# Need an explanation -- Consider pure water separated from....

• andkand97
In summary, after some time has elapsed, the concentration of starch solution separated from pure water by a semipermeable membrane will have decreased. This is because the water can freely pass through the membrane, leaving behind a higher concentration of starch solution. Water will not necessarily be completely gone, but the concentration of starch solution will have increased. This is because the volume of starch solution remains the same while the amount of water decreases.
andkand97
Please post this type of questions in HW section using the template.
Consider pure water separated from an aqueous starch solution by a semipermeable membrane, which allows water to pass freely but not starch. After some time has elapsed, the concentration of starch solution
a. will have increased
b. will have decreased
c. will not have changed
d. might have increased or decreased, depending on other factor.
e. will be the same on both sides of the membrane

The answer key says that the answer is b. will have decreased. This doesn't make sense to me. My understanding is that if the water is gone, the volume the starch is occupying has decreased but the amount of starch stayed the same. Intuitively, this would lead me to believe that the concentration has increased (assuming by concentration they mean something like molarity, where molarity = moles of solute/ volume ). My dad, who has a PhD in Chemistry, agrees with me. Can someone explain to me why it would decrease? Thanks!

Why do you think water will be gone?

Borek said:
Why do you think water will be gone?
Well, the question says "consider pure water separated from an aqueous solution by a semipermeable membrane". Isn't the water being separated from the starch solution?

What it means is you have two solutions and a semipermeable membrane between them (separating them as in "don't allowing a direct mix"), it doesn't say anything about what is happening to water (other than that it can freely move - but it is up to you to decide which way it moves).

Borek said:
What it means is you have two solutions and a semipermeable membrane between them (separating them as in "don't allowing a direct mix"), it doesn't say anything about what is happening to water (other than that it can freely move - but it is up to you to decide which way it moves).
Oh, okay that makes more sense. Thanks!

## 1. What is the meaning of "pure water" in this context?

Pure water refers to water that does not contain any impurities or contaminants. It is composed of two hydrogen atoms and one oxygen atom (H2O) and has a neutral pH of 7.

## 2. How is pure water separated from impurities?

Pure water can be separated from impurities through a process called distillation. This involves boiling the water, collecting the steam, and then condensing it back into liquid form. The impurities are left behind as the water turns into vapor.

## 3. What are some common impurities found in water?

Some common impurities found in water include minerals, chemicals, bacteria, and sediment. These impurities can come from various sources such as soil, pipes, and industrial pollution.

## 4. Why is it important to have pure water?

Pure water is essential for our health and well-being. It helps to maintain proper hydration and can prevent the spread of waterborne diseases. It is also important for industrial and agricultural purposes as impurities can affect the quality and effectiveness of products.

## 5. Is pure water safe to drink?

Yes, pure water is safe to drink as it does not contain any harmful impurities. However, it is important to note that some minerals and chemicals, although not harmful, can give water a different taste or odor. This does not affect the safety of the water.

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