Objects sink into water until the weight of water displaced is equal to the weight of the object. If the water weight that is displaced is less than the object’s total weight, the object sinks.
Soap sinks or floats depending on its density, or the amount of mass in given volume. Soap that has air whipped into it has a lower density. It's the same principle as comparing a ceramic cup to a Styrofoam one. The latter cup floats because of its air bubbles.
Objects comprised of material that is denser than water tend to sink; objects with a lighter density tend to float. Since air is lighter than water, hollow objects can float if their total density is less than water. Preventing objects from absorbing water is important.
Submarines float or sink by increasing or decreasing the air in their ballast tanks. These tanks have valves that allow them to fill with water, increasing the overall weight of the submarine and causing it to sink. The valves fill the ballast with compressed air to surface.
Submarines are able to control their depth via ballast tanks that take on or release water to make the submarine more or less buoyant than the water around it, explains How Stuff Works. This causes the submarine to float or sink.
Whether an object sinks or floats depends on its density and the density of the liquid into which it is placed. If the object has a greater density than the liquid, it sinks. If it has less density, the object floats.
It is good that ice floats on water because plants and fish can survive in water that is covered by the floating ice sheet. Water in its solid state becomes less dense than in its liquid state, unlike other matter types, due to its hydrogen bonding.
Density determines if an item sinks or floats. Items with a density greater than that of water (1 gram per cubic centimeter) sink in water, while those with a density less than water float.
The two forces that determine whether an object floats or sinks are buoyancy and pressure. Buoyancy is determined by the density of the object and the density of the medium.
Some ideas for good science projects include examining whether black light detects invisible stains and why insects are attracted to lamps. Another potential project explores what kind of automotive antifreeze is safest for the environment and why.