Why is it hard to create a completely closed system?
a) no closed systems exist in nature.
b) closed systems have very few elements.
c) scientists are not sure how to create a system that is closed
d) it is very difficult to have a system in which no outside gases enter.

To put it simply, this law states that in a closed system, energy cannot be created or destroyed, but it can be converted from one form to another. A closed system is a system that does not interact with other systems or one that is contained.

Let’s imagine a pot of water on the stove. It has a lid, so it is not interacting with other systems, and for the purposes of this example, let’s imagine that the lid seals perfectly. When you turn on the burner, you are adding energy to the water in the form of heat. That heat will enter the water. Once it is there, it will be converted as it is absorbed by the water, which will begin to boil as a result. As the water boils, some of that energy will separate the water into gas, or steam. That steam collects on the underside of the pot lid and forms droplets that return to the water.

In reality, a pot cannot be a completely closed system. If you have boiled water, you will have seen steam escaping along the edges of the lid. It is a good thing that the pot is not perfectly closed because letting off steam is what keeps the pot from exploding as the heat, and consequently the pressure, builds. When you take the lid off a pot that is boiling, you immediately open the system and allow the heat to escape and be converted as water molecules escape into the air. If heat is constantly added to a closed system, it will build pressure as the molecules move faster and faster. Enough pressure can cause a pot to explode because all of that energy has nowhere to go and will eventually push the boundaries of the system.

While the atmosphere is not a perfectly closed system, in many ways it acts as if it were. Much of the gases on the planet will move continuously around the troposphere, creating weather. Some of those gas molecules will escape into the far reaches of the atmosphere, which is why it cannot be a closed system. But if we think about the way the gases are cycled through the ocean and the atmosphere, it is helpful to keep the law of conservation of energy in mind.

So therefore the answer is D) It is very difficult to have a system in which no outside gases enter.