Question

1. Give an example that you have observed of surface tension in your own life.

2. Define cohesion in your own words. Give an example.

3. Define adhesion in your own words. Give an example.

4. How does water get to the leaves in the tops of the tallest trees against the force of gravity? Name the property responsible for this and explain how it works.

Answer 1

1. The phenomena of surface tension can be observed in your surroundings. For example- this one day in our school's physics laboratory, I accidentally dropped a mercury thermometer. You must be knowing that mercury is the only metal that is liquid at room temperature. But unlike any other liquid, mercury when spilled, formed into little spherical drops and did not flow anywhere like water or milk. This was due to high surface tension of mercury.

2. Cohesion and adhesion both words are made with the use of root word 'hesion'. Meaning 'to stick'. The attractive force between two similar molecules is termed cohesion. An example showcasing this phenomena could be water. Water can be kept in open containers because of the intermolecular attractive forces that exists tend to exhibit cohesive properties and hold it together.

3. Adhesion refers to the attractive force between two umlike molecules. For example, water drops sticking to the surface of window panes is an example of adhesion.

(You've heard of adhesive glue and tapes, right! What is it? Nothing but to stick two surfaces together) :D

4. The phenomena is known as capillary action or capillarity. It is the ability of liquids to move through thin and narrow spaces, in opposition to (and without) external force, like gravity.

This is why the color gets absorbed in the hair of paintbrush. And how an ink pen sucks back the ink you spill on a non- porous surface.

If you dip a really thin tube with narrow lumen in water, you could see the water automatically flowing upwards. The tube is called a capillary (should be available in lab).

The capirally action is caused by nothing other than the two forces namely, adhesion and cohesion. In the above case- adhesion between the molecules of liquid and solid tube, and inter-molecular cohesion (among the particles of water/liquid) tends to lift the liquid up in the cappilary.

Same property leads to drawing up of water to the top leaves in tall trees. Besides, other factors are

(i) vacuum - in tall trees, lower atmoespheric pressure creates vacuum causing suction of water.

(ii) electric charge in water molecules- molecules being charged stick to each other tightly causing the upper molecule to pull the lower one.

(iii) cohesion and adhesion- water molecules stick to each other because of the attractive force leaving no room for air and tighter strength. Adhesion causes sticking of water molexules to xylem walls.

(iv)negative and positive water potential- the difference in water potential, obviously makes it move from higher to lower gradient.