Problem 6 - Chemistry An Atoms First Approach

Question

If you put a drinking straw in water, place your finger over the opening, and lift the straw out of the water, some water stays in the straw. Explain.

Step-by-Step Solution

Answer

In short, when you lift a straw with your finger covering the opening, water stays in the straw due to the difference in air pressure inside and outside the straw. The trapped air inside the straw, as per Boyle's law, causes the pressure inside the straw to decrease as you lift it. This lower pressure inside the straw, compared to the constant atmospheric pressure outside, keeps the water from falling out. The finger plays a crucial role in trapping the air and creating this pressure difference.

Step 1: Identifying forces on the water inside the straw

First, we need to understand the forces acting on the water inside the straw when it's submerged in a liquid and when it's lifted with a finger covering the top. When the straw is submerged in water, there is atmospheric pressure acting on the water inside the straw as well as the water outside the straw. However, once we cover the top of the straw and lift it up, the air pressure inside the straw changes.

Step 2: Understanding air pressure inside the straw

When the top of the straw is covered with a finger and the straw is lifted, the air pressure inside the straw decreases. Since the finger blocks the air from flowing in and out, the volume of air in the straw becomes trapped and is forced to expand as the straw is lifted out of the water. According to Boyle's law, as the volume of a gas increases, its pressure decreases, keeping the product of the pressure and volume constant (P1V1 = P2V2).

Step 3: Comparing air pressure inside and outside the straw

As we lift the straw, the pressure inside the straw has decreased (due to Boyle's law explained in Step 2), while the atmospheric pressure outside the straw remains the same. Since the atmospheric pressure outside the straw is greater than the pressure inside the straw, it pushes the water up the straw and prevents it from falling out.

Step 4: Role of the finger

The finger covering the top of the straw is essential in this process. It traps the air inside the straw and creates the difference in pressure between the inside and outside of the straw. Without the finger, the pressure would equalize as the straw is lifted, and the water would spill out due to gravity.

Step 5: Conclusion

In conclusion, when you put a drinking straw in water, place your finger over the opening, and lift the straw out of the water, some water stays in the straw due to the difference in air pressure inside and outside the straw. The air pressure inside the straw is lower than the atmospheric pressure outside due to the trapped air and Boyle's law. This difference in pressure prevents the water from falling out of the straw, even though it's lifted out of the water.