When collecting a gas by displacement of water, the container of water must be inverted to prevent the gas from escaping. This is because gases are less dense than water, so if the container were not inverted, the gas would rise to the surface and escape.

By inverting the container, the gas is trapped at the bottom of the container and the water acts as a barrier to prevent it from escaping. As the gas is collected, it will displace the water and cause the water level in the container to drop.

It is important to note that the container must be completely inverted in order to prevent the gas from escaping. If the container is only partially inverted, the gas will still be able to escape through the opening at the top of the container.