Class 8 Science · Chapter 8 NotesNature of Matter: Elements, Compounds, and Mixtures

When two or more substances are mixed and each substance keeps its own properties, the result is a mixture. The substances that make it up are called its components, and they do not react chemically with each other. In a sprout salad you can easily pick out green gram, chickpeas, onion and tomato, so the mixture is non-uniform. In sugar dissolved in water the particles are spread evenly and cannot be seen separately, so the mixture is uniform. Stainless steel, brass and bronze are uniform mixtures of metals called alloys; stainless steel contains iron, nickel, chromium and a little carbon. Ancient Indian texts such as the Charaka Samhita and Susruta Samhita mention mishraloha, the name for a mixture of two or more metals with properties different from those metals.

• Mixturetwo or more substances mixed together, each retaining its properties
• Componentsthe individual substances that make up a mixture
• Uniform mixturecomponents evenly distributed and not distinguishable
• Non-uniform mixturecomponents generally visible to the naked eye or with a magnifying device
• Alloya uniform mixture of metals, such as brass (copper and zinc) or bronze (copper and tin)

Air is a uniform mixture of mainly nitrogen, oxygen, argon, carbon dioxide and water vapour. Nitrogen makes up about 78% of air and does not support combustion, while oxygen is needed by most living beings and helps in combustion. You can confirm that air contains carbon dioxide with lime water. Calcium oxide (quick lime) is added slowly to water to form calcium hydroxide and heat is released; the filtered solution is lime water. When this colourless lime water is left in a petri dish and stirred now and then, it turns milky because carbon dioxide from the air forms tiny insoluble particles of calcium carbonate. Dust particles suspended in air settle on a black sheet kept near an open window; they are pollutants, not an integral part of air.

• Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
• Lime water turning milky shows the presence of carbon dioxide in air
• Major air pollutantsparticulate matter (dust, soot) and gases such as carbon monoxide, ozone, nitrogen dioxide and sulfur dioxide
• Air quality index (AQI) is a tool used to describe air quality

On packets of milk, ghee or spices, the word pure means unadulterated — no cheaper or poorer quality substance has been added. Adulteration is an illegal process of adding such substances to increase quantity or reduce cost, and it can make a product hazardous to health. In science the meaning is different. A pure substance has no other substance present in it and consists of the same type of particles, all behaving identically. It cannot be separated into other kinds of matter by any physical process. Judged this way, a product that looks pure may still be impure if it is made of more than one substance. Pure substances are of two types: elements and compounds.

• Pure substancemade of the same type of particles and not separable into other kinds of matter by physical processes
• Adulterationillegal addition of cheaper or poor quality substances to a product
• Pure substances are either elements or compounds

When electric current is passed through water containing a few drops of dilute sulfuric acid, gas bubbles collect at both terminals. The gas that gives a pop sound with a burning candle is hydrogen; the gas that makes the flame glow brighter is oxygen. So water is made of two different constituents, hydrogen and oxygen. These two gases are pure substances called elements. An element is made of identical particles called atoms, which differ from the atoms of every other element, and it cannot be broken down into simpler substances. Gold, silver, sulfur and carbon are elements. Most atoms do not exist independently; two or more combine into a molecule, as in hydrogen and oxygen. Elements are classified as metals, non-metals or metalloids such as silicon and boron.

• Water → Hydrogen + Oxygen (on passing electric current)
• Hydrogen testpop sound with a burning candle
• Oxygen testflame glows brighter
• 118 elements are known at present; only mercury and bromine are liquid at room temperature
• Metalloidselements with properties intermediate between metals and non-metals

In water, hydrogen and oxygen particles are held so tightly that physical methods cannot separate them, which is why water is a compound. A compound forms when different elements combine chemically in a fixed ratio, giving a new substance with properties different from those of its elements. In water the ratio of hydrogen atoms to oxygen atoms is 2:1. Sodium, a soft metal, and chlorine, a hazardous gas, combine in a 1:1 ratio to form sodium chloride, common salt. Heating dry sugar turns it brown and then blackish, leaves charcoal (carbon) behind and forms droplets of water, showing that sugar is a compound of carbon, hydrogen and oxygen. Hydrogen is a fuel and oxygen supports combustion, yet water puts out fire.

• Compoundtwo or more elements combined chemically in a fixed ratio
• Constituent elements of a compound cannot be separated by physical methods
• Hydrogen to oxygen atom ratio in water = 2:1; sodium to chlorine ratio in sodium chloride = 1:1
• Sugar decomposes on heating to give carbon and water

Mix 5.6 g of iron filings with 3.2 g of sulfur powder to get Sample A. It is a non-uniform mixture in which black iron and yellow sulfur particles are still visible, a magnet attracts the iron, and dilute hydrochloric acid gives a colourless, odourless gas that burns with a pop sound, while sulfur stays behind as a yellow solid. Heat half of Sample A with stirring until a black mass forms, cool and grind it to get Sample B, iron sulfide. Sample B looks the same throughout, is not attracted by a magnet, and iron and sulfur can no longer be separated. With dilute hydrochloric acid it gives hydrogen sulfide, a colourless gas with a rotten egg-like odour. Sample B is therefore a compound.

• Iron + Dilute hydrochloric acid → Iron chloride + Hydrogen gas
• Iron sulfide + Dilute hydrochloric acid → Iron chloride + Hydrogen sulfide
• Sample A (iron and sulfur) is a mixture; Sample B (iron sulfide) is a compound
• Never smell a gas directly; waft the fumes towards your nose

Elements, compounds and mixtures are all around us. Air is a mixture of gases, water is a compound of hydrogen and oxygen, and iron and aluminium are used to build bridges, buildings and vehicles. Chemists use the way elements combine to invent medicines and vaccines and to make fertilisers that raise crop production, while engineers design stronger materials such as stainless steel. Wood, steel and concrete are mixtures. Most rocks are mixtures of minerals, which are natural solids with a fixed chemical composition. Native minerals such as gold, silver, copper, sulfur and carbon are pure elements; most minerals, such as quartz, calcite, mica, pyroxene and olivine, are compounds. Cement is made from calcite, quartz, alumina and iron oxide, and talcum powder from the mineral talc.

• Native mineralspure elements such as gold, silver, copper, sulfur and carbon
• Most minerals are compounds of more than one element
• Graphene aerogel, made from carbon, is described as the lightest material on earth and is highly porous
• Dhokra art of Bihar and Odisha uses molten brass or bronze poured into a clay mould made from a beeswax model