Omar Khalid
It is a common misconception that the ocean is covered with plastic, with floating 'islands' of plastic waste. In reality, less than 1% of plastic floats on the ocean's surface, with the rest sinking to the seafloor. The reason for this is density: if an object's density is greater than that of water, it will sink, and if it is less dense, it will float. While many plastics are denser than water, some plastics, such as plastic bags and containers, are less dense and therefore float.
| Characteristics | Values |
|---|---|
| Density of water | 1 |
| Density of plastic | Varies, e.g. acrylics ~1.2, nylon ~1.15, PVC ~1.388, PET ~1.333 |
| Buoyancy | If the density of an object is less than that of water, it will float |
| Plastic in the ocean | ~1% floats on the surface, ~50% floats deeper in the water column, the rest (~49%) sinks |
| Salt water density | Greater than pure water |
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What You'll Learn
Density is measured as mass per volume
Density is a measure of how much mass is contained within a given volume. It is calculated using the formula d = M/V, where d is density, M is mass, and V is volume. For example, the density of water is 1 gram per cubic centimetre, and Earth's density is 5.51 grams per cubic centimetre.
Different types of plastics have different densities. For instance, low-density plastics like LDPE (low-density polyethylene) are less dense than water and float. On the other hand, high-density plastics like PET (polyethylene terephthalate) and PVC (polyvinyl chloride) are denser than water and sink.
The density of a plastic item can also be affected by its shape. For example, small pellets or bricks of plastic may have a higher density than floating plastic items of the same material due to differences in shape.
In general, for an object to float on water, it must displace an equal or larger mass of water than its own mass. If an object displaces less water than its own mass when fully submerged, it is more dense than water and will sink. Conversely, if an object displaces an equal or larger mass of water, it is less dense than water and will float.
Density is a useful concept as it allows us to determine the mass of an object from its volume or vice versa. It is commonly used to understand the behaviour of objects in fluids, such as why some plastics float while others sink.
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Salt water is denser than pure water
The density of a substance is calculated by dividing its mass by its volume (M/V). If an object displaces less water than its own mass when fully submerged, it is denser than the water and will sink.
Salt water, or seawater, is denser than pure water. This is because the dissolved salts in the water increase the mass by a larger proportion than the volume. On average, seawater has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM), meaning that every kilogram (roughly one liter by volume) of seawater contains approximately 35 grams of dissolved salts, predominantly sodium (Na+) and chloride (Cl-) ions. The average density of seawater at the surface is 1.025 kg/L, while the density of pure water is 1.0 kg/L at 4 °C.
The higher density of seawater has several implications. Firstly, it affects buoyancy, making it easier for objects to float in the ocean. Additionally, the freezing point of seawater decreases as salt concentration increases. At typical salinity, seawater freezes at about −2 °C.
It is worth noting that seawater is not uniformly saline worldwide. Mixing with freshwater runoff from river mouths, melting glaciers, or high precipitation can result in substantially less saline seawater. Conversely, high evaporation rates, low precipitation, and low river runoff can lead to unusually salty water, as seen in the Red Sea.
Now, addressing the initial query about plastics and their buoyancy, it is important to understand that not all plastics are created equal. Some types of plastics, such as HDPE (high-density polyethylene), LDPE (low-density polyethylene), and PP (polypropylene), have densities lower than that of water, allowing them to float. On the other hand, plastics like PET (polyethylene terephthalate), PVC (polyvinyl chloride), and PS (polystyrene solid) are denser than water and will sink.
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Some plastics are denser than water
It is often believed that the ocean is covered with plastic, with 'islands' of plastic floating on the surface. However, this is not the case. In fact, less than 1% of plastic in the ocean floats on the surface. The rest is denser than seawater, so it sinks to the ocean floor, or floats deeper in the water column.
Plastic has a certain density, and if the density is greater than that of seawater, it will sink. If the plastic is less dense than water, it will float. Many plastics are denser than water, including Poly(vinyl chloride) (PVC), Poly(ethylene terephthalate) (PET), and Poly(methyl methacrylate) (PMMA).
However, some plastics are less dense than water and will float. These include HDPE (high-density polyethylene), LDPE (low-density polyethylene), and PP (polypropylene), which are commonly used for containers and plastic bags.
The reason why some objects float or sink depends on their density relative to water. If an object displaces less water than its own mass when fully submerged, it is denser than water and will sink. If the object displaces an equal or larger mass of water than its own mass, it will float.
It is also worth noting that salt water is denser than pure water, so things will float better in the ocean than in freshwater.
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Plastic bags float
Whether or not an object floats in water depends on its density relative to that of water. If an object is less dense than water, it will float; if it is more dense, it will sink.
Most plastics are denser than water. However, some types of plastics, such as HDPE (high-density polyethylene), LDPE (low-density polyethylene), and PP (polypropylene), which are often used to make plastic bags, are less dense than water and therefore float.
The density of an object is calculated by dividing its mass by its volume (M/V). The volume of water displaced by an object is equal to the volume of the object itself. If the mass of the object is less than the mass of the same volume of water, it will float.
The shape of an object can also affect its buoyancy. For example, a plastic bag can be shaped to hold air, which reduces its overall density and allows it to float more easily. Additionally, salt water is denser than pure water, so objects, including plastic bags, will float more easily in the ocean or other bodies of salt water.
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Only 1% of plastic floats on the ocean's surface
It was previously assumed that only 1% of plastic in the ocean floated on the surface. However, this assumption has been challenged by a recent study conducted by scientists from the University of Utrecht in the Netherlands. By developing a new numerical model, they estimated that the actual percentage of plastic floating on the ocean's surface is significantly higher, ranging between 59% and 62%.
The study focused on three types of plastics: polyethylene (PE), polypropylene (PP), and polystyrene (PS), which are known to make up the majority of items found on the ocean's surface, deeper layers, and beaches. The model considered a wide size spectrum of plastic particles, ranging from 0.1 mm to 1,600 mm, with microplastics classified as less than 5 mm and larger plastics as more than 25 mm.
The results of the study revealed that the total amount of buoyant marine plastic litter is much higher than previously estimated, with estimates ranging from 3,000 to 3,400 kilotons. It was found that the majority of this plastic mass is comprised of large plastic items, contributing between 90% and 98% of the total buoyant plastic mass. This finding highlights the significant impact of larger plastic objects on the total amount of plastic pollution in the ocean.
The study also addressed the misconception that the Great Pacific Garbage Patch, located between Hawaii and California, is a solid island of trash. Instead, it is more accurately described as a plastic soup, with an estimated mass of 100 million kilograms of floating plastic consisting of 1.8 trillion pieces larger than 0.5 mm. Additionally, it is important to note that the mass of plastics at the surface layer of the Great Pacific Garbage Patch is approximately 180 times higher than that of marine life, indicating the potential for plastic to be a primary food source for organisms in the region.
While the findings suggest that a substantial amount of plastic remains afloat on the ocean's surface, it is crucial to address the issue of plastic pollution at its source. This involves improving waste management practices, particularly in low-to-middle-income countries where mismanaged waste tends to be higher, and reducing the overall production and use of plastic. By addressing these issues, we can make significant progress in mitigating the impact of plastic pollution on our oceans and the delicate marine ecosystems that depend on them.
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Frequently asked questions
Not all plastics are denser than water. LDPE, HDPE, and PP plastics are less dense than water and float.
Less than 1% of plastic in the ocean floats on the surface. The rest is denser than seawater and sinks.
An object will float if it displaces an equal or larger mass of water than its own mass. If the object is more dense than the water, it will sink.
Yes, salt water is denser than pure water, so things float better in the ocean.
