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Law of Conservation of Matter

31st May 2019 @ 6 min read

Physical Chemistry

The law of conservation of matter is a fundamental law in science. It is also known as the law of conservation of mass. The later is used in physics while the former in chemistry. It is one of the laws of chemical combinations in chemistry. The law has huge applications in chemistry, physics, and engineering. In a closed system, the exchange of matter is restricted across its boundaries. So, there is no matter entering the system or leaving the system. Thus, the flow of matter in and out of the system is zero. These statements are true only for a closed system with no nuclear change. We can apply the law to systems which are subjected to physical and chemical changes, not nuclear changes. This will be better understood as we go through the article.

Principle

The law states matter is neither created nor destroyed in an isolated system. In other words, the amount of matter is conserved in an isolated system over time. The law can be mathematically expressed as:

mass in − mass out = 0

History

The origin of the idea of mass conservation is found in ancient Greek philosophy in the 4th century BC. Also, a similar idea existed in ancient Jain philosophy. Persian philosopher Nasir al-Din Tusi proposed the same idea that matter is always conserved. The notion of matter conservation came into attention in the 18th century when Russian scientist Mikhail Lomonosov demonstrated the law of conservation of matter through his chemical experiments. A few years later, the law was again brought into light by French chemist Antoine Lavoisier. He conducted the series of chemical experiments, and in 1773, he concluded that matter is conserved in a chemical reaction. Lavoisier studied the combustion reactions and he observed that there was no change in mass of the reaction mixture before and after the reaction. Which means the matter is neither created or destroyed in a chemical reaction.

Physical Change

Physical Change is a change in which the chemical identity of a substance is not altered. The examples of physical change are melting of ice, boiling of water, chopping of wood, drying of the wet cloth. The law of conservation is applicable in a closed system with a physical change. Consider an example of the melting of ice. When 100 g ice cubes melts, it transforms into 100 g liquid water. The amount of matter before the physical change is 100 g, which is equal to change the amount of matter after the physical change.

mass of ice = mass of water = 100 g
Ice melting to form water
Figure 1: The melting of ice is a physical change. The amount of ice cubes is equal to the amount of liquid water.

Chemical Change

A chemical change is a change in which the original chemical identity of a substance is lost permanently. The examples of chemical change are rusting of iron, combustion of petrol, burning of wood, photosynthesis. Chemical changes are nothing but chemical reactions. The law of conservation of matter is applicable in a chemical change. In chemistry, this principle has used extensively particularly in balancing chemical reactions, determining the amount of a reactant requires for forming the desired quantity of the product.

According to the law, for any chemical reaction, the amount of reactant at the beginning must equal the amount of product at the end. Thus, the sum of the mass of reactant equals the sum of the product at the end.

the sum of the mass of each reactant equals the sum of the mass of each product

In the above equation, we assume the complete consumption of reactants.

Consider an example of photosynthesis. This is a chemical reaction in which carbon dioxide present in the atmosphere reacts with water molecules absorbed by the plant from the soil. The main product of this reaction is sugar (glucose), and oxygen gas is also liberated in this process. The below figure illustrates the same.

Photosynthesis: carbon dioxide reacts with water in presence of sunlight to form glucose and oxygen.
Figure 2: In photosynthesis, carbon dioxide reacts with water to form sugar (glucose) and oxygen.
Photosynthesis reaction: CO2 reacts with H2O to form C6H12O6 and O2

From the above stoichiometrically balance reaction, 6 molecules of CO2 and 6 molecules of H2O reacts to form 1 molecule of C6H12O6 and 6 molecules of O2.

From the law of conservation of matter,

The sum of the mass of each reactant equala the sum of the mass of each product

Therefore, the amount of matter is conserved in the reaction.

Let consider one more example: combustion of methane (CH4). The chemical reaction is as follows

Combustion of methane: CH4 reacts with O2 to form CO2 and H2O.

From the above reaction 16 g of methane reacts with 64 g of oxygen to form 44 g of carbon dioxide and 36 g of water.

Applying the law of conservation,

Mass of CH4 + mass of O2 = mass of CO2 + mass of H2O 16+64 =44+36=80

Again, we observe the law of conservation is held true.

Fluid Mechanics

The law of conservation of matter is also used in fluid mechanics, but it appears differently in the form of the continuity equation. The continuity equation is derived using the principles of the law of conservation of mass. It has engineering applications.

Continuity equation

where:
ρ is the density,
t is the times,
∇ is the divergence,
v is the velocity field.

Limitations

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Basic Chemistry Laws Of Chemical Combinations

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Marufa Monir Mim
05th Oct 2020
Basic chemistry


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