#40Q1) Mechanics and Properties of Matter

Determination of the surface tension of water using a microscope slide

DifficultyMedium
Est. Time45 mins

Required Apparatus

A microscope slide, a petri dish, a four beam balance, a vernier calliper, few pieces of wire, a diluted, sodium hydroxide solution, and a diluted nitric acid solution, a micrometer screwgauge

Scientific Theory

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Let mg be the load that balances the surface tension force on the lower perimeter of the slide just touching the water surface when the slide is suspended from the balance. Also let T be the surface tension of water and a and b the length and thickness respectively of the slide. Then, 2 ( a + b ) T = m g

Experimental Method

Wash the microscope slide first with the diluted sodium hydroxide solution, then with the diluted nitric acid solution, and finally with pure water. Clean it thoroughly. Suspend the slide horizontally using clips and strings from the balance, and adjust the balance so that it is in equilibrium. Raise the beaker slowly and allow the lower surface of the slide to just touch the water surface in the beaker.

The equilibrium of the balance will be disturbed. Restore the equilibrium and determine the additional load required. Remove the slide and measure its length using a vernier caliper and its thickness using a micrometer screw gauge at three different points.

Let:

  • bb = mean thickness of the slide
  • aa = mean length of the slide
  • mm = excess load used

Using the theory, the surface tension of water is calculated by:

T=mg2(a+b)T = \frac{mg}{2(a + b)}

Important Points

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● Since the surface tension of water varies with temperature, it is more suitable to note down

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the temperature during the experiment and decide according to the temperature.

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● It has been assumed that water wets glass and hence the angle of contact is zero.

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● Care should be taken to use the situation that the slide just touches the water surface. If not

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an upthrust would exist and has to be accounted for.

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● For the fi nal washing of the slide tap water and not distilled water should be used. Since

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substances such as grease is used in the production of distilled water it could contain oily dirt.