Surface Tension Lab Worksheet

To Study the Effect of Detergent on Surface Tension of Water by Observing Capillary Rise

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Experiment Overview

This experiment investigates how adding detergent affects the surface tension of water by measuring the change in capillary rise in glass tubes. Surface tension is a fundamental property of liquids that determines many of their behaviors, and understanding how substances like detergents can modify it has important practical applications.

Surface Tension: Basic Concepts

Surface tension is a property of liquid surfaces that makes them behave like elastic sheets. It arises due to the cohesive forces between liquid molecules. At the surface, these molecules experience an inward pull since they have neighboring molecules only on one side.

Surface tension ($\gamma$) is defined as the force per unit length acting perpendicular to an imaginary line drawn on the liquid surface:

$$\gamma = \frac{F}{L}$$

where $F$ is the force and $L$ is the length of the line. The SI unit of surface tension is N/m or J/m².

Capillary Rise

Capillary rise occurs when a liquid rises in a narrow tube against gravity due to surface tension and adhesive forces between the liquid and the tube walls. For a liquid that wets the tube (contact angle < 90°), the height of capillary rise is given by:

$$h = \frac{2\gamma\cos\theta}{\rho g r}$$

where:

  • $h$ = height of capillary rise
  • $\gamma$ = surface tension of the liquid
  • $\theta$ = contact angle between liquid and tube wall
  • $\rho$ = density of the liquid
  • $g$ = acceleration due to gravity
  • $r$ = radius of the capillary tube

Effect of Detergents

Detergents are surfactants (surface-active agents) that reduce the surface tension of water. They have a hydrophilic (water-loving) head and a hydrophobic (water-repelling) tail. When added to water, they align at the water-air interface with their hydrophobic tails pointing away from water, disrupting the cohesive forces between water molecules and reducing surface tension.

As the concentration of detergent increases, surface tension decreases until it reaches a critical micelle concentration (CMC), beyond which additional detergent forms micelles in the solution rather than further reducing surface tension.

Apparatus

  • Capillary tubes of different diameters
  • Traveling microscope or vernier calipers
  • Laboratory stand with clamps
  • Beakers (250 mL) - 5
  • Measuring cylinder (100 mL)
  • Magnifying glass
  • White background card

Chemicals

  • Distilled water
  • Liquid detergent (e.g., dish soap)
  • Alcohol (for cleaning)
  • Tissues for cleaning

Other Tools

  • Thermometer
  • Stopwatch
  • Graph paper
  • Calculator
  • Ruler
  • Marker/pen

Experimental Apparatus

Capillary rise experimental setup showing capillary tubes in beakers with different detergent concentrations

Figure 1: Experimental setup for measuring capillary rise in tubes with varying detergent concentrations

Steps for Preparation

  1. Clean all glassware thoroughly with distilled water and alcohol. Dry completely.
  2. Clean the capillary tubes by rinsing with alcohol followed by distilled water. Ensure they are completely dry before use.
  3. Measure the inner diameter of each capillary tube using a traveling microscope or other appropriate methods.
  4. Prepare a stock solution of detergent in distilled water (e.g., 10% concentration).
  5. From the stock solution, prepare a series of detergent solutions with different concentrations (e.g., 0%, 0.5%, 1%, 2%, 5%).
  6. Label each beaker with the respective concentration.
  7. Set up the apparatus as shown in Figure 1, ensuring the capillary tubes are held vertically.

Procedure Steps

  1. Record the room temperature as it affects surface tension.
  2. Fill a beaker with distilled water (0% detergent).
  3. Take a clean capillary tube and immerse one end in the distilled water, ensuring it is vertical.
  4. Hold the tube steady and observe the rise of water in the capillary tube until it reaches equilibrium (approximately 2-3 minutes).
  5. Measure the height of the water column in the capillary tube from the water surface in the beaker. Use a white card behind the tube for better visibility.
  6. Repeat the measurement three times for accuracy and calculate the average height.
  7. Clean and dry the capillary tube thoroughly.
  8. Repeat steps 3-7 for each detergent concentration (0.5%, 1%, 2%, 5%).
  9. If multiple capillary tubes of different diameters are available, repeat the entire procedure for each tube.

Important Notes:

  • Keep the temperature constant throughout the experiment.
  • Ensure that capillary tubes are completely clean before each measurement.
  • Keep the capillary tube perfectly vertical during measurements.
  • Wait for the liquid level to stabilize before taking measurements.

Record Your Observations

Record the inner diameter of the capillary tube: ______________ mm

Room temperature: ______________ °C

S.No. Detergent Concentration (%) Height of Capillary Rise (mm) Average Height (mm)
Reading 1 Reading 2 Reading 3
1 0% (Pure Water)
2 0.5%
3 1%
4 2%
5 5%

For Multiple Capillary Tubes (Optional)

If using multiple capillary tubes with different diameters, create separate tables for each tube.

Graph Template

Graph template for plotting capillary height vs detergent concentration

Figure 2: Template for plotting capillary height vs. detergent concentration

Calculating Surface Tension

For each concentration, calculate the surface tension using the capillary rise formula:

$$\gamma = \frac{h \times \rho \times g \times r}{2\cos\theta}$$

Where:

  • $\gamma$ = surface tension (N/m)
  • $h$ = height of capillary rise (m)
  • $\rho$ = density of the liquid (kg/m³) - for water at 25°C, $\rho$ ≈ 997 kg/m³
  • $g$ = acceleration due to gravity (9.8 m/s²)
  • $r$ = radius of the capillary tube (m)
  • $\theta$ = contact angle (assumed to be approximately 0° for water in clean glass, so $\cos\theta$ ≈ 1)
Detergent Concentration (%) Average Height (m) Calculated Surface Tension (N/m) % Reduction in Surface Tension
0% (Pure Water) 0% (Reference)
0.5%
1%
2%
5%

Sample Calculation

Show your calculation for one concentration here:

For 0% detergent (pure water):

Given:

  • Average height ($h$) = ___ m
  • Radius of capillary tube ($r$) = ___ m
  • Density of water ($\rho$) = 997 kg/m³
  • Acceleration due to gravity ($g$) = 9.8 m/s²
  • Contact angle ($\theta$) ≈ 0° ($\cos\theta$ ≈ 1)

Substituting in the formula:

$$\gamma = \frac{h \times \rho \times g \times r}{2\cos\theta}$$ $$\gamma = \frac{\_\_\_ \times 997 \times 9.8 \times \_\_\_}{2 \times 1}$$ $$\gamma = \_\_\_\_ \text{ N/m}$$

Percent Reduction in Surface Tension

$$\text{Percent Reduction} = \frac{\gamma_{\text{water}} - \gamma_{\text{solution}}}{\gamma_{\text{water}}} \times 100\%$$

Results

  1. Plot a graph of capillary rise (mm) versus detergent concentration (%).
  2. Plot a graph of surface tension (N/m) versus detergent concentration (%).
Graph showing relationship between detergent concentration and surface tension

Figure 3: Graph showing relationship between detergent concentration and surface tension

Discussion Questions

  1. How does the addition of detergent affect the capillary rise in glass tubes?
  2. Explain the relationship between detergent concentration and surface tension.
  3. At what concentration does the detergent seem to have the most significant effect on surface tension? Why might this be the case?
  4. Why does detergent reduce the surface tension of water? Explain in terms of molecular interactions.
  5. What practical applications can be derived from understanding how detergents affect surface tension?
  6. What sources of error might have affected your results? How could the experiment be improved?

Conclusion

Write a concise conclusion based on your observations and analysis:

Practical Applications

  • Cleaning and Washing: Detergents lower surface tension allowing water to spread more easily and penetrate fabrics for more effective cleaning.
  • Emulsification: Reduced surface tension helps in forming and stabilizing emulsions in food and pharmaceutical industries.
  • Firefighting: Adding surfactants to water reduces its surface tension, making it more effective at penetrating burning materials.
  • Agriculture: Surfactants in pesticide formulations help the solution spread over and adhere to plant surfaces.
  • Medicine: Understanding surface tension is important in pulmonary medicine, particularly for surfactant therapy in premature infants.

Further Investigations

  1. Compare the effects of different types of surfactants (anionic, cationic, nonionic) on water's surface tension.
  2. Study how temperature affects the relationship between detergent concentration and surface tension.
  3. Investigate the effect of water hardness on the effectiveness of detergents in reducing surface tension.
  4. Compare the surface tension reduction properties of biodegradable versus non-biodegradable detergents.
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