Experiment to show that water is an insulator

This is one of my favourite experiments. We put ice into a boiling tube and then inserted a piece of gauze to hold the ice down. We then filled up the tube with water. Normally the ice would float - hence the gauze. We gently heated the water at the top of the tube and it boiled. The oddity is holding the tube in your hand to heat it rather than using test tube holders. You'll notice from the picture that we have boiling water at the top but the ice at the bottom has not melted. How is that possible? Thermal energy normally spreads best in fluids by convection. As water is heated, it expands, becomes less dense and floats up. So thermal energy is carried upwards. By heating the tube in the way shown, thermal energy transfer by convection is limited to the very top of the tube. The other way of transferring thermal energy here would be conduction ("heating by particles"). If the tube were metal, you'd burn your hand. But the molecules in water are not joined so they find it hard to pass on the thermal agitation hence thermal energy would take a long time to pass down the tube by conduction. Water is an insulator because it is a bad conductor.