Alexei Volkov
Whether an object sinks or floats depends on its density relative to the density of the fluid it's submerged in. If an object's density is greater than that of the fluid, it will sink. If its density is less, it will float. Given that water is denser than air, most objects that float in air will sink in water. However, salt water is denser than pure water, so things float better in the ocean. When it comes to plastic, most types are denser than fresh water, but some plastics are less dense and will therefore float. Over time, plastic in the ocean can become coated in algae and barnacles, which can cause it to sink to the ocean floor. Metal, on the other hand, is almost always denser than water, so it will usually sink.
| Characteristics | Values |
|---|---|
| Whether plastic sinks or floats | Depends on its density and shape |
| Whether metal sinks or floats | Depends on its density |
| Density of water | 1 g/cm3 |
| Density of seawater | Up to 1.1 g/cm3 |
| Density of plastic | Varies, some denser than water, some less dense |
| Density of metal | Varies, some metals like iron are denser than water |
| Plastic in the ocean | Mostly in the form of microplastics, found at all depths and on the seafloor |
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What You'll Learn
Density determines sinking or floating
Whether an object sinks or floats is determined by its density relative to the density of the fluid it is submerged in. If the object's density is higher than that of the fluid, it will sink, whereas if the object's density is lower, it will float. This principle applies to both plastic and metal objects.
The density of water varies depending on factors such as temperature and the presence of dissolved substances like salt. At 4°C, pure water has a density of 1 g/cm³. Most metals have densities greater than 1 g/cm³, which is why they typically sink in water.
Not all plastics are the same, and different types of plastics have different densities. Some common plastics, such as HDPE (high-density polyethylene), LDPE (low-density polyethylene), and PP (polypropylene), used in containers and plastic bags, have densities less than that of water and, therefore, float. On the other hand, plastics with higher densities, such as PET (polyethylene terephthalate), PVC (polyvinyl chloride), and PS (polystyrene solid), will sink.
It is important to note that the shape of an object can also influence whether it sinks or floats. For example, some plastics may have lower density than water but could be shaped in a way that causes them to sink because they cannot displace a volume of water equal to or greater than their own volume. Additionally, surface tension and the weight of an object relative to the force required to break the surface tension can also play a role in determining whether an object sinks or floats.
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Seawater density
The density of seawater is a function of temperature, salinity, and pressure. The density of surface seawater ranges from about 1020 to 1029 kg/m3, depending on these factors. The average density at the surface is 1025 kg/L. The density of seawater is expressed in units of mass per unit volume and is given in kilograms per cubic metre in the SI system of units.
Seawater is denser than both fresh water and pure water (density 1.0 kg/L at 4 °C (39 °F)) because the dissolved salts increase the mass by a larger proportion than the volume. Every kilogram (roughly one liter by volume) of seawater has approximately 35 grams (1.2 oz) of dissolved salts, predominantly sodium (Na+) and chloride (Cl-) ions. The salinity of seawater is about 3.5% (35 g/L, 35 ppt, 600 mM) on average. However, the vast majority of seawater has a salinity of between 31 and 38 g/kg, or 3.1–3.8%.
The density of seawater increases with increasing salinity and decreasing temperature. The temperature of maximum density for fresh water is 4 °C (39.2 °F). Below this temperature, further cooling of the surface produces less dense water. The freezing point of seawater decreases as the salt concentration increases. At typical salinity, seawater freezes at about −2 °C (28 °F).
Deep in the ocean, under high pressure, seawater can reach a density of 1050 kg/m3 or higher. The tremendous pressures present in the deep ocean contribute to the compressibility of seawater, which increases its density. The density of seawater also changes with salinity. Brines generated by seawater desalination plants can have salinity up to 120 g/kg, resulting in a density of 1088 kg/m3 at 25 °C and atmospheric pressure.
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Microplastics sink to the ocean floor
The presence of microplastics in the ocean is a pressing global issue. Marine debris, largely consisting of plastic, negatively impacts wildlife, tourism, and shipping. Of the 8 million tons of plastic that enter the ocean each year, only a small fraction is accounted for by floating debris. So, where is the rest of it?
Research has shown that microplastics are present throughout the entire water column, from the surface to the seafloor, and are even found buried in the ocean floor. These tiny particles, often invisible to the naked eye, are created when larger plastics break down due to sunlight, water, and wave action. Their small size makes them highly susceptible to ocean currents, which can cause them to accumulate in certain areas, such as submarine canyons. Additionally, processes such as dense shelf water cascading, coastal storms, and offshore convection contribute to the transfer of microplastics from shallow waters to deeper ocean layers.
The trajectory of microplastics to the seafloor is a critical yet understudied aspect of their impact. While most people envision plastic pollution as large pieces of trash floating on the surface, the reality is that microplastics are far more prevalent and insidious. As they sink, microplastics can be ingested by marine organisms, leading to potential ecosystem collapse and the leaching of poisons into our seafood.
The impact of microplastics on the ocean floor is an area that requires further investigation. The vastness of the deep sea and the prevalence of microplastics at all investigated sites indicate that the deep seafloor may be a significant repository for these pollutants. Studying the vertical distribution and movement of microplastics from the surface to the seafloor is crucial for understanding their impact on marine life and the ocean ecosystem as a whole.
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Plastic shape impacts sinking
The shape of plastic waste plays a crucial role in its sinking behaviour. While the relative surface area alone does not determine whether a piece of plastic sinks or floats, the shape and size of plastic waste influence its sinking velocity.
Research has shown that the attachment of biological matter, such as microbial biofilms, can significantly impact the sinking behaviour of microplastics. The colonisation of plastics by microbes and other organisms increases their density and causes them to sink. This process, known as biofouling, has been observed to affect polyethylene and polypropylene, the two most abundant buoyant plastics. However, the specific shape and size of the plastic waste also influence the extent of colonisation and, consequently, the sinking behaviour.
The interaction between plastic polymers and freshwater microalgae, for example, depends on the shape and size of the plastic particles. Different polymers can exhibit various characteristics, even when processed using the same methods. The shape and size of plastic waste can also affect its wettability, which may impact colonisation and transport.
The settling velocity of microplastic particles is influenced by their size, shape, and fractal dimension. Larger aggregates with higher biological content tend to sink faster due to their increased size and weight. On the other hand, smaller particles with higher surface area-to-volume ratios may be more susceptible to colonisation by microbes, which can also lead to sinking.
Overall, the shape of plastic waste, in conjunction with other factors such as size, density, and the presence of biological matter, plays a significant role in determining its sinking behaviour. Understanding these factors is crucial for developing effective remediation plans to address plastic pollution in aquatic environments.
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Metal density
The density of metals can vary depending on their composition and structure. For example, among common metals, aluminium is known for its lightweight properties, while gold is significantly denser. Alloys, which are mixtures of two or more metals, have densities that depend on the individual metals used and their relative proportions. If a denser metal is added in larger quantities to an alloy, it will increase the overall density, and vice versa.
Density is typically measured in grams per cubic centimeter (g/cm³) or kilograms per cubic meter (kg/m³). In the metric system, g/cm³ is commonly used for solids, while kg/m³ is often employed. The Imperial system uses pounds per cubic foot (lb/ft³). For metals, g/cm³ is a convenient unit because it is suitable for solid materials.
Temperature and pressure also influence the density of metals. As temperature increases, most materials expand, leading to increased volume and decreased density. Conversely, cooling a material generally results in higher density. Similarly, increasing pressure typically increases density, while decreasing pressure has the opposite effect. However, at extremely high pressures, even solids and liquids may experience a slight increase in density due to the compression of their atomic or molecular structures.
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Frequently asked questions
No, not all plastics sink. Whether or not a plastic sinks depends on its density. For example, LDPE (low-density polyethylene) and HDPE (high-density polyethylene) float as their density is less than that of water. However, higher-density plastics such as PET (polyethylene terephthalate) and PVC (polyvinyl chloride) sink. Additionally, microplastics can float in the ocean and sink to the seafloor.
No, not all metals rise. Metals with a density lower than that of water will rise, while those with a higher density will sink.
The density of the object and the density of the water are the main factors that influence whether an object sinks or floats. If an object has a lower density than water, it will float. If it has a higher density, it will sink. The shape of the object can also affect its buoyancy, as can the surface tension of the water.
