I am a Material Science and Engineering major.What I am having doubt with is how I am supposed to write long answers.Lets say someone asks,"What is a Lomer Cottrell barrier?" or may be,"What is solid solution strengthening?"Actually I write answers to these but those answers never fetch me a more than 50% marks.How do I get through?Please help.
Your question, and a comment you posted, suggest a misguided belief that “the art of writing long answers” in the context of an exam or homework assignment is somehow different than the general art of writing (and the even more general art of communicating your thoughts to others clearly and efficiently). Sorry, but that’s not true, and you’re looking for help in the wrong places. I, and everyone posting here, have never taken a class or read a manual on “writing long answers”. To write long answers well, you need to write well, and that’s something that a stack exchange answer can’t teach you. There are many books about it, but as people on the sister site writing.stackexchange like to say, the best way to develop good writing skills is to write, write, and write some more. Over time you will see your skills improve with practice.
Second, my experience with science and engineering is that the focus in these areas is usually on answers that are conceptually correct and show a good level of understanding of the material (even if they are not especially well-written or phrased) rather than on the quality of the writing itself. So you should also consider the possibility that if you’re losing 50% of the points or more on your answers, that may be because you don’t actually understand the material as well as you think you do (and as well as you should) - this may not be a writing issue per se. Even the best writer in the world will not be able to write a good answer to a question on a topic they don’t understand well. As others have said, it would be a good idea to review the feedback you get from your instructors and if necessary ask for additional feedback, to try to develop an understanding of where exactly the problem lies. Good luck!
As @GrotesqueSI said, go ask the teacher what s/he wants in an answer and the format of it. The length, how many references (you are adding references, right? in APA? from veritable sources?).
It also helps if you revise how you write. Given your field some tutorial/books about technical writing could be useful. In the first paraghph you give the concrete answer and then you explain the details in the next paragraphs. max 5 paragraphs and .5- 1 page should be enough.
For docs on technical writing:
Looking at your questions on the chemistry forum (from your profile), I get the impression your issues are much more from knowing the material than from exposition (writing ability). Although there's definitely an English language issue as well (if you are taking tests in that language).
My advice is to work more problems and get help with teachers (reviewing your drill problems). Basically what you are doing on the forums here, but with more intensity. Do homework problems until you puke and then wipe off the puke and do some more. Look into getting extra problem sets (with solutions or at least answers, so that you can check your work).
Get help for questions you don't understand--good that you are actively using the chem stack exchange. But you may want to get in person help as well. Also, perhaps you need to work more easy problems if you are struggling with all the hard problems you're showing on the site here. (Like working on my double back flips to get them perfect, if I'm having a problem with triples.)
In particular, many of your questions revolve around the trickier types of stoichiometry problems, especially in an applied (therefore complicated word problem) setting. The problems you're mostly showing seem to have some complication around them, but at their core they use freshman chemistry insights.
I have seen EXTREMELY poor English speaker/writers, still able to do well in classes like material science or inorganic chemestry. Even to ace the material. So, I really suspect your issue is more one of the subject matter content than of writing ability. Yeah, you need work on that too, but it's not the "limiting reagent" of the getting you a good grade reaction.