Students in a science class observed that when a strip of magnesium (Mg) metal was placed in a test tube containing hydrochloric acid (HCl), the magnesium began to bubble as hydrogen gas and magnesium chloride are formed. Which of the following choices show both the correctly balanced equation and identify the type of reaction? A.
Mg + 2HCl →2H2 + MgCl2 (single replacement)

B.
Mg + 2HCl →H2 + MgCl2 (single replacement)

C.
Mg + 2HCl →H2 + MgCl2 (synthesis)

D.
2Mg + HCl →H2 + 2MgCl2 (synthesis)

Visualize the problem and identify special cases first examine the problem by drawing a picture and visualizing the motion. apply newton's 2nd law, ∑f⃗ =ma⃗ , to each body in your mind. don't worry about which quantities are given. think about the forces on each body: how are these consistent with the direction of the acceleration for that body? can you think of any special cases that you can solve quickly now and use to test your understanding later? one special case in this problem is if m2=0, in which case block 1 would simply fall freely under the acceleration of gravity: a⃗ 1=−gj^.