Class 8 Science Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures Notes

What is this chapter about?

The staircase, the air, the water in a bottle, lunch, clothes and trees are all matter, made of tiny particles. But most things are not one substance; they are several substances together. The chapter sorts matter into mixtures and pure substances, splits pure substances into elements and compounds, and shows how to tell a mixture from a compound.

Mixtures

Poha and sprout salad are made by mixing ingredients, and so are lemonade and sugar water. In a mixture, each substance, or component, keeps its own properties, and the components do not react. In a sprout salad the green gram, chickpeas, onion and tomato are easy to see, so it is a non-uniform mixture. In sugar water the particles cannot be told apart even with a microscope, so it is uniform. Alloys are uniform mixtures of metals: stainless steel (iron, nickel, chromium and a little carbon), brass (copper and zinc) and bronze (copper and tin). Ancient Indian texts called them mishraloha. Read mixtures and pure substances.

Air is a mixture

Air is a uniform mixture, mostly nitrogen (about 78%) and oxygen, with argon, carbon dioxide and water vapour. Lime water left open turns milky, because carbon dioxide from the air forms insoluble calcium carbonate. A black sheet left near a window collects dust, but dust and soot are not part of air; they are pollutants, along with gases such as carbon monoxide and sulfur dioxide, and the air quality index (AQI) describes how clean the air is.

Pure substances

‘Pure ghee’ on a pack means nothing cheaper has been added; adding it would be adulteration, which is illegal and can be harmful. In science, pure means something stricter: a pure substance contains only one kind of particle, and no physical method can split it into other kinds of matter. Milk and fruit juice are mixtures, while sugar and baking soda are pure substances. Pure substances are of two kinds: elements and compounds.

Elements

When electricity from a 9 V battery passes through water with a little dilute sulfuric acid, bubbles rise at both terminals. One gas burns with a pop: hydrogen. The other makes a flame glow brighter: oxygen. Hydrogen and oxygen are elements, substances that cannot be broken into anything simpler. Each is made of its own identical atoms, and most atoms join into molecules, such as a hydrogen molecule of two hydrogen atoms. Of the 118 known elements most are solids; eleven are gases and only mercury and bromine are liquids. Elements are metals, non-metals or metalloids. Read elements and compounds.

Compounds

Water cannot be separated into hydrogen and oxygen by any physical method, because their particles are joined chemically in a fixed ratio of two hydrogen atoms to one oxygen atom. Such a substance is a compound, and its properties are new: hydrogen is a fuel and oxygen helps burning, but water puts out fire. Soft, reactive sodium and poisonous chlorine form harmless table salt. Heating sugar leaves black carbon and gives off water, so sugar is a compound of carbon, hydrogen and oxygen.

Mixture or compound?

Mix iron filings with sulfur powder: you can still see both, and a magnet pulls the iron out. With dilute hydrochloric acid the iron gives hydrogen and the sulfur is left behind. Now heat the mixture: it becomes a black mass, iron sulfide, that a magnet cannot separate and that gives foul-smelling hydrogen sulfide with the acid. Heating made a new compound. Read difference between mixtures and compounds.

Using elements, compounds and mixtures

Iron and aluminium build bridges and vehicles; chemists combine elements into medicines, vaccines and fertilisers; alloys such as stainless steel are stronger than pure iron; and wood, steel and concrete are all mixtures. Graphene aerogel, made from carbon, is so light that grass can hold it and may help clean oil spills. Most rocks are mixtures of minerals; native minerals such as gold are pure elements, but most, like quartz and calcite, are compounds. Dhokra artists of Bihar and Odisha cast brass or bronze figures in clay moulds made from beeswax models.

Before we move on

Anything with mass that occupies space is matter, made of elements and compounds, alone or mixed. Light, heat, electricity, thoughts and feelings are real but are not matter. Answers to every question are in the NCERT solutions for Chapter 8.