are grace marks given in hsc science for qualifying marks of 50%?

Pranav, To be honest, it is very difficult to predict whether you would be given grace marks or not as it entirely depends on your educational board and its' normalization norms. Now, if your exams of these subjects where you think you have under performed are done, then you cannot change anything and thinking about the marks or the chances of passing would not do any good, rather focus on other subjects so that you can compensate for that "not-so good" performed exams but if you are yet to appear for these exams and facing "exam anxiety", then stop overthinking and focus on what would help you that is, your focus should be on your preparation only. As regarding grace marks or what some technically refer to as normalization, it is very difficult to predict if there would be grace marks given, although most of the educational boards and the authorities have a policy of normalization of marks in case of marks being very close to passing marks, so you should hope for the same. In addition to it , you should not predict anything until the results are out as you never know the examiner's thought- process while checking your answers, so all you can do is to hope for the best and prepare for the other exams, if there are any.

Good Luck.