Yes, you can take definitely take pure science in the 11th standard itself even if you have opted for basic math in the 10th standard. Pure science generally means streams like Physics, Chemistry, and Biology (PCB). Though mathematics is essential for some science branches, it is not mandatory to have mathematics to get into PCB in the 11th standard.Most schools and boards offer flexibility in selecting subjects. This way, even without having studied standard mathematics in 10th class, a student can join science streams.  The course structure, however, varies from school to school or board to board.Though If you wish to have a career very reliant on mathematics (like, engineering ), you might be advised to take mathematics as well with your science.You can consult an academic counsellor for better understanding.