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Chapter 12: Problem 63

Explain how two different forces play a role in capillary action

Short Answer

Expert verified
In capillary action, two main forces play significant roles: cohesive forces and adhesive forces. Cohesive forces are the intermolecular forces that hold the molecules of a liquid together, providing surface tension. On the other hand, adhesive forces are the forces between different types of molecules, such as the attraction between a liquid's molecules and a solid surface. In capillary action, adhesive forces pull the liquid up a narrow tube due to the attraction between the liquid's molecules and the solid's molecules. Cohesive forces create surface tension, holding the liquid's molecules together and opposing the upward motion caused by adhesive forces. The balance between these forces determines how high the liquid will rise in the tube. This competition can be observed in everyday scenarios like water spreading through a paper towel, a sponge absorbing water, or plant roots drawing water from the soil.

Step by step solution

01

Understanding Cohesive Forces

Cohesive forces are the intermolecular forces that hold the molecules of a liquid together. In the case of water, these forces are mainly hydrogen bonds that form between the oxygen atom of one water molecule and the hydrogen atom of another water molecule. Cohesive forces provide the surface tension in liquids that allows them to resist external forces.
02

Understanding Adhesive Forces

Adhesive forces are the forces between different types of molecules, such as the attraction between a liquid's molecules and the molecules of a solid surface, for instance, the inside walls of a narrow tube. In capillary action, these forces allow the liquid to interact with and adhere to the surfaces of the solid material.
03

How Cohesive and Adhesive Forces Interact in Capillary Action

When a liquid enters a narrow tube in contact with a solid surface, adhesive forces attract the liquid to the surface, causing it to rise up the tube. At the same time, cohesive forces between the liquid's molecules resist this upward motion, creating a curvature (known as a meniscus) at the liquid's surface. In capillary action, the balance and competition between adhesive and cohesive forces determine how high the liquid will climb.
04

The Role of Adhesive Forces in Capillary Action

Adhesive forces pull the liquid up the tube's walls because of the attraction between the liquid's molecules and the solid's molecules. The stronger the adhesive forces, the higher the liquid will rise. This explains why some liquids, like water, rise significantly in narrow tubes, while other liquids with weaker adhesive forces, like mercury, do not rise as much or even sink.
05

The Role of Cohesive Forces in Capillary Action

Cohesive forces create surface tension in the liquid, and they tend to hold the molecules of the liquid together, opposing the upward motion caused by adhesive forces. When cohesive forces are stronger than adhesive forces, the liquid forms a convex meniscus and does not rise in the tube. Conversely, if adhesive forces are stronger, the liquid forms a concave meniscus and rises in the tube.
06

Capillary Action in Real-Life Scenarios

Capillary action can be observed in everyday situations, such as water spreading through a paper towel, a sponge absorbing water, or plant roots drawing water from the soil. In each case, cohesive and adhesive forces are responsible for the movement of the water through the material, enabling it to reach areas where it's needed.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Cohesive Forces
Imagine a family holding hands, sticking together. Similarly, cohesive forces are the molecular 'hand-holding' that keeps the liquid molecules united. Specifically, these are the forces at work within the same substance, like the attraction between water molecules through hydrogen bonding. These internal forces provide surface tension, a sort of skin on the liquid allowing it to form droplets and resist external forces. This tension is what enables insects to walk on water, and it's also responsible for the spherical shape of raindrops.

In capillary action, these forces work against the liquid being pulled apart, providing a delicate balance with adhesive forces. Understanding this balance is crucial, as it ensures accuracy in processes like using a thermometer, wherein the cohesive forces within the mercury must be precisely calculated to provide a correct reading.
Adhesive Forces
Now, envision that family reaching out to make friends. Adhesive forces are these friendly outward handshakes, the attractive forces between two different substances, such as water molecules and the glass of a capillary tube. When a paper towel touches a water spill, the liquid climbs up, defying gravity, because of these forces.

For understanding capillary action, one must comprehend the tug-of-war between these adhesive forces pulling the liquid onto a surface and cohesive forces keeping the liquid together. When adhesive forces win, the liquid crawls up the sides of containers or capillary tubes, being crucial in medical tests involving blood samples, where the blood must travel along channels for analysis.
Surface Tension
Surface tension is like a trampoline made of water molecules, created by cohesive forces at the surface of the liquid. It's this tension that forms the barrier separating the liquid from its environment, allowing it to resist external forces and maintain a certain shape.

In the context of capillary action, surface tension plays a pivotal role. It can be seen in the way water beads up on a waxed car hood, with the surface tension maintaining the bead shape against gravity's pull. Surface tension also influences how far a liquid can travel up a tube; the greater the tension, the more resistance there is to flow and vice versa. This understanding is instrumental in designing systems that rely on fluid movement, such as ink in pens or the transport of nutrients in plants.
Meniscus
The meniscus is the curved surface of a liquid in a tube or container and is a result of the battle between cohesive and adhesive forces. It's like looking at a rubber sheet stretched across a ring; the sheet dips or bulges depending on how it鈥檚 pulled from the sides.

For liquids that wet the container, like water in a glass, adhesive forces are stronger, causing the liquid to climb up the walls and form a concave meniscus. In contrast, with liquids like mercury, cohesive forces dominate, so it forms a convex meniscus. This curvature is vital for accurate measurements in laboratories because the bottom of the meniscus is used to read the volume of the liquid. Therefore, by analyzing the meniscus shape, scientists can infer the relative strength of cohesive versus adhesive forces of any liquid.

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Decide which of the substances listed can form hydrogen bonds. a. \(\mathrm{H}_{2} \mathrm{O}\) b. \(\mathrm{H}_{2} \mathrm{O}_{2}\) c. \(\mathrm{HF}\) d. \(\mathrm{NH}_{3}\)

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