Question

Several pieces of graphite from a mechanical pencil are (a) broken into tiny pieces. Then the pile of graphite is (b) ignited with a hot flame. Classify (a) and (b) as chemical or physical changes.

Short answer

Breaking graphite into tiny pieces is a physical change, and igniting graphite with a hot flame is a chemical change.

Step-by-step solution

Identifying Change (a)

To determine if breaking graphite into tiny pieces is a chemical or a physical change, consider whether the chemical identity of the graphite is altered. A physical change alters the form but not the composition of a substance. Breaking up graphite into smaller pieces does not change the chemical structure or the properties of the graphite; it only changes its size and shape.

Classifying Change (a)

Since breaking graphite into tiny pieces does not alter its chemical structure or properties, it is classified as a physical change.

Identifying Change (b)

To determine if igniting graphite with a hot flame is a chemical or a physical change, consider whether new substances with different chemical properties are formed as a result. A chemical change results in the formation of one or more new substances with different chemical properties.

Classifying Change (b)

Igniting graphite causes it to react with oxygen in the air to form carbon dioxide gas. This is a new substance with different chemical properties than graphite, indicating a chemical change has occurred.

Key concepts

Physical Change in Chemistry

Understanding physical changes in chemistry is vital for students who are diving into the fascinating world of matter and its transformations. A physical change is essentially a change in the physical properties of a substance without any alteration to its chemical composition.

When a substance undergoes a physical change, its size, shape, state, or appearance may be affected, but the molecules remain the same. For example, tearing a piece of paper changes its shape and size but does not affect its molecular structure—it's still paper. Similarly, when you snap a pencil into two, you still have pencil lead made of graphite in both pieces. This is a quintessential illustration of a physical change—you change something physically, but its identity remains unaltered.

In the classroom exercise, breaking graphite into tiny pieces is identified as a physical change because it involves altering the form (size and shape) of the graphite without changing its chemical identity.

Chemical Change in Chemistry

Chemical changes, on the other hand, are transformations that alter the chemical properties of a substance, resulting in the formation of one or more new substances. These changes involve making or breaking of chemical bonds, leading to a change in the molecular composition of the substance.

A simple example of chemical change is the rusting of iron. When iron reacts with oxygen and water, it forms iron oxide (rust), which is a new substance with properties different from the original iron. Chemical changes are often accompanied by observable indications, such as a change in color, temperature, the release of gases, or the formation of a precipitate.

Through igniting graphite, as demonstrated in our exercise, a chemical reaction occurs where graphite (carbon) reacts with oxygen to form carbon dioxide gas. This reaction results in entirely new substances and different properties, clearly indicating a chemical change.

Identifying Chemical and Physical Changes

Students can easily confuse chemical and physical changes, but identifying these changes can be simplified with a few markers. To distinguish between the two, one should observe if the process creates a different substance or if there are signs of a chemical reaction, such as a change in color, the production of light or heat, the formation of a gas, or the presence of a precipitate.

If the change only involves a difference in the physical properties such as form or state and no new substances are formed, we identify it as a physical change. Conversely, if the change results in new substances with different properties, it is a chemical change. An excellent way to remember this is to understand that physical changes could potentially be reversible (melting and refreezing water), while chemical changes are typically irreversible (burning wood).

Thus, in our provided exercise, (a) breaking graphite into tiny pieces represents a physical change, while (b) igniting graphite, which leads to the formation of carbon dioxide, represents a chemical change.

Reaction of Graphite with Oxygen

Graphite is a form of carbon that is used in a variety of applications, including pencil leads. When it undergoes a reaction with oxygen, especially when ignited, it demonstrates a fundamental chemical change. The reaction of graphite with oxygen to form carbon dioxide can be represented by the chemical equation:

\( C_{(s)} + O_{2(g)} \rightarrow CO_{2(g)} \).

In this reaction, solid carbon (graphite) reacts with oxygen gas to produce carbon dioxide gas. It is an exothermic reaction, which means it releases heat. This is one of the reasons why carbon compounds such as graphite can serve as fuels—because they release energy when they react with oxygen.

The reaction is also a perfect example of a chemical change because it results in new substances with properties that are different from those of the reactants, the graphite, and oxygen. Furthermore, this reaction is essential in explaining how combustible materials burn and the environmental considerations related to the release of carbon dioxide, a greenhouse gas.