Zoom Tool

Although not a selectable tool in the toolbar, there is a dedicated zoom tool in the top right of your screen.

Additionaly you can use the scrollwheel of your mouse to zoom as well. This behaviour can be turned off in the client settings.

A crucial difference between the scrollwheel and the scroll tool, is that the former zooms in relation to your mouse pointer whereas the latter scrolls in relation to the center of the screen.