[quote=Tyki]
then i guess ice is lava's opposite :P and matter is time's opposite. see you're solving your own riddles :P
[/quote]

Matter can AFFECT time, as can be seen during, for example, space travel. However, it does not counteract time. That's like saying that time is the opposite of ice, because ice can be affected by time.
And ice is simply another form of water. And it's not an opposite, as it can't cancel lava out. Instead, you're using a force that can affect lava, and then attempting to create a temperature so low it'll freeze the lava. However, time is required for this change to take effect. If you want to instantly freeze lava, that will take a lot more energy than counteracting it or creating it would, meaning you would rapidly tire yourself out.
If you want to expand this, name the opposite of stone, wood, or electricity. What's the opposite of diamond, or psychic energy. The opposite of love, There are tons of things which have no specific opposites, i can name a stack more.