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Do all gases weigh the same?

Science lesson in Year 4

Year 4 have been investigating states of matter. Their most recent question was 'Do all gases weigh the same?' Some children predicted yes, others predicted no. We tested this theory with two identical balloons: one filled with helium, and one filled with air (78% nitrogen, 21% oxygen, 0.9% argon, 0.04% carbon dioxide). The helium balloon floated but the air balloon fell to the floor - the children concluded that gases do not all weigh the same!