GRADE 8 Science MIXTURES,ELEMENTS AND COMPOUNDS – Elements and compounds Notes

Key ideas (simple)

• Element: a pure substance made of only one kind of atom (e.g., oxygen O, iron Fe, gold Au).
• Compound: a pure substance formed when two or more different elements react and join chemically (e.g., water H₂O, common salt NaCl).
• Atom: the smallest particle of an element that still has the properties of that element.
• Molecule: two or more atoms joined together. A compound has molecules made of different elements.

Simple visual ideas

How to tell them apart

• Elements: only one type of atom. They cannot be broken into simpler substances by chemical methods (e.g., heating alone).
• Compounds: two or more elements chemically joined in a fixed ratio. Compounds can be broken into elements or simpler compounds by chemical reactions (e.g., electrolysis of water).
• Example: Water is always H₂O (2 hydrogen atoms + 1 oxygen). Salt is always NaCl (1 sodium + 1 chlorine).

Examples from everyday Kenyan life

• Elements: Iron (Fe) — nails, tools; Copper (Cu) — some wires; Gold (Au) — jewellery; Oxygen (O₂) — the air we breathe.
• Compounds: Water (H₂O) — drinking, cooking; Table salt (NaCl) — food seasoning; Sugar (sucrose C₁₂H₂₂O₁₁) — tea and tea shops; Rust (iron oxide Fe₂O₃) — old iron sheets.

Short notes on chemical symbols and formulas

Each element has a chemical symbol (one or two letters): H (hydrogen), O (oxygen), C (carbon), Na (sodium), Cl (chlorine), Fe (iron). A formula shows which elements and how many atoms are in a molecule: H₂O (two H, one O), CO₂ (one C, two O).

Quick classroom activities (easy)

1. Collect pictures of things at home. Sort them into "elements" or "compounds" columns (use labels like Fe, Au for elements; H₂O, NaCl for compounds).
2. Make a model: use different-coloured beads for atoms and join them with string to show a molecule of water or carbon dioxide.
3. Test question: is charcoal an element or compound? (Answer: mostly carbon — an element, but real charcoal may have other impurities.)

Short quiz (write answers)

1. Is oxygen (O₂) an element or compound?
2. Is table salt (NaCl) an element or compound?
3. Water is H₂O. How many hydrogen atoms are in one molecule of water?
4. Give one example of an element you see at home and one compound you use every day.

(Answers: 1 element, 2 compound, 3 two H atoms, 4 student example)

Safety and small note

Some chemicals and reactions can be dangerous. Do experiments only with teacher supervision and use safe materials (e.g., water, salt, sugar).