 In this lesson, we will learn how to collect gases based on their density. Have you ever wondered why some balloons float and some don't? This is due to the density of the gas inside the balloon. Balloons that float contain helium, which is less dense than air. Balloons that we blow up ourselves contain mostly carbon dioxide, which is more dense than air. Therefore, these balloons fall to the ground. From the introduction, we already know that a gas less dense than air rises. These gases are collected using a method called upward delivery. The collection vessel is inverted so that the gas is able to displace the air inside it as it rises upwards. Since air is pushed downwards, this method is also called downward displacement. Hydrogen gas and ammonia are collected using this method. Recall the example of the balloon falling to the ground in the introduction. For gases more dense than air, they are collected via downward delivery. The collection vessel is placed upright so that the denser gas collected is able to displace the air inside it as it sinks. Since air is pushed upwards, this method is also called upward displacement. Carbon dioxide and chlorine gas are collected using this method. If we are unsure of the density of the gas, we can collect it over water. Since all gases are less dense than water, displacement of water is possible. Whatever gas produced is bubbled through water and collected in a gas jar. Note that this method will not work for gases which are soluble in water, such as ammonia. What do you think is the best method to collect hydrogen chloride? Let's pause the lesson momentarily to think about this. Hydrogen chloride dissolves in water to produce hydrochloric acid, so collecting over water is not possible. Since it is more dense than air, it can be collected via downward delivery. This brings us to the next topic and addresses a very important question. What if you are unsure of the solubility and the density of the gas produced? A method to collect a gas irrespective of these properties is by using a gas syringe. It is a closed container, so the density of the gas does not need to be taken into consideration. It can also be used when the volume of a gas produced needs to be measured. In summary, for gases less dense than air, they are collected via upward delivery. If a gas is more dense than air, we collect it via downward delivery. These gases can also be collected over water, but this method only works for gases which are insoluble in water. And finally, we can use a gas syringe to collect all gases and is useful for measuring the volume of a gas.