Water always contains hydrogen and oxygen combined in the fixed mass ratio 1:8. This experimental fact illustrates which law of chemical combination?

Introduction / Context:
Classical chemistry is built on several laws of chemical combination that describe how elements combine to form compounds. One of these laws explains why pure water from any source always has the same composition. Water contains hydrogen and oxygen in a fixed mass ratio of approximately 1:8, which does not change regardless of where the water comes from or how much water is tested. This question asks which law of chemical combination is demonstrated by this constant composition of water.

Given Data / Assumptions:

• Pure water has a definite composition: 2 hydrogen atoms and 1 oxygen atom per molecule (H2O).
• The mass ratio of hydrogen to oxygen in water is approximately 1:8.
• This ratio is constant for pure water samples from different sources.
• The options mention several classical laws: multiple proportions, definite proportion, conservation of mass, reciprocal proportions and gaseous volumes.

Concept / Approach:
The Law of Definite Proportion, also called the Law of Constant Composition, states that a given chemical compound always contains the same elements in the same fixed proportion by mass regardless of the source or amount of the compound. For water, this means that hydrogen and oxygen are always present in a mass ratio of 1:8 in pure samples. The other laws have different meanings: the Law of Multiple Proportions relates to different compounds formed by the same elements, the Law of Conservation of Mass states that mass is neither created nor destroyed and the Law of Reciprocal Proportions involves relationships between different compounds of the same elements. Therefore, the fixed 1:8 ratio in water is an example of the Law of Definite Proportion.

Step-by-Step Solution:
Step 1: Recall that the formula of water is H2O, meaning each molecule has two hydrogen atoms and one oxygen atom. Step 2: The relative atomic masses are approximately 1 for hydrogen and 16 for oxygen. Step 3: In one mole of water, the mass contribution is roughly 2 grams from hydrogen and 16 grams from oxygen, giving a mass ratio of hydrogen to oxygen of 2:16, which simplifies to 1:8. Step 4: This mass ratio remains constant for all pure water, no matter how much water is tested or where it comes from. Step 5: A law that states a compound always has the same elements in the same mass ratio is the Law of Definite Proportion. Step 6: The other laws listed do not describe this fixed mass ratio in a single compound. Step 7: Therefore, the correct answer is the Law of Definite Proportion.

Verification / Alternative check:
Historically, careful experiments measured the masses of hydrogen and oxygen that combine to form water during combustion reactions. These experiments repeatedly showed the same mass ratio of hydrogen to oxygen. Later, analysis of many different samples of water from rivers, lakes and distilled sources confirmed that the chemical composition of pure water is always H2O, which corresponds to the same 1:8 mass ratio. Textbooks explicitly present water as a classic example used to illustrate the Law of Definite Proportion, confirming that this law is the correct one for the situation described in the question.

Why Other Options Are Wrong:
Option A, the Law of Multiple Proportions, deals with different compounds formed by the same elements, such as carbon monoxide and carbon dioxide, not with the constant composition of a single compound. Option C, the Law of Conservation of Mass, states that total mass remains constant in a closed system during a chemical reaction; it does not specify fixed mass ratios in compounds. Option D, the Law of Reciprocal Proportions, relates to the ratios in which elements combine with a common third element. Option E, the Law of Gaseous Volumes, refers to volumes of gases under the same conditions, not mass ratios in compounds. None of these describe a fixed 1:8 mass ratio in water except the Law of Definite Proportion.

Common Pitfalls:
Students may confuse the Law of Definite Proportion with the Law of Multiple Proportions because the names sound similar and both involve ratios. Another common error is to think of conservation of mass simply whenever a mass quantity is mentioned, even though this law addresses total mass, not composition. To avoid such confusion, remember that definite proportion is about one compound always having the same mass ratio of elements, while multiple proportions involve several compounds made from the same elements. Keeping a clear mental example, such as water for definite proportion and carbon oxides for multiple proportions, helps you quickly choose the correct law in exam questions.

Final Answer:
The fixed 1:8 mass ratio of hydrogen to oxygen in water illustrates the Law of Definite Proportion.