lesson
A physical change keeps the substance
1 · Learn
Melting and freezing change arrangement and motion without changing the chemical identity of the substance. Evaporation occurs at a liquid's surface and can happen below its boiling point; boiling occurs throughout the liquid at its boiling temperature for the stated pressure.
During a pure substance's melting at constant pressure, energy transfer can change state without raising temperature. A simplified heating curve must label both temperature and time or energy supplied.
2 · Worked example
Water evaporating from a puddle is not necessarily boiling. It remains water in the gas state rather than disappearing into nothing.
3 · Your turn
Does evaporation require every liquid to reach its boiling point first?
Check your answer
No. Surface evaporation can occur below the boiling point.
4 · Apply your learning
Interpret a supplied heating curve and distinguish a temperature rise from a state-change interval.
Adult guidance and safety
Use supplied data and diagrams unless a qualified adult has approved the practical, equipment and risk assessment. No tasting substances, culturing unknown microbes, mains electricity, flames or hazardous chemical experiments at home. Models simplify reality; ask which assumptions affect the conclusion.
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Check a calculation · Year 5 onward
Use this only when your teacher or adult allows a calculator for the task. Show your thinking first. It does not replace mental or written arithmetic and does not mark your lesson answer.
Powers, roots and trigonometry · Year 7 onward
Use only when the task allows a calculator. Choose one operation at a time and record your working. Trigonometry here always uses degrees (DEG), not radians. Inverse sine, cosine and tangent return an angle, not a reciprocal. Keep exact fractions and π in answers when requested.
Approximate numerical results use up to 12 significant digits. Inverse sine returns −90° to 90°, inverse cosine 0° to 180°, and inverse tangent −90° to 90°; these are principal values, not every possible solution of a trigonometric equation.
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Follow your teacher's approved method and safety instructions. Record the sample or supplied dataset, conditions, measurements with units and uncertainties where appropriate. Distinguish observations from explanations. A described experiment is not permission to carry it out; do not improvise equipment or invent missing results.
Ask your teacher for support with the method and recording. A prediction is what you think might happen; an observation is what you actually see, hear or feel safely.
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