The law of conservation of mass states that matter cannot be created or destroyed in a chemical reaction. This principle ensures that the mass of the reactants equals the mass of the products in a closed system. Here’s how chemical reactions adhere to this law:

1. Conservation at the Atomic Level

• During a chemical reaction, atoms are rearranged, not created or destroyed.
• Bonds between atoms in the reactants are broken, and new bonds are formed to create products.
• Since the number of each type of atom remains constant, the total mass before and after the reaction remains the same.

Example: Combustion of methane:

$C{H}_4+2O_2\to C{O}_2+2H_{2}O$

• Reactants: 1 carbon atom, 4 hydrogen atoms, and 4 oxygen atoms.
• Products: 1 carbon atom, 4 hydrogen atoms, and 4 oxygen atoms.
• The mass of atoms on both sides is equal.

2. Balanced Chemical Equations

• Chemical equations are written to reflect the conservation of mass.
• Coefficients are adjusted to balance the number of atoms of each element on both sides of the equation.

Example: Formation of water:

$2H_2+O_2\to 2H_{2}O$

• Two hydrogen molecules (4 H atoms) react with one oxygen molecule (2 O atoms) to form two water molecules (4 H atoms and 2 O atoms).

3. Closed System Requirement

• For the law of conservation of mass to hold visibly, the reaction must occur in a closed system, where no mass is lost to or gained from the surroundings.
• In an open system, gases or other products may escape, making it seem like mass is “lost,” though it has merely dispersed into the environment.

4. Real-Life Demonstrations

• Precipitation Reactions: When solutions are mixed, and a solid forms, the combined mass of the solutions and the precipitate remains constant.
• Burning of Candle Wax: In a closed system, the mass of the wax and oxygen consumed equals the mass of carbon dioxide, water vapor, and other products.

5. Modern Validation

• The law has been validated by careful measurements using advanced tools.
• Antoine Lavoisier, the “father of modern chemistry,” first demonstrated this law by performing experiments in sealed containers, showing that the total mass remained unchanged.

In chemical reactions, the rearrangement of atoms and strict adherence to balanced equations ensure that the law of conservation of mass is upheld. This principle is fundamental to understanding chemical processes and serves as the basis for stoichiometric calculations in chemistry.

The Sun appears static while the planets revolve around it due to the principles of gravity and inertia as explained by Newton’s laws of motion and Kepler’s laws of planetary motion. Here’s a detailed explanation:

1. The Sun’s Gravity Holds the Planets in Orbit:

The Sun has an enormous mass, making it the most massive object in the solar system.

Due to its mass, the Sun exerts a strong gravitational pull on all the planets, keeping them in orbit around it.

The force of gravity decreases with distance, so planets farther from the Sun experience a weaker gravitational pull.

2. The Balance of Gravitational Force and Inertia:

Planets are in constant motion due to their inertia (an object’s tendency to keep moving in a straight line unless acted upon by an external force).

The Sun’s gravity continuously pulls the planets towards itself, preventing them from flying off into space.

This balance between the Sun’s gravitational pull and the planets’ inertia creates a stable orbit, causing the planets to revolve around the Sun in elliptical paths.

3. Why the Sun Appears Static:

The Sun is not completely static—it also moves slightly due to the gravitational pull of the planets, particularly massive ones like Jupiter and Saturn. However, this movement is minimal compared to the planets’ orbits.

The center of mass of the solar system (the barycenter) is very close to the Sun due to its massive size, making it seem stationary relative to the planets.

4. Role of Conservation of Angular Momentum:

The solar system was formed from a rotating cloud of gas and dust. As the cloud collapsed under gravity, the conservation of angular momentum caused the planets to form in orbits around the Sun.

This rotation and conservation of angular momentum are why planets continue to revolve around the Sun instead of falling into it.

5. The Heliocentric Model:

This understanding is based on the heliocentric model, proposed by Nicolaus Copernicus and later supported by Galileo and Kepler, which places the Sun at the center of the solar system.

In summary, the combination of the Sun’s gravitational force, the planets’ inertia, and the principles of angular momentum explains why the Sun remains at a relatively static position while planets revolve around it.