Question

In the equation: `A + B -> C+D`, what are A and B?

Answer 1

Reactants

Explanation:

In a chemical equation you have

Reactants `rarr` Products

The reactants are the compounds which react to make the products.

Answer 2

`A` and `B` are called the reactants of the chemical reaction.

Explanation:

For any chemical reaction, we write the chemical equation with the `sfcolor(red)("reactants"` on the `sfcolor(red)("left side"`, the `sfcolor(blue)("products"` on the `sfcolor(blue)("right side"`, and the reaction arrow between them, pointing most often to the right, to indicate that the reaction progresses from reactants to products.

[In the case where the reaction progresses to a state of equilibrium, you'll often see the arrow pointing in both directions (`rightleftharpoons`) to indicate that the rate of the forward reaction (reactants form the products) is the same as the rate of the reverse reaction (products form the reactants)].

Answer 3

`"Reactants" => "Products"`

Explanation:

`underbrace("A+B")_ "Reactants" => underbrace("C+D") _"Products"`

Example: for mass/mass calculations in equations...

`2Mg^o(s) + O_2(g) => 2MgO(s)`

`underbrace( overbrace("45.0 g")^(color(blue)("mass of Mg")) + overbrace(?color(white)(.)"g")^(color(blue)("mass of O"_ 2)))_ (color(purple)("the mass of the reactants")) => underbrace( overbrace("74.62 g")^(color(blue)("mass of MgO")))_ (color(purple)("the mass of the product"))`*

(*Underbrace formatting from Stefan V post.)