You would be amazed how many marks are lost by candidates in the GCSE theory papers and in the internal assessment exercise due to poor communication skills. Examiners and coursework moderators will never make assumptions about what you know or give you the 'benefit of the doubt' if your answer is not clear. Too many answers lack detail and/or clarity.

Unless the answer is obviously one word, try to write whole sentences, and consider how much space has been allowed for an answer. If you are given 5 lines and have only used half a line, then you probably need to write more! Read through what you've written and think if a reader might need to ask more questions of you to get a full understanding. When you are doing past papers at home, get a family member who isn't a science expert to read your answers and see if they can understand the answer you've written. If they can't, you need to alter it.

One final warning - this is about making your answer clearer and more detailed, not just longer. Clarity is good - waffle isn't!