Marie,
you have it exactly. There needs to be background understanding for inquiry to really help children learn science content. By taking their prior experience with these reactions into account, you can offer them a challenging design and make activity that uses that science in a concrete way.
We used to make lava lamps with visiting children in our birthday parties at Blackrock Castle, using food colouring, oil and water with fizzy vitamin tablets. My colleagues learnt pretty quickly which were the nicest colours to use as the vitamin C tablet changed the colour of the food colouring.