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Chemical compounds are substances formed when two or more elements combine in a fixed ratio. This combination of atoms is the base of all matter around us and represents the foundation of chemistry. When elements bond to form compounds, they create entirely new substances with unique properties. For instance:

• Water (H鈧侽) is a chemical compound formed by hydrogen and oxygen.
• Sugar (C鈧咹鈧佲倐O鈧�) is another compound, composed of carbon, hydrogen, and oxygen.

Each compound is distinct and cannot be broken down by physical means. This means each compound maintains a consistent chemical identity regardless of how large or small the sample is.
To understand chemical compounds, it's essential to know they follow specific laws, such as Proust's law, which help predict their behavior and composition.

The law of definite proportions, also known as Proust's law, states that a chemical compound always contains its component elements in a fixed ratio by mass. This was first recognized by French chemist Joseph Proust in the late 18th century. It highlights that:

• The composition of a compound does not change regardless of the quantity prepared or where it was sourced.
• For example, water always contains 11.2% hydrogen and 88.8% oxygen by mass.

This law is fundamental because it assures that chemical compounds have a consistent structure, allowing chemists to understand and predict the behavior of compounds.
It's important to note that Proust's law applies to individual compounds, not across different compounds. Different compounds, such as carbon monoxide and carbon dioxide, will have their own fixed compositions.

Carbon monoxide (CO) and carbon dioxide (CO鈧�) are both chemical compounds composed of carbon and oxygen, but they have different compositions and properties.
Carbon monoxide consists of:

• 42.9% carbon
• 57.1% oxygen

It is a colorless and odorless gas that is highly toxic and can be hazardous even in small amounts.
On the other hand, carbon dioxide is made of:

• 27.3% carbon
• 72.7% oxygen

It is a natural component of Earth's atmosphere and is not harmful at low concentrations. The differences in the composition of CO and CO鈧� demonstrate how Proust's law applies: each compound retains its specific proportion of elements. The law of definite proportions does not require different compounds to have the same elemental ratios, only that each maintains a consistent ratio within its own structure.