Tense of devoir

Question

My immediate instinct was to use "Attention ! Le mélange ne devrait pas trop chaud," but it was marked incorrect. In this particular context, is there a hard reason why it is better to use "Le mélange ne doit pas être trop chaud" instead?

Accepted Answer

The English text to translate is "The mix mustn't be too hot". 
So an obligation and not a recommendation to not do something must be expressed.
"ne devrait pas" means "should not".
Negated devoir in the indicative ("ne doit pas") on the other hand means "must not" (depending on the context, see below). 
An alternative to express must not is "il ne faut pas". So in this example:"Il ne faut pas que le mélange soit trop chaud".
Not sure, if this is listed as an option.
As a side note: devoir can also mean, like must in English, a logical implication. In this case negated devoir translates to "can't" and this is only clear from context. So "Le mélange ne doit pas être trop chaud." could also translate to "The mix can't (possibly) be too hot." e.g. because I only heated it for a minute.

Answer

Bonjour Winging,
Christian is correct!
Also, the option "Il ne faut pas que le ménage soit trop chaud" is a great example. However, it is not included in the exercise as it is an A2 level exercise and therefore the use of Le Subjonctif is too advanced (B2 level -> Il faut que + the subjunctive mood (Le Subjonctif) in French).
I hope this is helpful.
Bonne journée !