Elements, Compounds & Mixtures Class 8 PPT & Notes (Ch 8)

• Tell mixtures apart from pure substances
• See why elements are the building blocks of matter
• Learn how compounds form in a fixed ratio
• Do experiments: splitting water, heating sugar, and heating iron with sulfur
• Meet minerals and the everyday uses of matter

• What around us is made of matter, and what is not?
• How do elements join together to form a compound?
• Could a compound that absorbs carbon dioxide help the environment?

Curiosity always begins with the right questions.

• A mixture is two or more substances mixed together
• Each substance keeps its own properties
• The individual substances are called components
• The components do not react chemically
• Everyday examples: sugar in water, soup, lemonade

• Non-uniform: the parts can be seen separately. Example: sprout salad and poha
• Uniform: the parts are spread evenly and cannot be seen separately. Example: sugar dissolved in water

• Alloys are mixed so evenly that the parts cannot be seen, so they are uniform solid mixtures
• Stainless steel is iron, nickel, chromium and a little carbon
• Brass is copper and zinc
• Bronze is copper and tin
• Mishraloha is the ancient Indian name for a mixture of two or more metals. These alloys had new properties, unlike the pure metals. They are described in the Charaka and Susruta Samhita, and bronze (Kamsya) is copper 4 parts and tin 1 part, used long ago for utensils and health

• Air is a uniform mixture of gases
• It contains nitrogen, oxygen, argon, carbon dioxide and water vapour
• Nitrogen makes up about 78% of air and does not burn
• Oxygen keeps living things alive and supports burning

1. Add quicklime (calcium oxide) to water to make lime water
2. Leave the clear lime water exposed to air
3. It slowly turns milky white

Carbon dioxide from the air forms white calcium carbonate:

• Leave a clean black sheet of paper near a window
• Tiny dust particles settle on its surface
• Dust is a pollutant, not a true part of air
• Major pollutants include soot, carbon monoxide, ozone and sulfur dioxide
• The Air Quality Index (AQI) tells us how clean the air is

• In daily life we separate a mixture to get the part we want, and the other parts are simply thrown away
• In science we separate to find pure substances
• Separation is the doorway from mixtures to pure substances

On packs of milk, ghee and spices we see the word “pure”. In daily use it is about adulteration, which means adding cheaper or poor-quality substances, usually to add weight or cut cost.

In science the word means something different:

• A pure substance has no other substance in it
• It is made of only one type of particle
• It cannot be separated by any physical process
• To a scientist, even “pure” milk is impure, because milk is a mixture of water, fat, protein and more

Pure substances are of two kinds: elements and compounds.

1. Add a few drops of dilute sulfuric acid to water
2. Fill two small test tubes with this water
3. Place them upside down over the terminals of a 9 volt battery

What you see:

• Gas bubbles rise at both terminals
• These are not water vapour, because they did not turn back into water
• A pop sound with a flame means the gas is hydrogen
• A brighter glowing flame means the gas is oxygen

Electricity splits water into two different gases. This is a chemical change, and both gases are pure substances called elements.

• Elements are the simplest substances and cannot be broken down further
• They are the building blocks of all matter
• Each element is made of identical atoms
• One element’s atoms differ from every other element’s
• Examples: gold, silver, sulfur and carbon

Most atoms cannot exist on their own. Two or more atoms join to form a molecule. Two hydrogen atoms join to make one hydrogen molecule, and two oxygen atoms join to make one oxygen molecule.

• 118 elements are known today, and most of them are solids
• Only 11 exist as gases, like oxygen and nitrogen
• Just 2 are liquid at room temperature: mercury (a metal) and bromine (a non-metal)
• Gallium and caesium melt near 30 degrees, almost in your hand

• Different elements combine in a fixed ratio
• They form a brand-new substance
• A compound’s properties differ from those of its elements
• It cannot be separated by any physical method
• In water, hydrogen and oxygen atoms are in a 2 : 1 ratio

• E = one kind of atom only
• C = fixed ratio, new properties, cannot be split physically
• M = keeps its properties, any ratio, can be split physically
• Fixed ratio locks a compound; free mixing frees a mixture

Hydrogen is a fuel and burns easily, and oxygen supports burning. Yet water, which is made of both, puts out fire. The properties of a compound are completely new.

• Sodium is a soft, reactive metal
• Chlorine is a hazardous gas
• Together they form safe, tasty salt, and sodium and chlorine join in a 1 : 1 ratio
• Salt separates from water by evaporation, not into its elements

1. Heat a spoon of sugar gently in a boiling tube
2. It turns brown, then black as it chars
3. Water droplets form near the open end of the tube
4. Black charcoal (carbon) is left behind

So sugar is a compound of carbon, hydrogen and oxygen.

1. Take 5.6 grams of iron filings and 3.2 grams of sulfur powder, and observe them separately first
2. Mix them in a watch glass. This is Sample A
3. Heat half of Sample A until a black mass forms, then grind it. This is Sample B, iron sulfide, a compound

Sample A is a mixture. Iron and sulfur keep their own properties, and a magnet can separate the iron. Iron reacts with the acid, while sulfur stays as a yellow solid and does not react:

Sample B is a compound. Iron sulfide is one single new substance, with the same colour and texture throughout, and a magnet has no effect on it:

Hydrogen sulfide smells like rotten eggs. In iron sulfide, iron has lost its own magnetic property, because a compound has completely new properties.

• Air (a mixture) and water (a compound) keep us alive
• Iron and aluminium build bridges, buildings and vehicles
• Chemists make medicines, vaccines and fertilisers
• Alloys like stainless steel are strong and long-lasting
• Over 45 different elements sit inside a single mobile phone
• Fun fact: graphene aerogel is made from carbon and is described as the lightest material on Earth. It is so light that a blade of grass can hold it up, and it soaks up spills and can clean oil from sea and land

• Most rocks are mixtures of minerals
• Native minerals are pure elements: gold, silver, copper and sulfur
• Most minerals are compounds of many elements
• Examples: quartz, calcite, mica, pyroxene, olivine and talc
• Cement comes from calcite and quartz, and talcum powder from talc

• Elements and compounds are the building blocks of matter
• Matter has mass and takes up space
• Light, heat and electricity are not matter
• Even thoughts and feelings are not matter

• Mixture: substances mixed together, keeping their properties, with no reaction
• Pure substance: made of one type of particle
• Element: cannot be broken down further
• Compound: a fixed-ratio combination of elements with new properties
• Minerals: natural solids, mostly compounds and rarely pure elements

• Make comic strips comparing elements, compounds and mixtures
• Research discoveries: phosphorus, sodium, penicillin and brass
• Read snack and detergent labels, and list their compounds and mixtures
• Group role-play: which category matters most?

Tap an option to check it at once. Open Check answer for the reason.