CLASS 8 SCIENCE · CHAPTER 8 · STUDY NOTES
Nature of Matter: Elements, Compounds, and Mixtures
Understand nature of matter: elements, compounds, and mixtures through short explanations, labelled models and original practice. Read one topic, explain it in your own words, then check.
EXPLORE
A mixture keeps its different substances
Compare a uniform mixture with one that has separate regions.Distribution makes the difference
- Mixture
- Two or more substances mixed together without becoming one new compound.
- Uniform mixture
- A mixture with the same composition throughout at the scale being considered.
Salt dissolved evenly in water forms a uniform mixture. Sand in water can settle and leave different regions. A salad has easily visible ingredients; many other mixtures cannot be identified by eye alone.
The amounts of the components can vary. You can make salty water more or less salty while it remains a mixture of salt and water.
Let’s look a little closer.
Go deeper: Mixtures can occur in different states
Air mixes gases. Soda contains dissolved gas in a liquid. Alloys mix metallic elements and sometimes other elements. A mixture need not be made of two solids. Water and oil separate into layers rather than forming one uniform liquid mixture.
Take a moment. What do you think?
PAUSE & TRY · think first, then check with a free account
Question
Two glasses contain water with different amounts of dissolved salt. Must they be different compounds?
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 2, 3, 4, 5.
EXPLORE
Air is more than oxygen
Explain combustion, water droplets and suspended dust separately.Follow where the droplets come from
- Air
- A mixture mainly of nitrogen and oxygen, with smaller amounts of other gases and variable water vapour.
Oxygen supports burning; nitrogen makes up the largest share of dry air. Water vapour can condense as droplets on a cold surface. Carbon dioxide turns limewater milky in a suitable supervised test.
Dust is suspended material in air. A dust speck is much larger than a gas molecule. Breathing polluted air can harm health; do not deliberately inhale experimental gases.
PAUSE & TRY · think first, then check with a free account
Question
Droplets appear outside a sealed bottle of cold water. Did the water pass through the bottle?
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 3, 4, 5.
EXPLORE
Scientific “pure” has a precise meaning
Do not confuse a food label with a chemical classification.One substance or several?
- Pure substance
- A material consisting of one kind of chemical substance, with a definite composition.
Pure water consists of water; salt water contains both water and dissolved salt. Milk may be sold as unadulterated or “pure” in everyday language, but it is a mixture scientifically.
A pure substance may be an element or a compound. You cannot decide purity merely from whether a liquid looks clear.
PAUSE & TRY · think first, then check with a free account
Question
A clear liquid is labelled “pure milk.” Does the label prove that it is one chemical substance?
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 5, 6.
EXPLORE
An element contains one kind of atom
Separate the ideas “element,” “atom” and “molecule.”Single atoms and pairs can both be elements
- Element
- A substance that cannot be broken into simpler substances by ordinary chemical processes.
- Atom
- A basic unit of an element.
- Molecule
- Two or more atoms bonded together as a unit.
Iron and copper are elements. Oxygen gas is also an element, although its common molecules contain two oxygen atoms bonded together. A molecule does not have to contain two different elements.
Atoms of the same element have the same chemical identity. These pictures show identity by colour; atoms do not really have these classroom colours.
PAUSE & TRY · think first, then check with a free account
Question
Oxygen gas contains O₂ molecules. Why is it still an element?
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 6, 7, 8, 9.
EXPLORE
A compound has a fixed composition
Explain why new properties come from chemical combination.Boiling and decomposition are different
- Compound
- A substance formed by two or more elements chemically combined in a fixed proportion.
Each water molecule has two H atoms and one O atom. Hydrogen and oxygen gases have very different properties from water. Chemically combining elements does more than simply place them in the same container.
Sodium chloride has sodium and chloride ions in a fixed 1:1 ratio in a crystal. It is not a set of separate little NaCl molecules. Evaporating salt water recovers salt; it does not split salt into sodium and chlorine.
Fixed atom ratio
H₂O: H atoms : O atoms = 2 : 1This ratio is by number of atoms. It is not a 2:1 ratio by mass.
Go deeper: A teacher-led electrolysis model
A balanced representation is 2H₂O → 2H₂ + O₂. With suitable apparatus, hydrogen and oxygen gas volumes are approximately 2:1 under the same conditions. This is a chemical change; electricity and gas handling require teacher supervision.
PAUSE & TRY · think first, then check with a free account
Question
Does boiling water separate it into hydrogen and oxygen?
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 6, 7, 9, 10.
EXPLORE
Mixing iron and sulfur is not making iron sulfide
Use a before/after model to explain a reaction.Follow the change in identity
- Physical separation
- Separating mixture components without changing their chemical identities.
- Chemical reaction
- A process that produces substances with different chemical identities.
Before heating, iron and sulfur retain their identities in a mixture. A magnet can remove suitable iron particles. After reacting under suitable heating conditions, iron sulfide has new properties.
A compound cannot be separated into its elements by the ordinary physical methods that separate a mixture. Treat these as teacher-led demonstrations, not home heating experiments.
Go deeper: Tests must be safe and interpreted carefully
Different acid reactions can help a trained teacher distinguish materials. Some products are dangerous gases. Never smell, taste or deliberately inhale them. The important revision idea is the change in composition and properties, not memorising an unsafe procedure.
PAUSE & TRY · think first, then check with a free account
Question
A magnet removes iron particles from an iron–sulfur mixture. Does this prove iron sulfide has formed?
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 11, 12, 13.
EXPLORE
Choose materials by their useful properties
Connect elements, compounds and mixtures to real objects.A lost-wax casting sequence
- Alloy
- A material made by combining a metal with one or more other elements.
- Mineral
- A naturally occurring solid material with a characteristic composition and structure.
Brass contains copper and zinc; bronze commonly contains copper and tin. These alloys can have useful properties different from their main metal. Rocks commonly contain mixtures of minerals, such as quartz and mica.
Dhokra crafts use metal casting and often alloys. A lost-wax process makes a mould around a wax model, removes the wax and fills the space with molten metal. The craft is skilled work; hot-metal casting is not a classroom DIY activity.
PAUSE & TRY · think first, then check with a free account
Question
Why might a craftsperson choose an alloy rather than its main metal alone?
Sign in to see the answer
Write your own answer and compare it with ours. It’s free.
Sign inNew here? Sign up freeNCERT reference: chapter PDF pages 3, 4, 14, 15.