Matter and Its Interactions
What you must know
Grade 8 matter content centers on the particle model and on how substances combine and change. Understand that matter is made of atoms, that atoms of an element are alike in proton number, and that molecules form when atoms bond. Distinguish physical changes (state, shape, dissolving) from chemical changes (new substances with new properties, often signaled by gas, precipitate, color change, or heat/light release). Know the three common states and how particle spacing and motion explain properties such as compressibility and fixed versus variable shape.
Conservation of mass is the backbone of reaction questions: atoms are rearranged, never created or destroyed. Read a balanced equation as a recipe of particles. Also know that pure substances have fixed composition (elements and compounds) while mixtures can vary and can often be separated by physical means (filtration, evaporation, magnetism, distillation).
Traps to watch
- A change of state is not a chemical change, even when it looks dramatic (boiling, melting).
- "Mass disappeared" in a fizzing reaction is usually gas escaping into the air; mass is conserved in a closed system.
- Dissolving is physical; the substance keeps its identity.
- A coefficient multiplies the whole formula; a subscript belongs only to the atom before it.
Rule of thumb
Count atoms on each side of an equation. If the counts match for every element, mass is balanced. For changes, ask: "Is a new substance formed with new properties?" If yes, it is chemical.
Worked example
2H₂ + O₂ → 2H₂O. Left: 4 H, 2 O. Right: 4 H, 2 O. Balanced - and 4 g of hydrogen reacting with 32 g of oxygen gives 36 g of water.