Substances composed of two or more different elements combined in a fixed proportion by mass and having a definite chemical formula are called what?

Introduction / Context:
Classifying matter into elements, compounds, and mixtures is a fundamental step in learning chemistry. Compounds differ from mixtures because their composition is fixed and they can be represented by a chemical formula. Many exam questions test whether students can correctly apply these definitions. This question asks you to identify the term used for substances made of two or more different elements united in a fixed proportion by mass.

Given Data / Assumptions:

• The substance is made of at least two different elements.
• The elements are present in a fixed proportion by mass.
• The substance has a definite chemical formula, such as H2O or CO2.
• The properties of the substance are different from those of the constituent elements.

Concept / Approach:
A compound is defined as a pure substance composed of two or more elements chemically combined in a fixed proportion by mass. Water, H2O, for example, always contains hydrogen and oxygen in the same ratio. A mixture, by contrast, consists of two or more substances mixed in variable proportions without chemical bonding, so its composition can change. Elements are pure substances made of only one type of atom. A solution is a special type of mixture, and alloys are mixtures of metals. Therefore, the correct term for a substance meeting the conditions in the question is compound.

Step-by-Step Solution:
Step 1: Focus on the phrase "two or more different elements combined in fixed proportion by mass." Step 2: Recall that compounds have fixed composition and definite chemical formulas, such as NaCl or H2SO4. Step 3: Remember that mixtures do not have fixed composition and cannot be represented by a single chemical formula. Step 4: Recognise that elements contain only one type of atom and thus do not fit the description of multiple elements combined. Step 5: Identify "compound" as the term that matches the definition given.

Verification / Alternative check:
Consider water, H2O. It is made from hydrogen and oxygen in a fixed ratio of two hydrogen atoms to one oxygen atom by number, and about 1:8 by mass. This composition does not vary and is always the same for pure water. Similarly, sodium chloride always has a 1:1 ratio of sodium to chloride ions. These substances are called compounds. Air and brass, on the other hand, have variable composition and are mixtures, not compounds. These examples confirm that a fixed proportion by mass is a hallmark of a compound.

Why Other Options Are Wrong:
A mixture has components that can be present in any proportion and are not chemically combined, so it does not meet the fixed ratio condition. An element contains only one kind of atom, so it cannot involve two or more different elements. A solution is a type of homogeneous mixture, in which solute and solvent can vary in proportion. An alloy is also a mixture, usually of metals, and can have variable composition. None of these terms match the strict requirement of a fixed mass ratio and definite chemical formula.

Common Pitfalls:
Students sometimes confuse mixtures and compounds because both can have more than one element present. Another mistake is to think that any homogeneous material is a compound, even if its composition can change. To avoid these errors, remember that compounds have a fixed composition and can only be separated into elements by chemical methods, whereas mixtures can often be separated by physical methods and have variable composition.

Final Answer:
Substances made of two or more elements in a fixed proportion by mass and with a definite formula are called compounds.