Scientists are curious and students can model being a scientist by exploring things that can affect whether an object floats such as its shape, weight or the amount or type of liquid used to float it. This investigations are a perfect opportunity for students to develop experimenting and reporting skills.
This story could provide opportunities for students to raise further questions about floating and sinking. Our website uses a free tool to translate into other languages. This tool is a guide and may not be accurate. For more, see: Information in your language. You may be trying to access this site from a secured browser on the server. Please enable scripts and reload this page.
Skip to content. Do you know why oil floats on water? Would an object that sinks in oil be able to float in water? Try this experiment to find out and learn more about density. Add 2 or 3 drops of food coloring.
Watch what happens. The layers should separate so that the corn syrup is on the bottom, the oil is on top, and the water is in the middle. Guess where each of your objects will land when dropped into the cup, then test them out one at a time. The corn syrup was the most dense liquid, so it sank to the bottom of the cup.
The water was less dense than the corn syrup, but more dense than the oil, so it settled on top of the corn syrup. The oil was the least dense, so it floated on top of the water! The objects that you dropped into the cup had different densities. Each object sank into the cup until it got to a liquid that was more dense than it. The cork was not very dense at all, so it floated on the surface of the oil. The wax fell into the oil, but not all the way to the water, so it was more dense than the cork, but not as dense as water.
The grape and the raisin fell to the bottom of the water layer, but not into the corn syrup. That means that they were less dense than the corn syrup, but more dense than the water! So it's clearly it's this water weighs more now this time, right? The box of water will be more, because of that, that means our buoyant force this weight will be larger than the weight of the box. And as a result, our box will float. And so right in front of our eyes, we're seeing that weight doesn't decide whether something will float or sink.
We are having an extremely heavy object, which is floating and an extremely light object, which is sinking. Why is that happening? Because we're seeing that something which is extremely heavy, can displace even heavier amount of water, which is why it is floating and something which is light is displacing even lighter amount of water because of which, it is sinking.
You see the secret to floating is that your objects should be able to displace even heavier amount of water. And when will that happen? That can only happen provided per centimeter cube your object is lighter than water.
Otherwise it will sink. So the secret to floating is per centimeter cube objects should be lighter than water. In other words, the density of the object should be smaller than the density of the water. Then it will float. So with water, if you wanted to figure it out, if something's going to float or not calculate its density and check whether it's lighter than water. So general, the floating condition is that density of the object should be smaller than the density of the fluid.
It's not just for water. It can be for any liquid or gas. And by the way, how do we calculate density? Well, the density is usually calculated as mass divided by its volume.
See, mass divided by volume uses the density of that object, as long as it's smaller than that of the fluid, it will float. And of course, if you need more clarity on this, where this comes from, we've talked a lot about that density in previous videos.
So great to go back and watch videos on density. Anyways, now we can answer our original question. So why is it that if I put a spanner, it will sink? A spanner are made mostly of steel and steel has a density of eight grams per centimeter cube, roughly. Which is way more than that of water, and that's why if you put it in water, it's going to sink, larger density than water. On the other hand, what about a ship?
A ship, which is made of the same material. Why doesn't that sink? Even it should be heavier than water per centimeter cube, right? No, it's not because a ship is not completely made of metal, not completely made of steel. Unlike the spanner, a ship has a lot of empty space in it because of that, the metal occupies a much larger volume.
Just think about it. I take that metal and I put a lot of empty space in between. I increase its volume. The density starts decreasing, right? And that's the secret for ships. Molecules can be packed in close together like in a rock or more spread out like in bubble wrap. The positioning of molecules affects the density of an object. Objects with tightly packed molecules are more dense than those where the molecules are spread out.
Density plays a part in why some things float and some sink. Objects that are more dense than water sink and those less dense float. Hollow things often float too as air is less dense than water. This is partly why huge heavy ships float. Another thing to consider is the shape of an object. Generally the more of the outside of an object that is touching the water the more buoyant it is.
Water pushes back up against objects so the more surface area an object has the more water pushes back against it helping it to float.
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