Objects like coins, rocks, and marbles are more dense than water. They will sink. Objects like apples, wood, and sponges are less dense than water. They will float.

Density is a measure of how heavy something is compared to its size. If an object is more dense than water it will sink when placed in water, and if it is less dense than water it will float.

What do sinking objects have in common? All of the sinking objects have a density more than 1.0 g/cm^3. Does mass or volume alone determine whether an object will sink or float? No, you need both mass and volume (mass divided by volume) to find density (needed to figure out if the object will sink or float).

The density of an object determines whether it will float or sink in another substance. An object will float if it is less dense than the liquid it is placed in. An object will sink if it is more dense than the liquid it is placed in.

When an object is placed in water, there are two primary forces acting on it. The force of gravity yields a downward force and a buoyancy force yields an upward force. If the gravitational force is less than the buoyancy force then the object floats (a boat), otherwise it sinks (a rock).

The air that is inside a ship is much less dense than water. That’s what keeps it floating! The average density of the total volume of the ship and everything inside of it (including the air) must be less than the same volume of water.

The volcanic rock is called ‘pumice stone’ and is very lightweight, meaning it can float on water. Pumice is made when magma from a volcano cools really quickly in water. The rock raft is made up of trillions of tiny pieces of pumice stone, so it looks like a floating island when they’re all together.

“The whole ocean was matte.” The couple had sailed upon what scientists call a “pumice raft,” a mass of floating, porous rock that forms when a volcano erupts from the ocean floor. The hot lava, full of gas bubbles, cools in the ocean water and produces pumice, which floats to the surface above the eruption.

Landscapers know this rock as lava rock. Pumice is a froth of felsic volcanic glass. It is rock foam with so much air in its structure that it often floats on water. The basaltic lava starts out black, but oxidation of iron during eruption and emplacement of the scoria turns it red.

Once ejected into the atmosphere, the sulfur dioxide spewed from volcanoes can react to form sulfate aerosols, which are tiny particles suspended in the air. Both ash and aerosols can scatter the sun’s rays, giving a sunset its apparent color. And under certain circumstances, purple sunsets will result.

Pumice