Make your own lava lamp
Equipment: empty plastic bottle, water, oil, food colouring, effervescent tablet.
How to: fill a third of the bottle with water; fill another third with oil; add 10 drops of food colouring; watch how the different liquids settle – what do you notice? Break the tablet in half and add it to the bottle. What happens? Add the remaining half table to continue the results.
What is happening? The oil sits on top of the water because the oil is lighter than the water or, more specifically, less dense than water. When the tablet is added it sinks to the bottom and starts dissolving in the water, creating a gas. As the gas bubbles rise, they take the coloured water with them. When the bubbles of coloured water reach the top, the gas escapes and the coloured water sinks down through the oil.
Taking it further: children could explore what happens if they put the lid on the bottle when the lava lamp is working, whether different oils and food colourings create different effects and whether the ratios of water to oil make a difference.