the eye, a magnifying glass, or a microscope. Some minerals, called native minerals, are pure elements rather than compounds — metals like gold, silver, and copper, or non-metals like sulfur and carbon. Most minerals, however, are compounds made of more than one element, such as quartz, calcite, mica, pyroxene, and olivine.

Many everyday items are made from minerals or elements extracted from them — cement is made from calcite, quartz, alumina, and iron oxide, while talcum powder is made from the mineral talc.

Elements and compounds are the building blocks of matter — everything that has mass and takes up space. But not everything around us is matter: light, heat, electricity, and even thoughts and emotions are important parts of our world without being made of matter. Understanding what matter is — and what it is not — helps us better understand the world around us.