Four months back, I had installed Ubuntu on my laptop and completely forgot about that after the installation. Yesterday, when I tried to install Java and Netbeans IDE on Ubuntu, I had to do some admin works, but I dont remember what password I gave for the 'Ëœroot' user during the installation. So, I thought I forgot the root password and I would have to re-install Ubuntu.
But, when searching on the Internet, I came to know that during the installation, Ubuntu do not ask any password for the root user (but I forgot this!). So, by default the root password is set to blanks. So, now I am happy that I don't need to re-install Ubuntu. 
But to change the root password in Ubuntu, which blanks by default, follow this step:
- Open Terminal/Command
- Type sudo passwd
- Ubuntu will ask you for current user password. Type in the current password and press enter.
- Now Ubuntu will ask you to enter the new password for the root. Type in the new password, confirm it again and you are done.PS: It is not possible to log-in as a 'Ëœroot' user. I guess this feature is de-activated in Ubuntu.