Why do things float in water?
All objects, both those that sink and those that float, are made up of molecules. These molecules are groups of atoms and, depending on the thing, can be packed either super tightly or more spread out. The layout of the molecules within an object affects its density, which, in turn, affects its ability to float. If the molecules within an object are tightly packed together, the thing has a higher density than an object in which the molecules are more spread out.
So, how does this relate to floating? Well, water itself also has its density. If an object’s density is higher than the density of water, it will sink. On the flip side, if a thing is less dense than water, it will float.
What is floating?
Floating is what we call it when objects, either wholly or partially, rest on the surface of the water. Floating does not only occur in water; it can also occur in air, which we will dive into a little later. Many people associate floating with the weight of an object, but this isn’t necessarily true. As discussed above, floating is a lot more to do with density than it is to do with weight.
The science behind what things float and why
Density is not the only factor that plays into what things float; an object’s buoyancy also plays a role in this. Buoyancy, derived from the Spanish word ‘boyar,’ which means ‘float,’ is a force on an object that causes it to move upwards. It is created by the difference in pressure placed on the thing by either the liquid or air it is in. Buoyancy pushes less dense stuff to the surface of the fluid they are in, allowing them to float.
Have you ever wondered how massive metal ships can float quickly on the water? The answer is buoyancy. This force allows super-heavy objects, like ships, to seem much lighter than they are and rest on the surface of the water.
Misconceptions around density
In the initial discussions you will have with your students about what things sink and float, they may think they have it sussed. However, you will likely understand that heavy items sink and light things flow. While this is true in some cases, the principle of this statement is incorrect. For instance, a large ship will float while a tiny pin will sink. This is because, as previously mentioned, sinking and floating are to do with the density of an object, not its weight. Knowing this is one thing, but how do you explain density to your class?
To begin with, you must introduce your class to the idea of mass. Mass is simply the amount of matter that an object contains. The more matter an object has, the more it will weigh. Density involves two measurements: mass and volume and the relationship between the two.
This can be a confusing learning area for kids, as they are likely being introduced to many brand-new terms at once. For this reason, the best way to explain density to your kids is to show them. Get two objects, one of which should have a large mass but low density. This can be something like a plank of wood. The other object should have a smaller mass but a higher density. For this, you can use something like a pin or an earring. Let your kids pass around the objects to get a feel of each size and weight. Then, based on their observations, get them to make predictions about which entity will sink and what will float. After this, lower the objects into a water basin and let density do its thing. This will allow kids to see density at work and challenge their idea of what things sink.
What is the density of water?
The density of water is most commonly given as 1g/cm3. However, the density of water is subject to change depending on its temperature. For the density of water to change significantly, temperatures must be either extremely cold or hot. Water only has an exact density of 1g/cm3 at 4.0 °C. Once the water temperature drops below freezing point, the thickness of the water starts to decrease. This is because ice has a lower density than water. While you may expect the density of water to have the opposite reaction at boiling point, this is not the case. When the water reaches boiling point, it also decreases in thickness. This is because when water is heated, it increases in volume and becomes less dense when it rises in volume.
What do things float on water?
So, we know what floating is and what phenomena cause objects to float, but what are some examples of things that float on water? Before we get into that, let’s run through the basics one more time. For an object to float, it must have a lower density than water and be capable of water displacement. Water displacement allows massive ships, which are technically denser than water, to float. Water displacement is when an object occupies enough space by displacing its weight or removing enough water to maintain its buoyancy. This is why dense things with a large surface area can also float.
They must also be made of hydrophobic materials. A molecule possesses hydrophobicity when it appears to be repelled by a mass of water. These molecules are called hydrophobes. Common hydrophobic molecules include oils, fats, and greasy substances.
Now we’re ready to run through some examples! Here are some examples of what things float on water:
- Leaves,
- Wooden sticks,
- Paper,
- Ships,
- Plastic bottles,
- Bubble wrap,
- Sponges,
- Oil.
What do things float in the air?
Water is not the only substance that things can float in. There are also tons of examples of objects floating in the air. The same rule applies to standing in the same way that objects float when they have a lower density than water. When an object has a lower density than air or can cause enough air displacement to make room for its surface, it can float.
Here are some examples of things that float in the air:
- Paper,
- Kites,
- Balloons,
- Hot air balloons,
- Birds,
- Airplanes,
- Bubbles,
- Feathers.
Just like ships utilize buoyancy and displacement to float on water, airplanes use these same phenomena to float in the air. The low pressure on the upper side of the plane and the high pressure on the downward side of the wings create buoyancy, which allows it to float.
What do things float? – Examples
There are loads of examples of objects that float in our day-to-day lives. These objects are great for including in some class experiments to teach your kids about floating and sinking.
Here are some examples of objects that can float because they have a lower density than water:
- Ice cubes
If you put some ice into a glass and then fill it with water, you will notice that all ice cubes float to the surface. This is because ice, the solid form of water, is less dense than water in its liquid form. As a result, the molecules are forced to spread out when water is frozen, which lowers its density.
- Oils
Most oils have a lower density than water, allowing them to float. This is why, when you pour oil into a glass of water, the two liquids will remain completely separate and not mix.
- Wood
One of the most common examples of a material that floats on water is wood. Wood has a lower density than water, which allows it to float. For this reason, wood was one of the primary materials for making ships and other water-bound vessels.
Another factor that impacts an object’s ability to float is the substance that it is filled with. Balloons, for instance, would not float on their own. They can only swim in air and water when pumped full of air or gas.
Here are some examples of things that float because they are filled with air:
- Hot air balloons,
- Footballs,
- Bottles,
- Inflatable toys.
Food can also float! Many different fruits and vegetables can float because they have a lower density than water. For example:
- Apples,
- Oranges,
- Lemons,
- Leafy vegetables.
Lastly, rubber, wax, and plastic objects also float on water. Sponges also float on water, but there’s a catch. Much like paper, sponges only float on water when they are dry. The sponge will slowly absorb the surrounding water as they sit on the surface. The more water the sponge absorbs, the denser it becomes; eventually, it will sink below the surface. This same principle applies to paper. A dry sheet of paper will float, but as it sits and soaks up the water, it will get denser and sink.
What things sink? – Examples
We’ve looked at the science behind floating and sinking, and the role density plays in this. Next, let’s run through a few examples of things that sink if placed in water.
- Coins
Have you ever wandered past a fountain and seen people throwing coins in? If you have, you’ll also have noticed that the coins don’t float on the water’s surface, turning the whole fountain copper. Instead, the coins sink to the bottom of the fountain, where they remain until someone moves them. This is because coins are small, dense objects that sink instantly when placed in water.
- Stones
It is not always true that the larger an object is, the more likely it is to sink. Some things, regardless of their size, are too dense to float. Stones are a prime example of this. Whether you have a tiny pebble from your garden or a huge boulder, both will sink when placed into a water body. You may be thinking. Indeed ships are more extensive than stones, so how can they float? The reason boats can sail, despite their large mass, is that they are hollow. This means that ships have a lower density than stones despite being more significant than stones.
- Sponges
Unlike the previous two examples, this one is not so straightforward. Depending on its state, a sponge can either float or sink. A dry sponge has a super low density, allowing it to flow on the water’s surface. However, due to the porous nature of sponges, they soon begin to soak up water, which increases their density. At a certain point, the sponge’s density will overtake that of the water and begin to sink.
Fun facts about floating and sinking
Let’s take a look at some fun facts about floating and sinking:
- Just because an object floats in water does not mean it will float in all liquids. This is because all things and liquids have different levels of density. Therefore, if an object floats in water, it may sink into liquids like vegetable oil or alcohol.
- The shape of an object affects its ability to float. The larger the surface area of an object, the more likely it is to float, as its buoyancy increases the more surface area it has.
- An increased surface area makes it easier to float, so humans can float more quickly when spread out on their backs. This is why kids are taught to float in a ‘starfish position.’ It is also why we are told to lie on our backs and stretch out our arms and legs if we are ever in danger of drowning.
- Buoyancy makes objects seem lighter than they are. This is why it is easier to lift heavy objects in the water.
- Ships float at different heights depending on how heavy and dense the water is.
- Seawater, made up of salt water, has a much higher density than fresh water. For this reason, ships float higher in salt water than they do in fresh water.
- Due to the higher saltwater density, eggs can float in the sea but not in normal, fresh water.
- Ships also float higher in colder water than in warm water. Again, this is because of the increased density of colder water.
- Hollow objects can also float because air is much less dense than water. This is a big reason why ships can sail.
