In order to pass students need to get a grade 4 or above. In order to achieve this students must ensure they are carrying out effective revision strategies, such as using active recall techniques to consolidate knowledge and completing practice papers/questions.
This varies depending on the exam board and on the difficulty of the paper. If the questions in the paper are very hard one year then the boundary is lowered, and if they are slightly easier in another year then it is raised. In general, this level can sit at around 70-80%. Exam boards publish the details of their past grade boundaries online.
Getting a nine is not just based on memorising and regurgitating all the course content in the paper. In order to get a nine you must be able to apply your knowledge to new scenarios and must also have additional skills such as mathematics and data interpretation, and also knowledge of practical investigations. The best way to improve these skills is to complete practice questions, either through past papers or using resources such as those found at Save My Exams. Repetition is a good way to improve, as well as studying mark schemes and taking the time to learn from your mistakes in order to improve.
This varies depending on the exam board and whether you're taking combined or separate science. In general, most courses have two papers with different ways that the content is split across the two papers. Always check your specifications for the exact details of your board.