Chemistry
Chemistry in the Pak Army Soldier Test checks atomic structure, the periodic table, chemical formulas, physical and chemical changes, acids and bases, and simple mixtures. Items are usually definition- or formula-based.
Core rules
- Atomic number is the number of protons in an atom. In a neutral atom, protons equal electrons.
- A compound is formed by chemical combination in fixed ratios; a mixture can be separated by physical methods.
- Acids have pH below 7, bases above 7, and neutral solutions at 7.
- Common formulas: H2O water, CO2 carbon dioxide, NaCl sodium chloride, O2 oxygen.
- A chemical change produces a new substance; a physical change does not.
Worked example
How many atoms are in 2H2O? The coefficient 2 multiplies the whole molecule. Each H2O has 2 hydrogen atoms and 1 oxygen atom, so one molecule has 3 atoms. Two molecules have 2 x 3 = 6 atoms: 4 hydrogen and 2 oxygen.
Traps
Do not multiply only the first element after a coefficient; the coefficient applies to the whole formula. Do not read the subscript 2 in H2O as two oxygen atoms. Do not call dissolving salt in water a chemical change if no new substance forms. Check pH direction: lower pH means more acidic.
Exam focus
Practice reading formulas, counting atoms, and identifying acids, bases, and mixtures. Learn the first 20 elements' symbols and common compounds used in daily life.