Problem 101 - Chemistry The Science in Context

Question

One of two glass capillary tubes of the same diameter is placed in a dish of water and the other in a dish of ethanol \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\right) .\) Which liquid will rise higher in its tube? Explain your answer.

Step-by-Step Solution

Answer

Answer: Water will rise higher in the capillary tube compared to ethanol, due to its stronger surface tension and stronger adhesion with the glass tube, both of which are influenced by the presence of hydrogen bonding. These properties contribute to a stronger capillary action in water as compared to ethanol.

Step 1: Understanding capillary action

Capillary action is the ability of a liquid to flow against gravity in narrow spaces without the assistance of, or even in opposition to, external forces like gravity. This phenomenon is due to the interactions between the liquid molecules and the walls of the narrow space (in this case, the capillary tube) as well as the surface tension of the liquid.

Step 2: Factors influencing capillary action

There are two main factors that influence capillary action in this exercise: (1) the surface tension of the liquid, which causes the liquid to contract and resist deformation and (2) the adhesion between the liquid and the glass tube, which determines how strongly the liquid is attracted to the walls of the tube.

Step 3: Comparing surface tension

Water has a higher surface tension than ethanol due to the presence of hydrogen bonding between the water molecules. A higher surface tension results in a strong cohesive force, causing the liquid to contract and rise.

Step 4: Comparing adhesion with the glass tube

The adhesion between water and glass is stronger due to the presence of hydrogen bonding between the water molecules and the glass surface. Ethanol, although it can also form hydrogen bonds, has weaker interactions with the glass surface compared to water. A stronger adhesion will promote liquid to rise higher in the capillary tube.

Step 5: Conclusion

Water will rise higher in the capillary tube compared to ethanol. This is because water has stronger surface tension and stronger adhesion with the glass tube due to the presence of hydrogen bonding. These properties together contribute to a stronger capillary action in water compared to ethanol.

Key Concepts

Surface Tension

Surface tension is a fundamental property of liquids that results from the cohesive forces among molecules at the surface. It's like an invisible skin that resists external force. This phenomenon is most pronounced in water due to its high level of surface tension. Water molecules are strongly attracted to each other, primarily through hydrogen bonds. This strong attraction pulls the surface molecules into a tighter formation, creating a firmer surface.

In contrast, liquids with lower surface tension, like ethanol, spread out more easily because the molecules do not pull as tightly toward each other. Surface tension plays an essential role in capillary action. The higher the surface tension, the higher the liquid can rise in a capillary tube.

• Water has a high surface tension due to strong hydrogen bonds.
• Ethanol has a lower surface tension as its molecules have weaker inter-molecular attractions.
• Surface tension counteracts gravity, allowing liquids to rise in narrow spaces.

In the context of the exercise, this means that water, with its greater surface tension, will rise higher than ethanol in a capillary tube.

Adhesion

Adhesion refers to the attraction between different substances, such as a liquid and a solid surface. In the case of capillary action, we look at how liquids are attracted to the walls of the capillary tube. The strength of these interactions can determine how much a liquid climbs up the walls.

In our example with water and ethanol, adhesion comes into play significantly. Water exhibits strong adhesive forces when interacting with glass due to hydrogen bonding. This means water molecules are more likely to "stick" to the glass surface, drawing more water upward.

• Adhesion is crucial for liquid rise in a capillary.
• Glass attracts water more strongly than it does ethanol.
• Stronger adhesion can cause the liquid to rise higher within the tube.

Thus, the adhesive forces between water and glass are stronger than those between ethanol and glass, aiding water in achieving a greater height in a capillary tube.

Hydrogen Bonding

Hydrogen bonding is a specific type of attraction between molecules, often considered stronger than other types of van der Waals forces but weaker than covalent bonds. This type of bond occurs when a hydrogen atom covalently bonded to a highly electronegative atom, like oxygen, interacts with another electronegative atom.

Water is a prime example of a molecule that heavily relies on hydrogen bonds. Water molecules consist of hydrogen and oxygen (H₂O), where each hydrogen atom is attracted to an oxygen atom of another water molecule, creating a robust network.

• Hydrogen bonds are strong due to the polarity of water.
• They are responsible for water's unique properties, like high boiling point and surface tension.
• Ethanol also has hydrogen bonds but they are weaker, hence less impact on its properties.

These strong hydrogen bonds make water particularly sticky and cohesive, leading to higher surface tension and stronger adhesion with surfaces like glass, unlike ethanol which has weaker hydrogen bonding. Therefore, hydrogen bonding significantly contributes to water's ability to rise higher in capillary action compared to ethanol.