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Author: Subject: can you instruct me ??

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[*] posted on 27-1-2012 at 08:26
can you instruct me ??

hello everyody
this is my first topic at this site....

I want to know which is pour first
water to acid
or acid to water

and why???

my friends said that i must ask experts to avoid problems
i don't know anything about chemestry

thanks for all
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[*] posted on 27-1-2012 at 08:33

"Acid to wata, just like you outta"

If you pour water into acid, the acid really likes to be in the water, so it may splash up as it tries to get into the water.

If you pour acid into water, the acid quickly disperses and no splashes tend to happen.

thats how I learned it.
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[*] posted on 27-1-2012 at 08:34

This belongs in the beginners thread.

I assume you’re referring to the mixing of concentrated strong acids with water. Always add the acid to water and not the other way around. Strong acids strongly dissociate and solvate when dissolved in water. During that process much heat is released, known as solvation enthalpy. As a result, when you pour for instance concentrated sulphuric acid into water, the newly formed solution heats up considerably because the water acts as a heat sink.

By contrast pouring water into concentrated sulphuric acid the released heat can cause the acid to locally overheat, causing splattering.

Even when adding a conc. acid to water, do this slowly and with constant stirring to avoid local overheating which could cause the solution to locally start boiling and thus also splatter.

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[*] posted on 27-1-2012 at 08:35

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[*] posted on 27-1-2012 at 08:43

Acid in water.

That rule applies for few acids in concentrated form, namely sulphuric acid. Selenic acid is another example. The reason is enthalpy of hydration, but rule of thumb is that concentrated acids that resemble the motion of oil, in the sense of greater viscosity than water, are the ones that apply for this rule. Sulphuric and selenic acid resemble oil, except they're miscible with water, of course.

If you were to drop water in such acids, there would be so much heat liberated in a small volume of the drop so its temperature would quickly rise above boiling point, leading to popping and splattering few drops of acid, to probably even a detonation, depending on the scale of what you're doing.

If you're completely oblivious of what you're doing, and have no idea which concentrated acid is prone to such dangerous scenarios, always apply the rule, just to be on the safe side.

One thing I really loathe is when I see chemists and alike (people with a friggin degree!) being careful not to pour water in 5% hydrochloric acid. Not only HCl does not exibit described behaviour, but it's just 5%. You can wash hands with it, FFS.

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[*] posted on 12-3-2012 at 01:18

BTW, the spelling of chemistry is chemistry and not chemestry!
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[*] posted on 12-3-2012 at 02:55

The story of heating is true for the following concentrated acids:

- sulphuric acid of concentration above appr. 70%
- selenic acid of appr. 80%
- fuming nitric acid, not for the normal 65% stuff
- perchloric acid of concentration well above 70%
- phosphoric acid of nearly 100%, not for the normally available 85%
Anhydrides of these acids are MUCH more reactive and these react very violently with water. Normally you'll never encounter these anhydrides, except maybe the anhydride P4O10 of phosphoric acid.

I also tried with glacial acetic acid and 98% formic acid, but these do not show strong heating when mixed with water.

A similar rule is also true for strong bases. Alkalies like NaOH, KOH and CsOH produce a lot of heat when water is added to them. Even stronger is the effect with the oxides and peroxides of alkali metals.

An interesting observation is that very conc. HCl (37% by weight) also shows the heating effect, but only weakly. If you mix very conc. HCl with half its volume of water, then the liquid noticeable heats up. When one starts with the liquids at room temperature, then after mixing the resulting liquid is luke-warm, e.g. like warm skin.

[Edited on 12-3-12 by woelen]

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[*] posted on 14-3-2012 at 01:03

you must add only acid to water . if you add water to acid it may heat up a lot and everythin will be splashed up and you will be in great trouble
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[*] posted on 14-3-2012 at 09:45

I can attest with everybody above. Always add small amounts of acid to larger volumes of water very slowly and with plentiful stirring.

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