Identify the law that explains why a water molecule in a raindrop falling on Phoenix, Arizona, and a water molecule in the Nile River in Egypt are both made of two hydrogen atoms for every oxygen atom.

Imagine you have a favorite cookie recipe that requires exact amounts of flour, sugar, and chocolate chips to consistently create the same delicious taste every time. In a similar way, chemical composition refers to the recipe of a substance, indicating which elements and how much of each element are present to form a chemical compound. Just like with baking cookies, if the proportions of ingredients in a chemical compound change, the end product is not the same.

Chemical compounds are substances formed when two or more elements combine in a fixed ratio. This is a crucial aspect to understand because it underscores the stable and predictable nature of the substances around us. Whether it is a compound found in nature or synthesized in a lab, its chemical composition remains constant. This is due to a fundamental principle known as the Law of Definite Proportions, which states that no matter the amount or source of a compound, the proportion of each element within the compound remains the same.

Water is essential to life, and the roles it plays on Earth are as diverse as they are vital. From making up about 60% of the human body to regulating the planet's climate, understanding the water molecule is key to grasping how these functions are possible. A single water molecule, consisting of two hydrogen atoms and one oxygen atom, exhibits a bent shape due to the way the atoms bond with each other. This unique shape makes it a polar molecule, enabling it to dissolve various substances and interact with other molecules effectively.

The cohesive and adhesive properties of water, which are important for processes such as the transport of nutrients in plants, also arise from its molecular structure. Furthermore, as a universal solvent, water supports countless chemical reactions both within organisms and in the environment. In every instance, the configuration and consistency of the water molecule are what make it such a critical and versatile player in the chemistry of life.

A molecule's chemical formula offers a lot of information in a concise package. The chemical formula H2O tells us that each molecule of water is made up of two atoms of hydrogen (H) and one atom of oxygen (O). Delving deeper, the subscript '2' after hydrogen in the formula specifies the exact number of hydrogen atoms present. This precise ratio is significant not only because it defines water but also because it reflects the Law of Definite Proportions in action.

The simple representation of H2O belies the complex interactions that give water its unique properties, such as its ability to exist in three states – solid, liquid, and gas – under Earth's environmental conditions. Beyond the elementary construction illustrated by its formula, the water molecule's polarity, hydrogen bonding, and thermal properties reveal a nuanced character that underpins water's role in chemistry and biology. Its consistency across all locations and situations means that whether it's part of a vast ocean or a single drop of dew, the formula H2O always reliably describes water's fundamental makeup.