#container_2 {
    border: 9px solid;
    padding: .5em;
    width: 5em;
    height: 5em;
    transition: .3s all;  /* rotate gradually instead of instantly */
}

#container_2:hover {
    -webkit-transform: rotate(90deg);  /* to support Safari and Android browser */
    -ms-transform: rotate(90deg);      /* to support IE 9 */
    transform: rotate(90deg);
}