how can i know if a First order logic sentences is valid or unsatisfiable or neither? i have trouble with understand this type of questions , 

i know how i can determine if the sentences is valid or unsatisfiable in Propositional logic , but in FOL i can't 

for example , i have the following sentences in FOL : 

1 = 2 


the professor said this is neither valid nor unsatisfiable 

but , i think it is always unsatisfiable , because 1 != 2 !!! 

can someone explain to me how can i answer this type of question 

thank you all
A formula is valid if all of the possible truth-assignments to its variables evaluate to true.

A formula is satisfiable if at least one of the truth-assignments to its variables evaluates to true.

The formula you gave is :


  $1 = 2$


Since this contains no variables, under any truth-assignment, the expression is $1 = 2$, which is false.

Neither of the above conditions have been met.

Therefore the formula is neither valid nor satisfiable. Since it is not satisfiable, we can say that it is unsatisfiable.