Can you please explain in detail what The Law of Conservation of Mass is?

Dear Student,

The law of conservation of mass states that during in a physical or chemical change, the mass of products remains equal to the total of reactants.

For example: When 100g of mercuric oxide is heated, 92.6 g of mercury and 7.4 g of oxygen are obtained as products.

i.e. HgO→ Hg + O2

In this case mass of reactant =100g

Mass of products = Mass of Hg + Mass of O2 = 92.6 + 7.4 = 100g

Thus, we obtain Mass of product = Mass of reactants

This verifies the law of conservation of mass.

Hope this helps.

Best Wishes!

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 The law of conservation of mass, also known as principle of mass/matter conservation is that the mass of a closed system (in the sense of a completely isolated system) will remain constant over time. This is much like the conservation of energy in a sense that both keep the energy/mass enclosed in the system (hence, "conservation"). The mass of an isolated system cannot be changed as a result of processes acting inside the system. A similar statement is that mass cannot be created/destroyed, although it may be rearranged in space, and changed into different types of particles. This implies that for any chemical process in a closed system, the mass of the reactants must equal the mass of the products.

Opposed to conservation, the principle of matter conservation may be considered as an approximate physical law, that is true only in the classical sense, without consideration of special relativity and quantum mechanics. Another difficulty with the idea of conservation of "matter," is that "matter" is not a well-defined word scientifically, and when particles which are considered to be "matter" (such as electrons and positrons) are annihilated to make photons (which are often notconsidered matter) then conservation of matter does not take place, even in isolated systems.

Mass is also not generally conserved in "open" systems (even if only open to heat and work), when various forms of energy are allowed into, or out of, the system (see for example, binding energy). However, the law of mass conservation forclosed (isolated) systems, as viewed over time from any single inertial frame, continues to be true in modern physics. The reason for this is that relativistic equations show that even "massless" particles such as photons still add mass and energy to closed systems, allowing mass (though not matter) to be conserved in all processes where energy does not escape the system. In relativity, different observers may disagree as to the particular value of the mass of a given system, but each observer will agree that this value does not change over time, so long as the system is closed.

The historical concept of both matter and mass conservation is widely used in many fields such as chemistrymechanics, and fluid dynamics. In relativity, the mass-energy equivalence theorem states that mass conservation is equivalent to energy conservation, which is the first law of thermodynamics.

 

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Law of Conservation of Mass is a relation stating that in a chemical reaction, the mass of the products equals the mass of the reactants.

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he Law of Mass Conservation states that

  • "mass can neither be created nor destroyed"

law of mass conservation

The inflows, outflows and change in storage of mass in a system must be in balance.

The mass flow in and out of a control volume (through a physical or virtual boundary) can for an limited increment of time be expressed as:

dM = ρi vi Ai dt - ρo vo Ao dt  (1)

where

dM = change of storage mass in the system (kg)

ρ = density (kg/m3)

v = speed (m/s)

A = area (m2)

dt = an increment of time (s)

If the outflow is higher then the inflow - the change of mass dM is negative -

  • the mass of the system decreases

And obvious - the mass in a system increase if the inflow is higher than the outflow.

The Law of Mass Conservation is a fundament in fluid mechanics and a basis for the Equation of Continuity and the Bernoulli Equation.

Example - Law of Mass Conservation

Water with density 1000 kg/m3 flows into a tank through a pipe of 50 mm inside diameter. The velocity in the pipe is 2 m/s. The water flows out of the tank through a pipe with inside diameter 30 mm with a velocity of 2.5 m/s.

Using equation (1) the change in the tank content after 20 minutes can calculated as:

dM = (1000 kg/m3)(2 m/s)(3.14 (0.05 m)2/4) (20 min 60 s/min)

  - (1000 kg/m3)(2.5 m/s)(3.14 (0.03 m)2/4)(20 min 60 s/min)

  = 2590.5 kg

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