When a gas is collected over water by the downward displacement method, the jar to collect the gas must be held upside down and filled completely. This is because gas is less dense than water. Water will not let gas pass through it as gravity will pull the water into space occupied by the gas.
Therefore, carbon dioxide being denser than air, can simply be collected by placing an open jar over the source and allowing the gas to flow into it, displacing the air. As carbon dioxide is heavier than air, it settles at the bottom of the container and can be collected without the risk of it escaping or mixing with the surrounding air.
