Assuming that you're using Word, what you should be doing at the end of every chapter is this:
Insert > Break > Page Break > OK.
I'm actually a bit confused by what you've written. Surely, if Chapter 1 finishes half way down a page, you want to leave a space at the bottom of the page so that Chapter 2 starts on the next page? I can't quite see how it would be possible to close up that space and still have the Chapter 2 heading on a fresh page.
As I've stated, I'm not entirely clear what your problem really is. Perhaps you've already got a page break in between the chapters? If you then try to add something to Chapter 1, by typing after the page break, your new text will appear on the next page (above the Chapter 2 heading). If that's the cause of your problem, you either need to remove the existing page break and insert a new one or (probably simpler) move the text to the correct side of the break. To do this, highlight the text which has strayed to the top of page 2. Right-click and select 'copy'. Place the cursor at the exact end of the text which forms Chapter 1. (Not further down). Right-click and select 'paste'. Then adjust the spacing, as required.
Chris