Chemical Reactions & Conservation of Mass
Aligned to MS-PS1 — Next Generation Science Standards.
What this lesson teaches
In a chemical reaction, atoms rearrange to form new substances, but no atoms are created or destroyed — so the total mass before equals the total mass after.
Worked example
When iron rusts, iron + oxygen → iron oxide. The rust weighs more than the iron alone because oxygen atoms joined it; total mass (iron + oxygen) is conserved.
Practice questions
- In a reaction, 10 g reactants form how many grams of product? Why?
- How do you know a chemical reaction occurred?
- Why does a closed jar's mass stay the same during a reaction?
Watch the lesson
Every lesson comes with a video taught in English and Spanish — the same video the QR code in the printed workbook opens.
▶ Watch this lessonEn español
Reacciones Químicas y Conservación de la Masa
En una reacción química, los átomos se reorganizan para formar nuevas sustancias, pero ningún átomo se crea ni se destruye — por lo tanto, la masa total antes de la reacción es igual a la masa total después.
Ejemplo: Cuando el hierro se oxida, hierro + oxígeno → óxido de hierro. El óxido pesa más que el hierro solo porque se le unieron átomos de oxígeno; la masa total (hierro + oxígeno) se conserva.
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