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.

Sound waves travel through different mediums (such as solids, liquids, and gases) by causing particles in the medium to vibrate. The way sound waves propagate depends on the properties of the medium, including its density, elasticity, and temperature. Here’s how sound waves travel through each medium:

1. Through Solids:
   • In solids, sound travels quickly because the particles are closely packed together. When sound energy is applied, the particles vibrate and transfer energy to neighboring particles. The stiffness (elasticity) of solids allows sound to travel at high speeds. For example, sound travels faster through metal than through wood or rubber.
2. Through Liquids:
   • In liquids, sound waves also propagate by causing particles to vibrate. However, since particles in liquids are more spread out than in solids, sound travels slower through liquids than solids. For instance, sound travels faster in water than in air but slower than in solids like steel.
3. Through Gases:
   • In gases, sound travels the slowest because the particles are far apart and have less interaction with each other. In air, for example, sound travels at a speed of about 343 meters per second at room temperature. The speed of sound in gases depends on factors like temperature and pressure—higher temperatures generally result in faster sound transmission, as the particles move more rapidly.

Sound travels fastest in solids (due to close particle proximity and high elasticity), slower in liquids, and slowest in gases (due to greater particle distance and less efficient energy transfer).

The three laws of motion, formulated by Sir Isaac Newton, are fundamental principles describing the relationship between the motion of an object and the forces acting on it. They are:

1. First Law (Law of Inertia):
   • An object at rest stays at rest, and an object in motion continues in motion at a constant velocity, unless acted upon by an external force.
   • This law highlights the concept of inertia, which is the tendency of an object to resist changes in its state of motion.
2. Second Law:
   • The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.
   • It is mathematically expressed as $F=maF\; =\; ma$, where $FF$ is the net force applied to the object, $mm$ is the mass of the object, and $aa$ is its acceleration.
3. Third Law:
   • For every action, there is an equal and opposite reaction.
   • This means that if one object exerts a force on another object, the second object exerts an equal and opposite force back on the first object.

These laws form the foundation of classical mechanics and are essential for understanding the behavior of objects in various physical situations.