Question

State whether the following describes a physical or a chemical change: changing chemical formula, but not physical state.

Short answer

This describes a chemical change.

Step-by-step solution

Understand the Concept of Physical and Chemical Changes

A **physical change** involves changes in physical properties or states of matter without altering the composition or identity of the substance. Examples include changes in state, shape, or size. A **chemical change**, however, involves a change in the chemical composition of the substance, resulting in the formation of new substances.

Analyze the Given Description

The problem states "changing chemical formula, but not physical state." When the chemical formula of a substance changes, it means that the substance's molecular composition has been altered, signifying a transformation that produces a new chemical substance.

Apply the Concepts to the Description

Because a change in a chemical formula directly implies an alteration in the chemical structure and composition of a substance, this description indicates a chemical change. The physical state not changing is unrelated to the classification of this change as chemical.

Key concepts

Physical Change

A physical change occurs when a substance undergoes a transformation that impacts its physical properties without altering its core chemical composition. This can include adaptations in shape, size, or physical state, such as melting, freezing, or cutting.
It's important to note that during a physical change, no new substances are formed. The molecules stay the same, meaning the arrangement of those molecules might change, but their internal structure is unaffected.

• Melting ice into water
• Boiling water into steam
• Breaking a piece of chalk

In all these examples, the substances maintain their original chemical composition. Understanding this helps differentiate them from chemical changes.

Chemical Composition

Chemical composition refers to the types and quantities of atoms present in a substance. When chemical composition changes, it results in the formation of entirely new molecules, hence new substances.
This transformation occurs during a chemical reaction, where bonds between atoms break, rearrange, and form new bonds. Reacting substances, or reactants, transform into products with a different composition. This process is fundamental in chemical changes.
For example, when hydrogen gas reacts with oxygen, water is formed. The reaction can be represented by the equation: \[ 2H_2 + O_2 \rightarrow 2H_2O \] Understanding chemical compositions is crucial because it explains why substances have their particular characteristics and reactivity.

Physical Properties

Physical properties are characteristics of a substance that can be observed or measured without changing its identity. These include aspects like color, odor, boiling point, melting point, density, and hardness.
Physical properties can be used to identify substances and to separate mixtures into their components. They help categorize materials based on their inherent characteristics that don't alter the substance at a molecular level.
Different substances can share similar physical properties, but distinctive properties often aid in identification. For instance, water is unique as it is colorless, odorless, and has a boiling point of 100°C under normal atmospheric pressure.
Recognizing physical properties is essential, as they help us understand how a substance might interact in different environments and conditions.