wow, that is a very interesting question! I wish Duncan was still around as he would have been able to answer this!
In all honesty, I am not sure. But, i do know that lava contains potassium so it might not have a great effect. Have you tried putting potassium in water at school? If so you will know how reactive it is so possibly, adding it to lava would cause a great big chemical reaction to happen. I am going to have a look and try and find out but if anyone else knows then please help me out!