Ich kaufe Mehl.
Wo ist ein / das Mehl?
In my textbook, I found 2 sentences one of which uses an article before Mehl, but previously I had learnt the rule that we don't use any articles with uncountable nouns such as Mehl, Wasser, etc.
Could you please clear it up?

Question

Answer

As already mentioned, you do not use indefinite articles with uncountable nouns, but in spoken German there are some exceptions:
Ein (Glas) Wasser bitte! Einen Reis bitte! When ordering in a restaurant.

Answer

The rule of not using any articles with uncountable nouns applies only to the indefinite articles (unbestimmte Artikel), that is "ein / eine / etc.".
Thus you cannot say "Ich möchte ein Mehl kaufen". You have to say "Ich möchte eine Packung Mehl kaufen. / Ich möchte 1kg Mehl kaufen. / Ich möchte Mehl kaufen. / ..."

But, as DonHolgo said, you can ask "Wo ist das Mehl?". Or "Kannst du mir bitte das Mehl geben? / Das Mehl ist alle."
Very often, however, you say "Mehl" without any article, e.g. "Ich brauche Mehl für den Kuchen."

Answer

It works pretty much like in English: I can ask my girlfriend "wo ist das Mehl?" because I'm talking about *the* flour I know we have somewhere, I just don't remember where. I can't really think of a context where "wo ist ein Mehl?" would work, though.

Answer

Du kannst sagen ein Päckchen Mehl. Das wären dann meistens 1000g Mehl. Aber ein Mehl gibt es nicht. 
Einen Löffel Mehl, ein Glas Mehl, ein Kilo Mehl, einen Sack Mehl, einen Haufen Mehl.....

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