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Basically, what are some mistakes that you can make while using the downward displacement of water method and how to fix them? This is actually a homework question and I'm not too sure what it means either. The original question:

State and explain Dalton’s Law of Partial Pressures. Why is it necessary to correct the pressure of a gas that is collected over water? {Background: Gases formed in a chemical reaction are collected using a technique called the downward displacement of water, feel free to research this technique before you respond to this question}

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    $\begingroup$ Could you please add more context? It's not quite clear how (and why) one would "correct" the pressure in the first place. Posting an example of an experimental setup or citing a textbook where you encountered this info would help to understand your question better. $\endgroup$
    – andselisk ♦
    Commented Jul 22, 2019 at 7:54
  • $\begingroup$ Here's the full question: State and explain Dalton’s Law of Partial Pressures. Why is it necessary to correct the pressure of a gas that is collected over water? {Background: Gases formed in a chemical reaction are collected using a technique called the downward displacement of water, feel free to research this technique before you respond to this question} $\endgroup$
    – Rumaysa Khan
    Commented Jul 22, 2019 at 8:22
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    $\begingroup$ This comment should be an edit to your question. Since it's a homework question, you are expected to demonstrate your effort in solving the problem (literature/internet research also counts). $\endgroup$
    – andselisk ♦
    Commented Jul 22, 2019 at 8:26
  • $\begingroup$ He never actually taught us this so I was hoping to find some help somewhere else. But the question also says that we can research before actually answering. $\endgroup$
    – Rumaysa Khan
    Commented Jul 22, 2019 at 8:29
  • $\begingroup$ see eg quora.com/What-is-downward-displacement-of-water $\endgroup$
    – Buck Thorn ♦
    Commented Jul 22, 2019 at 9:37

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Generally, gases which are insoluble in water are collected by putting them into an inverted vessel via a tube, so that the gas rises to the top and 'displaces' the water downward. Take a look at the following picture—

Downward displacement of water

Now, the problem is that water has a certain vapour pressure at any temperature. In other words, whenever water is exposed to a gaseous phase, it will start to evaporate until the partial pressure of the water vapour in the gas phase reaches a certain value (this value depends on the temperature). This value is known as the 'vapou pressure of water'.

In this case, the gas you collected by displacing the water will be contaminated by water vapour. Therefore, the pressure of the gas which you measure will be the sum of the partial pressure of the product gas, and partial pressure of water vapour. $$\begin{align*} Measured\;pressure &= Partial\;pressure\;of\;product\;+\;partial\;pressure\;of\;water\;vapour \\ &=Actual\;pressure + Vapour\;Pressure \end{align*}$$ This means that you must correct the pressure, by subtracting the vapour pressure of water from the measured value to get the actual pressure of the produced gas.

Note: There are standard tables which list the value of vapour pressure of water at different temperatures. These tables are generally used for the correction.

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