Chemistry - Substances and Chemical Changes
This domain tests how the Niigata examination distinguishes physical from chemical change, selects separation methods, and explains acids, bases, neutralization, oxidation, and gas tests. Expect questions built on experimental flowcharts: dissolve, filter, evaporate, test gas, and compare masses.
Pure substances and mixtures
- Elements cannot be decomposed; compounds have fixed mass ratios; mixtures retain component properties.
- Pure substances melt and boil at fixed temperatures; impurities lower melting point and raise boiling point.
- Separation: filtration for insoluble solids; distillation for liquids with different boiling points; evaporation for dissolved solids; chromatography for dyes.
Physical versus chemical change
- Physical changes alter state or size without new substances; dissolving is physical.
- Chemical changes produce new substances: gas, precipitate, color change, heat, or light.
- Exothermic releases heat; endothermic absorbs heat.
Acids, bases, salts
- Acids turn blue litmus red and react with metals to give hydrogen; bases turn red litmus blue.
- Neutralization: acid + base → salt + water; H⁺ + OH⁻ → H₂O.
- pH below 7 is acidic, 7 neutral, above 7 basic.
- Gas tests: O₂ relights a glowing splint; CO₂ turns limewater milky; H₂ burns with a pop.
Worked example
An 8.0 g mixture of sand and sodium chloride is stirred with water, filtered, and the residue dried. The dry sand weighs 5.5 g; evaporating the filtrate gives 2.2 g salt. Percentage sand = 5.5 ÷ 8.0 × 100 = 69%. The 0.3 g difference is not a new substance; it is salt left in the wet sand or lost during evaporation. This is physical separation, so total mass should be conserved if drying is complete.
Traps
- Calling dissolving or melting a chemical change.
- Using distillation for sand.
- Assuming neutralization always gives pH 7; the salt may be acidic or basic.
- Confusing the limewater test with the burning-splint test.