I did a practical to determine the density of water and oil, the practical succeed but some quesions were arise. So I hope your help for solve these problems.

I did this practical using a u shaped tube, and the density was determined by the height of each liquid due to the pressure.

When I do this practical my text books says we should add high density liquid first and then lower density liquid.

Also when we messure the heights of the liquids (due to pressure) if I want to change the heights to get various messurements(the messurements are taking to draw the chart) the text book says we should add the lower density liquid.

Can you say the reasons for these two situations?

In first situation why firstly add high density liquid?

In second situation why we add the lower density liquid to get various messurements? Why we can't add water to get various messurements? What is the problem? Thank you


2 Answers 2


Speaking as a (retired) experimental scientist I would always do the experiment. So I would have tried adding the low density liquid first to see what happened. Even if the experiment fails done that way (which it will :-) you'll usually learn something in the process.

Anyhow, the problem with adding the low density fluid first is that when you add the high density fluid it will fall through the low density fluid to the bottom of the U tube:

Light fluid first

So this won't give you the arrangement you require with oil on one side and water on the other.

As for your second question, there's no reason why you can't add water to the water side as well as oil to the oil side. I suspect the instructions assume you have added all the water first, so the experiment consists just of adding the oil bit by bit and measuring the heights.

  • $\begingroup$ Sir, I thought that the reason for the second, such like this. The heights are taking from the level of water, in the left side (I determine that oil is in left side ) so when I add water, the height of the oil doesn't change , so by the equation of pressure it gives the same height for water and oil every time. So can't to get various measurement s to draw the chart.(every time same measurements) As that I thought the reason for the second situation. Is there any problem in my thinking? $\endgroup$ Nov 12, 2017 at 8:34
  • 1
    $\begingroup$ @OsalSelaka: If you are just measuring the height difference between the oil/water and water/air interfaces then it certainly makes sense to keep the amount of water constant. Then you're just measuring the pressure difference due to the weight of the oil. Adding water won't change the height difference between the interfaces because it would just move both interfaces up by the same distance. $\endgroup$ Nov 12, 2017 at 8:38

The answer for your second question is that when you add the higher density liquid to one arm, the increase and decrease in the common surface of the two liquids will be equal on both limbs therefore it does you no help. So it is recommended to add the less dense liquid:)


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