Problem 56 - Master Resource Book in JEE Main Physics

Question

While measuring surface tension of water using capillary rise method the necessary precaution to be taken is/are (a) capillary tube should be clean while water should have some grease (b) both capillary tube and water should be clean (c) no need to take care of temperature of water (d) None of the above

Step-by-Step Solution

Answer

(b) Both capillary tube and water should be clean.

Step 1: Understanding the problem

We need to identify the correct precautions to take when measuring the surface tension of water using the capillary rise method. It's important to consider cleanliness and temperature factors.

Step 2: Analyze option (a)

Option (a) suggests the capillary tube should be clean while the water should have some grease. Surface tension measurements require both the tube and the liquid to be clean to ensure accurate results, so this option is incorrect.

Step 3: Analyze option (b)

Option (b) states that both the capillary tube and water should be clean. Clean surfaces help ensure that the water rises primarily due to capillary action without interference from contaminants. This is a correct precaution.

Step 4: Analyze option (c)

Option (c) claims there's no need to care about the water temperature. This is incorrect because surface tension can vary with temperature, so keeping water at a consistent temperature is important for accurate measurements.

Step 5: Evaluate option (d)

Option (d) suggests that none of the above precautions are necessary. Given our analysis, option (b) is a correct precaution, so this option is incorrect.

Key Concepts

Surface Tension

Surface tension is a fascinating phenomenon that occurs at the surface of a liquid. It happens because molecules at the surface experience a different set of forces compared to those within the liquid. These molecules tend to get pulled inwards, which creates a 'skin' on the liquid's surface. This surface tension can easily be observed when small objects, like certain insects, are able to float on water without sinking. When we discuss surface tension in a scientific sense, it is important to remember:

• It is affected by the nature of the liquid. Different liquids have different surface tensions due to the strength of cohesive forces among their molecules.
• Any contamination, like dirt or oil, will change the way that molecules interact, thereby affecting surface tension. This is why having clean surfaces in experiments is so vital.
• Surface tension is measured in force per unit length, typically in Newtons per meter.

Understanding these points allows us to realize the importance of accuracy and cleanliness when measuring surface tension using methods like the capillary rise method.

Cleanliness in Experiments

The cleanliness of equipment and substances used in experiments is crucial for obtaining reliable results. In the context of measuring surface tension through the capillary rise method, cleanliness cannot be overstated. The presence of contaminants can drastically alter the results in the following ways:

• Modified Surface Tension: Even a small amount of grease or dirt can disrupt the cohesive forces among the water molecules, leading to incorrect surface tension readings.
• Capillary Tube Functions: The effect of a dirty tube can result in erratic water rise, as impurities could affect adhesion differently at various points in the tube.
• Reproducibility: Clean conditions help ensure that experiments can be repeated, and results can be reliably compared.

As a best practice, always ensure that both the liquid and the measuring equipment are thoroughly clean before beginning any experiment. Consistent cleaning and testing under identical conditions help in achieving accurate and reliable measurements.

Temperature Effects on Surface Tension

Temperature plays a significant role in determining the surface tension of liquids. As illustrated by the properties of water, the surface tension decreases as temperature increases. This is because higher temperatures give more energy to the liquid molecules, making them move more freely and reduce their cohesive forces. Why does temperature matter?

• Functional Variability: If the temperature fluctuates, surface tension measurements will vary, leading to potential inaccuracies in data.
• Precision in Experiments: Maintaining a consistent temperature throughout an experiment is crucial for obtaining valid results. Without temperature control, results may not truly reflect the properties being measured.
• Standardization: Knowing the relationship between temperature and surface tension allows scientists to standardize methods and compare results more effectively across different studies.

Thus, it is imperative to carefully manage and maintain the temperature during such experiments to ensure the precision and accuracy of the surface tension measurements obtained.