What does gas weigh? A group of children investigate whether fizzy lemonade is heavier than flat lemonade. Although gas is very light, it still has mass. The fizzy drink, which contains carbon dioxide gas, is therefore heavier than the flat drink. Gases can be either heavier or lighter than air.
Show the equipment to the class and ask them what you are going to find out. Discuss, predict and carry out the test. Discuss what will happen to balloons when they are inflated by helium and carbon dioxide.