Class 9 Science- Chapter 1- Matter In Our Surroundings – Activity 1.8: Testing the Surface Tension of Water Using Fingers

Activity 1.8: Testing the Surface Tension of Water Using Fingers

Objective

To demonstrate the concept of surface tension in water and understand why the surface of water resists external forces

Materials Needed

• A container filled with water (e.g., a bowl or glass).
• Your fingers.

Steps to Perform the Activity

1. Fill a container with water and let it settle so the surface is calm.
2. Gently place your finger on the surface of the water and try to “cut” through it using a downward motion.
3. Observe whether you are able to break or cut the water surface easily.

Observations

• When you press your finger on the surface of the water, the surface resists breaking.
• Even though the water moves slightly, it seems to hold together as if there is an invisible film on the surface.

Also Check – Chapter 1 -MATTER IN OUR SURROUNDINGS -Class 9 – Simplified notes

Concepts Behind the Activity

1. What is Surface Tension?
   Surface tension is a property of liquids caused by the cohesive forces between liquid particles. These forces are especially strong at the surface because the particles at the surface are not surrounded by similar particles on all sides and are pulled inward by the liquid below.
2. Why Does the Surface Resist Cutting?
   The cohesive forces between water molecules create a “film-like” effect on the surface. This is why the surface resists breaking when you press it with your finger.
3. Intermolecular Forces:
   The particles of water are attracted to each other due to intermolecular forces. At the surface, these forces act inward, making the surface behave like a stretched elastic sheet.

Learning Outcomes

1. Understanding Surface Tension:
   The surface of water acts like a stretched membrane due to cohesive forces, which is why it resists cutting.
2. Nature of Liquids:
   Liquids exhibit unique properties, like surface tension, due to the arrangement and interactions of their particles.
3. Real-World Applications:
   • Surface tension allows insects, like water striders, to walk on water.
   • Droplets of water form a spherical shape due to surface tension minimizing the surface area.

Applications of Surface Tension

• Understanding how detergents and soaps reduce surface tension to clean surfaces more effectively.
• Explaining why certain small objects, like paper clips, can float on water even though they are denser than water.

Intext and additional Question Answers on Activity 1.8: Testing the Surface Tension of Water Using Fingers

Question 1: Were you able to cut the surface of water with your fingers?

No, the surface of water resists breaking because of surface tension. This property makes the water surface behave like a stretched elastic film.

Question 2: Why does the surface of water remain together?

The cohesive forces between water molecules create a strong attraction at the surface, forming a “film” that resists external forces.

Additional Question: What real-life applications rely on water’s surface tension?

• Water striders can walk on water due to surface tension.
• Droplets of water form a spherical shape because surface tension minimizes the surface area.
• Surface tension allows capillary action in plants, helping water move through narrow tubes.

Conclusion

This activity demonstrates the concept of surface tension. The cohesive forces between water molecules create a resistance at the surface, making it behave like a thin elastic film. This property is essential in various natural and practical phenomena.