Wordpress post Loop not working as expected

Hi guys im using a theme that uses the blog page to display all of its content on the blog page rather than an excerpt and then when i click into the post i want to show the whole content.

im using the following code:

$postId = get_the_ID();
        $ex = the_excerpt();

        if($postId == 19){

            echo $ex;


        }

        else{

            echo $content;

        }

the blog page is located at post =19

i would expect only the excerpt to show on the blog page and the content to show on the post page. however both show. also it doesnt matter if i change the number 19 in my if statement as the same happens. can anyone see where i am going wrong?