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Dalton's Atomic Theory is foundational to understanding chemical reactions. It proposes that all matter is composed of tiny indivisible particles called atoms. According to this theory, atoms cannot be created or destroyed; they only rearrange during chemical reactions. This concept is critical when examining reactions like the decomposition of water.
In Dalton's view, chemical reactions are merely the rearrangement of atoms. For instance, when water decomposes, hydrogen and oxygen atoms simply reorganize into different molecules. Dalton suggested that all atoms of a particular element are identical in mass and properties, which is why they form predictable combinations in chemical reactions, such as forming water from hydrogen and oxygen.
This theory laid the groundwork for the modern understanding of chemistry and helped scientists predict the outcome of chemical reactions with accuracy.

The Law of Conservation of Mass is an essential principle in chemistry. It states that in any chemical reaction, the total mass of the reactants—the starting substances—must equal the total mass of the products—the substances formed. This is apparent in the chemical equation for the decomposition of water: \[2H_2O \rightarrow 2H_2 + O_2\].
In this reaction, we start with water molecules and end with hydrogen and oxygen molecules. Throughout the process, the number and mass of the atoms are conserved. This means no atoms are lost or gained from start to finish; they only rearrange to create new molecules.

• The reactants have 4 hydrogen atoms and 2 oxygen atoms.
• The products have 4 hydrogen atoms and 2 oxygen atoms as well.

This constant balance is why the mass remains unchanged, proving the law holds true in every chemical reaction, just as Dalton's theory suggests.

Decomposition of water is a classic example of a chemical reaction where water molecules break down into hydrogen and oxygen gases. This reaction can be expressed by the equation: \[2H_2O \rightarrow 2H_2 + O_2\].
Here, water starts as the reactant, and the products are hydrogen gas and oxygen gas. By observing this reaction, we see how atoms separate and reassemble into new compounds.

• Initially, two water molecules contain 4 hydrogen atoms and 2 oxygen atoms.
• After decomposition, they form 2 hydrogen molecules (2Hâ‚‚) and 1 oxygen molecule (Oâ‚‚).

This decomposition is a straightforward demonstration of Dalton’s notion of atomic rearrangement during chemical reactions. As any chemical reaction, it reflects the law of conservation of mass, maintaining equal mass before and after the reaction, as the atoms are simply redistributed in different molecular forms.