In basic chemistry, which term refers to a pure substance that cannot be broken down into simpler substances by ordinary chemical reactions?

Introduction / Context:
Classifying matter into elements, compounds, molecules and mixtures is one of the first tasks in chemistry. An element is a fundamental chemical substance, while compounds and mixtures are formed by combining elements in different ways. This question asks which term describes the simplest form of matter in the sense that it cannot be decomposed into simpler substances by ordinary chemical means.

Given Data / Assumptions:

• We are comparing the meanings of element, molecule, atom, compound and mixture.
• Ordinary chemical reactions are considered, not nuclear reactions.
• We look for the term that defines a basic chemical substance.
• Atoms are recognised as the smallest units of elements that retain their properties.

Concept / Approach:
An element is a pure substance composed of only one type of atom. It cannot be chemically decomposed into simpler substances. A compound consists of two or more different elements chemically combined in a fixed ratio. A molecule is a group of atoms bonded together and may belong to an element, such as O2, or a compound, such as H2O. An atom is the smallest unit of an element that retains its chemical identity, but the term that describes the pure substance itself is element. A mixture is a physical combination of substances that can be separated by physical methods.

Step-by-Step Solution:
Step 1: Consider the definition of an element: a pure substance consisting of only one kind of atom and which cannot be further chemically broken down.Step 2: Recognise that compounds like water can be decomposed into hydrogen and oxygen by chemical methods such as electrolysis.Step 3: Understand that a molecule is a structural unit and can refer to molecules of elements (for example O2) or compounds (for example CO2), so it is not the category of simplest substance.Step 4: Note that an atom is a single particle of an element, but the question asks for the simplest form of matter as a pure substance, which is the element itself.Step 5: Mixtures can always be separated by physical or simple chemical processes into their components.Step 6: Conclude that element is the correct term for a pure, chemically indivisible substance.

Verification / Alternative check:
In the periodic table, each entry corresponds to an element such as hydrogen, oxygen or iron. Textbooks define an element as the simplest chemical substance that cannot be decomposed by ordinary chemical means. They then define compounds as substances formed when elements chemically combine. This hierarchy confirms that element is the correct answer for the simplest form of matter as a chemical substance.

Why Other Options Are Wrong:
Option b, molecule, is a structural unit and not always a basic substance; for example, a water molecule contains two elements. Option c, atom, refers to the smallest particle of an element but the category of pure substance is the element. Option d, compound, can be chemically broken down into elements. Option e, mixture, can be separated by physical means and is clearly not the simplest form of matter.

Common Pitfalls:
Students sometimes confuse atom and element, using the terms as if they were identical. While closely related, an element is a macroscopic substance made of many atoms of the same type, whereas an atom is an individual particle. Exam questions that ask about pure substances that cannot be decomposed usually expect the term element, not atom. Keeping this distinction clear helps avoid wrong choices in classification questions.

Final Answer:
The simplest form of matter that cannot be decomposed by ordinary chemical reactions is an element.