Mass of Water
In oceanography, water mass is defined as the body of water with a common formation history and also has physical properties distinct from the surrounding water. The physical properties of water mass include salinity, temperature, isotopic ratios, and other physical properties which are conservation flow tracers (Flow tracer is any fluid property used to track direction, magnitude, flows, and circular patterns). Water mass is also recognized through its non-conservative flow tracers such as intricate, silicate, oxygen, and phosphate.
Water masses are categorized not only based on their respective tracers but also by their locations in the World's ocean. Water masses are also categorized based on their vertical position so that there are deep water mass, intermediate water mass, and surface water mass.
Water Mass Formation
Water masses are formed as a result of different climatic conditions and effects in specific regions. When ice is formed in a cold climate like Antarctica, the cold temperatures separate the molecular bond of water causing it to become less dense. However, the ice becomes less dense than water when water increases its volume by 9% when frozen. This in turn makes the water less saline. The salinity of the water makes the water freeze at a lower temperature than freshwater. Freshwater freezes at an average of 28.4°F whereas saline water freezes at an average of 32°F.
Atomic Mass of Water
The Atomic mass ( mₐ or m) is the mass of an atom. The atomic mass is often represented in the non - SI unit dalton ( da or u), where 1 dalton is defined as the 1/12 mass of a single carbon atom, at rest. Water (H₂O) contains 2 hydrogen atoms and 1 oxygen atom. This implies that the atomic mass of water is 18.02 amu. This is because
The atomic mass of Hydrogen: 1.00784 amu 2 = 2.01568
The atomic mass of Oxygen = 15.999 amu
Therefore, the atomic mass of water = 2.01568 + 15.999 = 18.015 amu
Molecular Mass of Water
The molecular mass of any substance is defined as the sum of atomic masses of all the atoms present in each molecule of a given substance. It is measured in Daltons (Da or u). Molecular mass is calculated by adding up the atomic masses of all the atoms present in each molecule of a given substance. For example, if we need to calculate the molecular mass of water, H₂O, we need to add the atomic mass of hydrogen 2 times and atomic mass of nitrogen one time:
Therefore, the water molecular mass is 18.015 u.
Relative Molecular Mass
Relative molecular mass (Mr.) is defined as the sum of the relative atomic mass (Ar.) of each atom present in the given molecule.
How to Find Relative Molecular Mass?
Determine the molecular formula of a given molecule.
Determine the number of atoms of each element present in the given molecule.
Determine the relative atomic mass of each element.
Multiply the number of atoms of each element with the relative atomic mass.
Add the total values to get the relative molecular mass.
Relative Molecular Mass of Water
Step 1: Determining the molecular formula of water
The molecular formula of water is H₂O.
Step 2: Determining the number of atoms of each element in the molecule.
A water molecule (H₂O) is made up of two atoms of hydrogen and one atom of oxygen.
Step 3: Determining the relative atomic mass of each element.
Ar of hydrogen is 1
Ar of oxygen is 16
Step 4: Multiplying the number of atoms of each element with the relative atomic mass.
A water molecule contains two hydrogen atoms and one oxygen atom. Accordingly,
2 H = 2 1 = 2
1 O = 1 16 = 16
Step 5: Adding the total values to get the relative molecular mass
2 + 16 = 18
Hence, the relative molecular mass (Mr.) of water is 18.
Mass of One Mole of Water
The mass of one mole of any substance is equivalent to that substance's atomic molecular mass in grams. For example, the mean molecular weight of water is 18.015 amu. Hence, the mass of one mole of water is 18.015 grams.
Did You Know?
The first scientists to introduce atomic mass were John Dalton and John Berzelius.
Molar mass is the mass of a substance that consists of 6.022x10²³ atoms.
The unit of molar mass is gram per mole, abbreviated as g/mol.
The Antarctic water bottom is a very important water mass. The Antarctic water bottom is the remaining portion when sea ice is being formed. It is extremely cold but not quite freezing so that water moves down along the ocean floor.
FAQs on Water Mass
1. What is a water mass in geography?
A water mass is a large, distinct body of ocean water that has a uniform temperature and salinity profile. These characteristics are acquired at the ocean's surface in a specific source region. Once formed, the water mass sinks and moves throughout the ocean basins at a depth determined by its density, retaining its core properties for long distances and periods.
2. What are the key characteristics used to identify a water mass?
The primary characteristics used to identify and distinguish water masses are temperature (T) and salinity (S). These two properties together determine the water's density, which dictates its vertical position in the ocean. Oceanographers often use a T-S diagram to plot these values and visually identify different water masses based on their unique T-S signatures.
3. How are water masses formed?
Water masses are formed at the ocean surface through prolonged interaction with the atmosphere in specific geographical locations. Key processes that determine their characteristics include:
- Cooling and Heating: Air-sea temperature exchange alters the water's temperature.
- Evaporation: Increases salinity as freshwater evaporates, leaving salt behind.
- Precipitation and Ice Melt: Decreases salinity by adding freshwater.
4. What are some major examples of water masses found in the world's oceans?
Some of the most significant water masses include:
- Antarctic Bottom Water (AABW): The coldest and densest water mass, formed near Antarctica, which fills the bottom of most ocean basins.
- North Atlantic Deep Water (NADW): A salty and cold water mass formed in the North Atlantic Ocean that flows southward at deep levels.
- Antarctic Intermediate Water (AAIW): A relatively low-salinity, cold water mass formed in the subantarctic region that flows northward above the deep waters.
- Mediterranean Intermediate Water (MIW): A very warm and highly saline water mass that forms in the Mediterranean Sea and flows into the Atlantic Ocean at an intermediate depth.
5. What is the difference between a water mass and an ocean current?
The key difference lies in their definitions. A water mass is a large body of water defined by its static properties (temperature and salinity), like a specific type of air. An ocean current is the movement or flow of ocean water. Essentially, ocean currents are responsible for transporting water masses. A single current can be composed of one or more distinct water masses.
6. Why do water masses sink and form layers in the ocean?
Water masses form layers, a process known as stratification, due to differences in density. Water that is colder or saltier is denser than water that is warmer or less salty. When a parcel of surface water becomes denser than the water beneath it (through cooling or evaporation), gravity pulls it downwards. It continues to sink until it reaches a depth where the surrounding water has the same density. This creates a stable, layered structure in the ocean, with the densest water masses at the bottom.
7. How do water masses influence global climate and marine ecosystems?
Water masses are crucial drivers of Earth's systems. For global climate, they are a key component of the thermohaline circulation (or global conveyor belt), which transports enormous amounts of heat from the tropics towards the poles, regulating global temperatures. For marine ecosystems, sinking water masses transport dissolved oxygen from the surface to the deep ocean, supporting life. They also carry essential nutrients that fuel marine food webs, especially where different water masses meet and mix.
8. What is the significance of a T-S diagram in studying water masses?
A Temperature-Salinity (T-S) diagram is a fundamental tool in oceanography. By plotting temperature on the y-axis and salinity on the x-axis, scientists can create a unique fingerprint for each water mass. Its significance lies in allowing researchers to:
- Clearly identify and classify different water masses present in a water column.
- Analyse how different water masses mix with one another.
- Trace the origin and movement path of a water mass through the ocean.
