Why pumice rock float on water

Objectives Demonstrate how the distribution of particles in a substance determines its density. What To Do Show the students the two sticks and two rocks. Ask them to predict what will happen when each object is placed in the water.

Ask them what they base their predictions on. Put the objects in the water individually or as pairs. The regular rock and piece of ironwood will sink; the pumice stone and regular stick will float.

Let the students handle the objects to feel the weight mass of each. Extensions Explain that the density of a substance or an object remains regardless of its size. Cut or break off a small piece of wood from the larger piece. Predict which brands if any will sink and which brands if any will float. Should the number of dislikes on YouTube videos be hidden? Home Menu. Giant volcanic pumice rock raft floating in Pacific ocean.

Getty Images. Pumice stone is very light and full of air bubbles. Michael Hoult and Larissa Brill were some of the first people to report seeing the raft - and they took samples to study. Your Comments Join the conversation.

To use comments you will need to have JavaScript enabled. Floating pumice can also be a hazard for boats, as the ashy mixture of ground-up pumice can clog engines. While scientists have known that pumice can float because of pockets of gas in its pores, it was unknown how those gases remain trapped inside the pumice for prolonged periods. If you soak up enough water in a sponge, for example, it will sink.

They then used an X-ray imaging technique at the ALS known as microtomography to study concentrations of water and gas — in detail measured in microns, or thousandths of a millimeter — within preheated and room-temperature pumice samples. To tackle this problem, Zihan Wei, a visiting undergraduate researcher from Peking University, used a data-analysis software tool that incorporates machine learning to automatically identify the gas and water components in the images.

So surface tension really dominates. The team also found that a mathematical formulation known as percolation theory, which helps to understand how a liquid enters a porous material, provides a good fit for the gas-trapping process in pumice.

And gas diffusion — which describes how gas molecules seek areas of lower concentration — explains the eventual loss of these gases that causes the stones to sink. The study showed that previous estimates for flotation time were in some cases off by several orders of magnitude.