# Determine the density of oil and water using a u tube

#### osalselaka

Hi, I have a question within physics practical, so I hope your help for solve this priblem.

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?

#### studiot

I assume you realise that the liquids must be immiscible and must be added gently?

What happens when you add one liquid to another in an ordinary measuring cylinder, not a U tube?

Describe both ways round. (more dense to less dense and less dense to more dense)?

Now can you answer your own question about the U tube version?

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#### osalselaka

OK, according to your logic I could find the answer for the first question. In second situation mainly I can't to describe. According to the text book I add oil for messurenent. When the oil level is increasing then the oil level is going to the opposite side of the u tube. While the water is more density ,as that reason oil is goin to up of the water* so how the secon situation is correct.....

#### Woody

The high density liquid will try to find a position below the low density liquid.
If you add the low density liquid first, the high density liquid will try to move through and/or around the low density liquid to get to the bottom, and you are likely to end up with the two liquids mixing.

#### studiot

The high density liquid will try to find a position below the low density liquid.
If you add the low density liquid first, the high density liquid will try to move through and/or around the low density liquid to get to the bottom, and you are likely to end up with the two liquids mixing.
It's worse than that.

Since the liquids are immiscible, they will eventually separate.
When they do, if you add water second, you could end up with some oil on each side of the U tube, held apart by the water plug in the bottom.

#### Woody

Thinking about your second point.

I think that you could do the experiment either way.
Either keep the oil constant and vary the water,
Or keep the water constant and vary the oil
(assuming both ends of the U tube are OK for adding fluid).

I am guessing that your text book has just chosen one of the options,
this makes it possible for the author of the book to define a single expected result
rather than having to try to follow the two possible outcomes.

It is sometimes useful to read on a few problems further on from the immediate question in the text book.
Sometimes the later problem presented will be something like "repeat the experiment varying the water rather than the oil...",
This then gives you a clue to the direction the book is trying to guide you and thus a clue to answering the first question.

#### studiot

A sketch makes it easier to see.

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