The old volcano with baking soda and vinegar thing always makes a splash.
A couple of other ideas that use kitchen items would be
What is an emulsion? One clear container with oil and vinegar, maintaining their separation, even with shaking (like a salad dressing); then another container with the same amounts of oil and vinegar, and a packet of regular mustard (yep, mustard!). Add the mustard to the oil and vinegar and shake it up - the oil and vinegar can mix because the mustard helps form an emulsion.
What is surface tension? Again, you'll use a clear container with water in it and a small amount of oil to float on the surface. Add a drop of dishwashing liquid in the middle and watch the oil scoot away to the edges of the container. The soap changes the surface tension of the water so that the oil disperses. Another example would be where a paper clip (or a pin, if they are allowed at school) can be made to float on top of water. Or how water will form beads on a freshly waxed surface - use a white wax crayon and a small spray bottle to mist water - you'll get a great effect if you use a smooth plastic cup or a mirror and wax one half of the surface but not the other.
You can look up the definitions for the terms on wikipedia and give other examples of emulsions or discuss why surface tension is important.