Balance the following redox equation in acidic solution

H₃PO₂ + Cr₂O₇^2- → H₃PO₄ + Cr³⁺

Step #1 - Write the half-reactions

H₃PO₂ → H₃PO₄

Cr₂O₇^2- → Cr³⁺

Step #2 - Balance the elements that are NOT oxygen and hydrogen

H₃PO₂ → H₃PO₄

Cr₂O₇^2- → Cr³⁺

Step #3 - Balance oxygen with water

H₂O + H₃PO₂ → H₃PO₄

Cr₂O₇^2- → Cr³⁺ + H₂O

Step #4 - Balance hydrogen with hydrogen ions, H⁺

H₂O + H₃PO₂ → H₃PO₄ + H⁺

H⁺ + Cr₂O₇^2- → Cr³⁺ + H₂O

Step #5 - Balance charges with electrons, e^-

H₂O + H₃PO₂ → H₃PO₄ + H⁺ + e^-

e^- + H⁺ + Cr₂O₇^2- → Cr³⁺ + H₂O

Step #6 - Balance electrons lost and gained

H₂O +H₃PO₂ → H₃PO₄ + H⁺ + e^-

e^- + H⁺ + Cr₂O₇^2- → Cr³⁺ + H₂O

Step #7 - Sum the two half-reactions

H⁺ + H₃PO₂ + Cr₂O₇^2- → H₃PO₄ + Cr³⁺ + H₂O