Задача · 8 класс Задача №3760266
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Quick answers
Task 1
1 → B
2 → C
3 → A
Task 2
- Oxygen gas, O₂ — Element
- Pure water, H₂O — Compound
- Sea water — Mixture
- Iron filings, Fe — Element
- Carbon dioxide, CO₂ — Compound
- Air — Mixture
Task 3
- NaCl — 2 elements: sodium (Na), chlorine (Cl)
- H₂SO₄ — 3 elements: hydrogen (H), sulfur (S), oxygen (O)
- CaCO₃ — 3 elements: calcium (Ca), carbon (C), oxygen (O)
Task 4
- Filtration — pour the mixture through filter paper. Sand stays on the paper as residue; water passes through as filtrate.
How to think about similar tasks
- Particle pictures: one ball = atom; two joined identical balls = molecule of an element; different-colored balls joined = compound; different balls loose = mixture.
- Element / compound / mixture: an element has only one type of atom, even if it is diatomic like O₂. A compound has different atoms chemically bonded in a fixed ratio, like H₂O or CO₂. A mixture has substances not chemically bonded and can be separated physically, like sea water or air.
- Counting elements in a formula: each capital letter starts a new element symbol; lowercase letters belong to the same symbol. Subscripts count atoms, not element types. So H₂SO₄ has H, S, O — 3 elements, not 7.
- Separating mixtures: filtration for an insoluble solid like sand; evaporation for a dissolved solid like salt; magnetism for iron.
Check: O₂ is an element, H₂O and CO₂ are compounds, and sea water and air are mixtures. The matches and formula counts are consistent.
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Task 4 — Separating mixtures
Evaporation
Heat the salt solution; the water evaporates and the dissolved salt remains.
(If you also need to collect the water, use simple distillation.)Magnetic separation
Use a magnet: iron filings are attracted to it, while sulfur powder is not.
Task 5 — True or False?
- True — a compound has different properties from the elements it is made from.
- True — mixtures can be separated by physical methods without a chemical reaction.
- False — water is a compound, not an element. It is made of two elements: hydrogen and oxygen.
Task 6 — Element Symbols Challenge
- H → Hydrogen
- C → Carbon
- N → Nitrogen
- Na → Sodium; symbol: Na
- Cl → Chlorine; symbol: Cl
- Fe → Iron; symbol: Fe
Key idea: dissolved salt cannot be separated by filtration, so evaporation is used. Iron is magnetic, so a magnet separates it from sulfur.
