SQL & PL/SQL

Why does the IS NULL stament work diferent for Varchar2,varchar and char?
The next procedure can explain what I'm talking about:

CREATE OR REPLACE PROCEDURE try_isnull IS
n VARCHAR(10):='';
n2 VARCHAR2(10):='';
n3 CHAR(10):='';

BEGIN
--Without space
IF n IS NULL THEN
DBMS_OUTPUT.PUT_LINE('N is NULL');
END IF;

IF n2 IS NULL THEN
DBMS_OUTPUT.PUT_LINE('N2 is NULL');
END IF;

IF n3 IS NULL THEN
DBMS_OUTPUT.PUT_LINE('N3 is NULL');
END IF;

n:=' ';
n2:=' ';
n3:=' ';

--With space
IF n IS NULL THEN
DBMS_OUTPUT.PUT_LINE('N is NULL');
END IF;

IF n2 IS NULL THEN
DBMS_OUTPUT.PUT_LINE('N2 is NULL');
END IF;

IF n3 IS NULL THEN
DBMS_OUTPUT.PUT_LINE('N3 is NULL');
END IF;

n:= NULL;
n2:=NULL;
n3:=NULL;

--With NULL
IF n IS NULL THEN
DBMS_OUTPUT.PUT_LINE('N is NULL');
END IF;

IF n2 IS NULL THEN
DBMS_OUTPUT.PUT_LINE('N2 is NULL');
END IF;

IF n3 IS NULL THEN
DBMS_OUTPUT.PUT_LINE('N3 is NULL');
END IF;

END;

Why Do they work different ? For me NULL is nothing, So, the rigth behavor should be the char behave.
Does somebody have any answer?