Photo means light, synthesis means combine with light. When plants photosynthesize, they give off oxygen. People need oxygen to live. When we exhale, we give off a gas called carbon dioxide. Our bodies do not use this, but plants do for use during photosynthesis.
Plants provide oxygen to people and animals through photosynthesis.
Plants purify the air by taking out carbon dioxide. Animals take in oxygen and give off carbon dioxide as they breathe. Plants remove carbon dioxide from the air and add oxygen during photosynthesis. Keep plant in water until you are ready to do the experiment. Cut off a leaf on top of the plant and submerge in a test tube.