Chemistry practice: atoms and equations
Practise reading chemical formulas and balancing a simple equation. Check worked answers and download an original Kurdish revision worksheet.
Read subscripts carefully
In H₂O, the subscript 2 applies to hydrogen, so one water molecule contains two hydrogen atoms and one oxygen atom. In CO₂, one molecule contains one carbon atom and two oxygen atoms. A missing subscript means one. First count each element separately; do not add subscripts as though they were a single total attached to the whole formula.
Use coefficients to balance an equation
Start with H₂ + O₂ → H₂O. There are two oxygen atoms on the left and one on the right. Put 2 before H₂O, giving four hydrogen atoms on the right. Then put 2 before H₂. The balanced equation is 2H₂ + O₂ → 2H₂O. Both sides now contain four hydrogen atoms and two oxygen atoms. Change coefficients, not chemical formulas.
Try, then check
Exercise 1: count each element in 3CO₂. Exercise 2: balance N₂ + H₂ → NH₃ using the smallest whole-number coefficients. Answers: 3CO₂ contains three carbon atoms and six oxygen atoms. The second equation is N₂ + 3H₂ → 2NH₃. Write the atom count on each side to check your answer. These are paper exercises, not instructions to carry out reactions.
Frequently asked questions
May I change H₂O to H₂O₂ when balancing?
No. That changes the substance. Keep the formula fixed and change the number of molecules using a coefficient in front.
Is this a complete exam revision course?
No. This short worksheet practises one foundation skill. Follow your course outline and use your teacher’s assigned material for the rest of the syllabus.