TITLE: Is there a law that you can add or multiply to both sides of an equation?
QUESTION [31 upvotes]: It seems that given a statement $a = b$, that $a + c = b + c$ is assumed also to be true.
Why isn't this an axiom of arithmetic, like the commutative law or associative law?
Or is it a consequence of some other axiom of arithmetic?
Thanks!
Edit: I understand the intuitive meaning of equality. Answers that stated that $a = b$ means they are the same number or object make sense but what I'm asking is if there is an explicit law of replacement that allows us to make this intuitive truth a valid mathematical deduction. For example is there an axiom of Peano's Axioms or some other axiomatic system that allows for adding or multiplying both sides of an equation by the same number? 
In all the texts I've come across I've never seen an axiom that states if $a = b$ then $a + c = b + c$. I have however seen if $a < b$ then $a + c < b + c$. In my view $<$ and $=$ are similar so the absence of a definition for equality is strange.

REPLY [5 votes]: As all the others have said, from $a=b$ you can conclude that $f(a)=f(b)$ for any function defined on a domain containing $\{a,b\}$.
There is a subtle point, however, that has not been addressed so far: An equation
$$\Phi(x)=\Psi(x)\tag{1}$$
implicitly defines a solution set $S$ in the agreed domain of discourse. "Solving" such an equation means producing an "explicit" description of $S$, e.g.,  in the form of a list or of a parametric representation of $S$.
Applying some function $f$ to both sides of $(1)$ (say, squaring both sides) in order to obtain a simpler equation may very well alter (meaning: enlarge) the solution set $S$.