Students will investigate the properties of, and interactions among, liquids and solids.
Associated Curriculum Topic
Properties of Liquids and Solids
- Alka-Seltzer® or generic effervescent-antacid-and-pain-relieving tablets
- Vegetable oil and water
- Tall clear plastic containers (a 1 or 2 liter bottle for each group)
- Food colouring (neon colors work well – ask the groups to share the food colouring)
- Funnels (optional – useful in pouring the oil into the container)
- turkey baster (optional – useful for removing oil and returning to bottle after experiment)
- Balloon (optional)
The main idea is the concept of density, in particular, that things that are denser than a liquid sink and things that are less dense than a liquid float. This activity is interesting because gas is added to and taken away from big drops of water, which changes their density and causes them to float or sink in oil. Before the Alka-Seltzer® is dropped in the bottle, the oil floats on the colored water. Oil floats because it is less dense than water. Oil and water don’t mix because water molecules are not attracted to oil molecules. When the Alka-Selzer® tablet is dropped in the oil and water, it sinks to the bottom because it is denser than oil and denser than water. In the water layer, the tablet begins to dissolve and the chemicals in the tablet react with each other creating bubbles of carbon dioxide gas. When enough gas enters an area of water, the water-gas combination in this spot becomes less dense than the water around it, so it floats up through the water. If this water-gas mixture is less dense than the oil, it floats up through the oil too. Since the water is attracted to itself and not to the oil, the water-gas mixture moves through the oil in a ball-shape. Once a ball of water-gas gets to the surface, some bubbles of carbon dioxide gas pop, releasing the gas into the air. When enough bubbles pop, the water-and-remaining gas becomes denser than the oil. So the ball of water sinks down through the oil and joins the rest of the water.
- ¼ fill container with water
- carefully add vegetable oil until container is nearly full
- add two or three drops of food colouring and observe
- add half or a quarter of an Alka-Seltzer tablet
- immediately cover the top of the bottle with the balloon and observe
Ask students to predict what will happen at each step:
- Will the oil and water mix?
- How will a drop of food colouring behave in oil? in water?
- Will the tablet float or sink in the oil and how else might it behave?
- Will the tablet float or sink in the water and how else might it behave?
Observing the lava lamp:
- What is happening to the tablet in the water?
- What makes the coloured blobs of water float up in the oil?
- What makes the coloured blobs of water fall back down again to the bottom?
- What happens to the balloon while the tablet is dissolving?
- What happens once the tablet has dissolved entirely?
Ask the students to write an instruction manual for building a lava lamp based on their experiences