Chemistry (គីមីវិទ្យា)
What the paper tests
Grade 9 chemistry introduces matter and its states, atomic structure, the periodic table, chemical formulas, simple reactions, acids and bases, and salts. Questions test both recall and the ability to balance equations and interpret particle diagrams.
Core rules and concepts
- An atom has protons and neutrons in the nucleus and electrons in shells.
- The atomic number equals the number of protons; in a neutral atom it also equals the number of electrons.
- Groups in the periodic table have similar chemical properties; Group 1 metals are reactive, Group 0 gases are unreactive.
- A chemical equation must be balanced: the number of atoms of each element is the same on both sides.
- Acids have pH below 7, alkalis above 7; neutralisation produces a salt and water.
Worked example
Balance CH₄ + O₂ → CO₂ + H₂O. Carbon: 1 on each side, so CO₂ is correct. Hydrogen: 4 on the left, so place 2 before H₂O. Oxygen: right side now has 2 (CO₂) + 2 (H₂O) = 4, so place 2 before O₂. Final equation: CH₄ + 2O₂ → CO₂ + 2H₂O. Count again: C 1 = 1; H 4 = 4; O 4 = 4. Balanced.
Traps that cost marks
Never change subscripts to balance an equation; change coefficients only. Remember that oxygen is O₂, not O. Do not write H₂O as HO₂. In acid–base questions, name the salt from the metal and the acid, not from the indicator colour.
Exam focus
Practise counting atoms element by element, then re-check the total after writing coefficients.