Why did the Titanic sink?

my daughter has a science project. it is supposed to be a green project as well. she feels that ships/boats sink when the air in the bulkheads are replaced with water. she is particularly interested in the titanic, as it had 16 bulkhead compartments, and still sunk. she is wondering if a different material other than air could be used as flotation. combining this with green technology, she feels old styrofoam could be used instead, and thinks it could have saved the titanic from sinking. Any ideas?