Which of the following substances is an element rather than a compound or mixture?

Introduction / Context:
A key skill in basic chemistry is distinguishing between elements, compounds and mixtures in everyday substances. Elements are pure substances made of only one type of atom, while compounds consist of atoms of different elements chemically combined in fixed proportions, and mixtures contain more than one substance physically combined. The options in this question include several common industrial and household materials. You are asked to identify which one is actually an element rather than a compound or mixture, testing your understanding of these classifications and some common names.

Given Data / Assumptions:

• Alumina (aluminium oxide) is a well known ceramic material.
• Brass is a familiar metal alloy used in hardware and decoration.
• Graphite is a form of carbon used in pencils and lubricants.
• Silica is the main component of sand and quartz.
• Common salt refers to sodium chloride used in food.

Concept / Approach:
Elements contain only one kind of atom and are represented by a single chemical symbol, such as C for carbon, O for oxygen or Na for sodium. Compounds have specific chemical formulas with more than one element, such as Al2O3 for alumina, SiO2 for silica and NaCl for common salt. Mixtures like brass have variable composition and are made from two or more elements that are not chemically bonded in fixed proportions. Graphite is a crystalline form, or allotrope, of the element carbon, so it is an element. The approach is to examine each substance and determine whether it is made of a single element or a combination of elements.

Step-by-Step Solution:
Step 1: Consider alumina. Its chemical formula is Al2O3, indicating that it contains aluminium and oxygen, so it is a compound, not an element. Step 2: Brass is an alloy, typically containing copper and zinc, sometimes with small amounts of other metals. Because it has more than one element and variable composition, it is a mixture, not an element. Step 3: Graphite is a crystalline form of carbon. Its atoms are all carbon atoms arranged in layers, making it an allotrope of the element carbon. Step 4: Silica has the chemical formula SiO2, which contains silicon and oxygen, so it is a compound. Step 5: Common salt (table salt) is sodium chloride, NaCl, a compound containing sodium and chlorine. Step 6: Therefore, the only substance in the list that is actually an element is graphite, which is pure carbon.

Verification / Alternative check:
Examining the formulas confirms the classification. Al2O3, SiO2 and NaCl each contain two different elements in definite proportions and behave as compounds with fixed properties. Brass is typically described not by a simple formula but by its composition, such as 70 percent copper and 30 percent zinc, varying with application, which is characteristic of mixtures. Graphite, on the other hand, is described simply as carbon in a particular allotropic form. In the periodic table, carbon is an element, and all its natural allotropes, such as diamond and graphite, are still the element carbon. This confirms that graphite is the correct choice.

Why Other Options Are Wrong:
Option A, alumina, is aluminium oxide, a compound of aluminium and oxygen with a definite formula and not an element. Option B, brass, is a mixture (alloy) mainly of copper and zinc, so it is not a pure element. Option D, silica, is silicon dioxide, a compound of silicon and oxygen and the main ingredient of sand and glass. Option E, common salt, is sodium chloride, a compound of sodium and chlorine. Only option C, graphite, consists solely of carbon atoms and is therefore an element.

Common Pitfalls:
Students may sometimes think that because graphite looks like a manufactured material it must be a compound, or they may confuse silica with silicon, which is an element. Another common mistake is to think that any shiny or metallic looking substance must be a pure element, when in fact many are alloys or mixtures. To avoid these errors, focus on the chemical formula and whether it contains more than one type of atom. Remember that graphite and diamond are both allotropes of carbon, so they are still the element carbon, while alumina, silica and common salt are compounds.

Final Answer:
The substance that is an element rather than a compound or mixture is Graphite (an allotrope of carbon).