Question

One popular demonstration in chemistry labs is performed by boiling a small quantity of water in a metal can (such as a used soda can), picking up the can with tongs and quickly submerging it upside down in cold water. The can collapses with a loud and satisfying pop. Give an explanation of this crushing of the can. (Note: If you try this demonstration, do not heat the can over an open flame.)

Short answer

The can is crushed due to the sudden change in pressure inside and outside the can caused by the rapid cooling and condensation of gas inside it. The atmospheric pressure outside the can becomes much greater than the low or almost zero pressure inside the can, causing the can to collapse under the external pressure.

Step-by-step solution

Understanding the Physical Principles

There are two important physical principles to understand before diving into the explanation: Charles's Law and the principle of atmospheric pressure. Charles's Law states that the volume of a gas is directly proportional to its temperature. This means if a gas is heated, its volume will increase and if it is cooled, its volume will decrease. Atmosphere pressure plays a role in moving things from high pressure to low pressure.

Initial Conditions inside the Can

When the can is heated, the water inside it forms steam which pushes out most of the air originally inside the can. This causes the pressure inside the can to increase, which isn't a problem initially because the pressure inside the can is balanced by the atmospheric pressure outside.

Change in Conditions

When the can is quickly placed in the cold water, the steam inside cools rapidly and condenses back into liquid water. This drastically reduces the volume of the gas inside the can, creating a partial vacuum. With the gas volume decreased, the pressure inside the can drops rapidly.

Crushing of the Can

The pressure outside the can (atmospheric pressure) is now significantly greater than the pressure inside the can. Therefore, the greater external pressure crushes the can to equalize the pressure difference, resulting in the 'loud and satisfying pop'.

Key concepts

Atmospheric Pressure

Atmospheric pressure is the force exerted by the weight of the air in the Earth's atmosphere. It's all around us and affects everything we do, although we often don't notice it because we're used to it. In this experiment, atmospheric pressure becomes very noticeable when the can is submerged in cold water.

Here's how it works:

• When the can is first heated, the atmospheric pressure outside the can is balanced by the pressure inside created by the steam.
• Once the can is submerged, the pressure inside decreases quickly because the steam condenses, but the atmospheric pressure outside stays the same.
• This greater external pressure, unopposed by equivalent pressure inside, causes the can to collapse."

Understanding atmospheric pressure helps us see why the can is crushed so dramatically. The atmosphere is more powerful than it often seems.

Gas Volume

Gas volume is an important concept in this experiment because it explains part of what happens when conditions in the environment change. Volume refers to the amount of space that a gas occupies. According to Charles's Law, if a gas is heated, its volume increases as the molecules spread out. Conversely, when a gas cools, its volume decreases as molecules come closer together.

In the demonstration:

• Heating the can causes water to turn into vapor, increasing the gas volume inside the can.
• When the can is immersed in cold water, the volume of gas rapidly decreases as the vapor condenses into liquid water.

These changes in gas volume are crucial to creating the partial vacuum inside the can, leading to the crushing effect due to atmospheric pressure.

Thermal Expansion

Thermal expansion refers to the increase in volume that occurs when a substance is heated. This is a direct consequence of Charles's Law, as gases expand on heating. In this experiment, when the can is heated, the water inside turns into vapor, and the gas molecules inside the can spread out more.

Here's how thermal expansion plays a role:

• At high temperatures, the molecules move faster and tend to occupy more space, increasing the pressure inside the can as the gas expands.
• This thermal expansion is temporary as long as the can remains hot. The moment it is cooled, the expansion reverses as gas molecules lose energy and take up less space.

Understanding thermal expansion helps explain how heating the water initially increases the internal pressure of the can, before cooling causes the internal collapse.

Vacuum Formation

Vacuum formation is a critical principle in this experiment. A vacuum is a space devoid of matter, and in this case, it refers to the low pressure inside the can once it is submerged in cold water. Here's how the vacuum is formed in this demonstration.

• Initially, the can is filled with steam, which has a large volume while hot. When cooled quickly by the cold water, the steam condenses into a much smaller volume of liquid water.
• This rapid condensation significantly reduces the pressure inside the can, creating a partial vacuum.
• A vacuum forms because the number of gas molecules inside the can is drastically reduced and cannot balance the atmospheric pressure outside.

This is why a vacuum formation leads to the can being crushed. Without sufficient internal pressure, the atmospheric pressure outside forces the can inward with a loud pop.