I learned we cannot live without chemistry: shampoo, soda, drugs, toothpaste, and all material in our houses are chemistry.
...goal with hard work and dedication.
After I started my high school life, I encountered different science classes such as biology, physics, and chemistry, in which chemistry is my favor. I felt so excited about the chemical reactions and atomic structure. I learned we cannot live without chemistry: shampoo, soda, drugs, toothpaste and all material in our houses are chemistry. Chemistry like penicillin saves people's lives and changes the world into the age of antibiotic. Chemistry can create a beautiful world. Thus, in the second year I was in the United States while my English is still so limited, I chose AP chemistry as my first AP class to learn more new knowledge of chemistry.
Repetition is not bad at all, for its one aim is to emphasize your point. However,
if you improperly use it , that will only make the readers tired reading your paragraph. Try to count how many times you wrote the word "chemisrty" which I have highlighted above. Please minimize reiterating that word as much as possible. You can use pronoun "it" to replace it :)