disappear off my page code

I am new to wordPress and I am writing some code for a particular page.

I am writing in the code window, not the wysiwyg window. When I want a line break between paragraphs I put in <p></p>, which works when I save and preview the page. But when I shut the page down and go back to it the <p></p> is gone and I no longer have line breaks. Also, the <p> I put in at the start of paragpraphs and </P> at the end also disappears.

Example code when I type it:
<h3>Heidelberg 74 five colour offset press</h3>
<p></p>
<p>520mm x 740mm convertible to perfecting (1/4)</p>
<p>equipped with CP \Tronic and CP 104</p>
<p>coating unit with axis control colour management system</p>
<p>Alcolor V Variosystem and inkline cartridge system</p>

When I close the page editor and re-open, I get this:
<h3>Heidelberg 74 five colour offset press</h3>

520mm x 740mm convertible to perfecting (1/4)
equipped with CP \Tronic and CP 104
coating unit with axis control colour management system
Alcolor V Variosystem and inkline cartridge system

All paragraph code is gone and I no longer have any line breaks.

Very frustrating!!!