lesson
Reactions rearrange atoms
1 · Learn
In a chemical reaction, atoms are rearranged into new combinations. In ordinary chemical reactions, atoms are not created or destroyed, so a balanced equation has equal numbers of each element on both sides.
The mass of a closed reacting system is conserved. An open container may appear to lose mass when a gas escapes, or gain mass when material from the air reacts. Include the surroundings before claiming matter vanished.
2 · Worked example
2 H₂ + O₂ -> 2 H₂O has four H atoms and two O atoms on each side. The equation describes proportions; it is not an instruction to perform this reaction.
3 · Your turn
Why could an open reaction vessel lose measured mass even though atoms are conserved?
Check your answer
A gaseous product may leave the measured vessel, carrying mass into the surroundings.
4 · Apply your learning
Balance a supplied paper model by changing whole-molecule counts, never by secretly changing the substance formula.
Adult guidance and safety
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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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