Weighing a Change
Aligned to 5-PS1-2 — Next Generation Science Standards.
What this lesson teaches
When matter changes — melting, dissolving, or reacting — the tiny particles get rearranged but none are lost. If nothing escapes, the total weight before and after a change stays exactly the same.
Worked examples
Example 1
Freeze a cup of water and weigh it, then let it melt and weigh it again. The scale reads the same number both times, because the same water particles are still there — just rearranged as ice or liquid.
Example 2
Weigh a sealed bag of bread, then crush it to crumbs and weigh it again. Same mass — crushing rearranged it, it did not remove anything.
Example 3
A wet towel gets lighter as it dries, but the water did not vanish. It became invisible water vapor in the air.
Practice questions
- Weigh a sealed jar with a fizzy tablet and water inside, then weigh it again after it fizzes. What do you predict?
- Make a bar graph of the 'before' and 'after' weight for two different changes.
- If the jar weighed less after fizzing, what must have happened to some of the matter?
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
Pesando un Cambio
Cuando la materia cambia — al derretirse, disolverse o reaccionar — las partículas diminutas se reacomodan pero ninguna se pierde. Si nada escapa, el peso total antes y después del cambio sigue siendo exactamente el mismo.
Ejemplo 1: Congela un vaso de agua y pésalo, luego déjalo derretir y pésalo otra vez. La balanza marca el mismo número las dos veces, porque las mismas partículas de agua siguen ahí — solo reacomodadas como hielo o líquido.
Ejemplo 2: Pesa una bolsa sellada de pan, luego aplástalo hasta hacerlo migas y pésalo otra vez. La misma masa — aplastar lo reacomodó, no le quitó nada.
Ejemplo 3: Una toalla mojada se aligera al secarse, pero el agua no desapareció. Se volvió vapor de agua invisible en el aire.
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