Question

Classify each of the following as either a physical change or a chemical change. (a) dissolving sugar in water (b) crushing rock salt (c) burning sulfur (d) digesting a chili dog

Short answer

(a) Physical change; (b) Physical change; (c) Chemical change; (d) Chemical change.

Step-by-step solution

Understanding Physical Changes

Physical changes are changes in the form or physical properties of a substance, without a change in its chemical composition. These changes are reversible and do not result in the production of a new substance.

Understanding Chemical Changes

Chemical changes involve changes in the chemical composition of a substance, leading to the formation of new substances. These are generally irreversible and involve chemical reactions.

Classification of Dissolving Sugar in Water

When sugar dissolves in water, it disperses into individual sugar molecules within the water but retains its chemical identity as sugar. This is a physical change because the chemical structure of sugar remains unchanged, and the process is reversible by evaporating the water.

Classification of Crushing Rock Salt

Crushing rock salt involves breaking it into smaller pieces, but the composition of the salt (sodium chloride) is unchanged. This is a physical change since the identity of the salt remains the same and no new substances are formed.

Classification of Burning Sulfur

Burning sulfur involves a chemical reaction with oxygen to form sulfur dioxide, a new compound. This is a chemical change because it results in the formation of a substance with a different chemical composition than the original sulfur.

Classification of Digesting a Chili Dog

Digesting a chili dog involves enzymatic breakdown and chemical reactions that transform the food into new substances that can be absorbed by the body. This is a chemical change because the food is chemically altered.

Key concepts

Physical Changes

Physical changes occur when the form or appearance of a substance is altered, but its chemical identity remains the same. These changes are typically reversible, meaning the original state can be restored without changing the chemical structure of the substance. Examples of physical changes include:

• Changing the state of matter, such as melting ice into water.
• Breaking substances into smaller pieces, like crushing a rock.
• Dissolving substances in a solvent, assuming no chemical reaction occurs.

While these processes may change a substance's shape, size, or state, the molecules themselves do not change. For instance, when sugar dissolves in water, you still have sugar molecules, just spread throughout the water. This reversible process exemplifies a classic physical change.

Chemical Changes

Chemical changes entail a transformation at the molecular level, creating a new substance with different properties and composition from the original material. Unlike physical changes, chemical changes are usually irreversible.
A key to recognizing chemical changes is the occurrence of a chemical reaction. During a chemical reaction:

• Old chemical bonds break, and new ones form.
• The process may release or absorb energy, such as heat or light.
• New substances are produced, which may exhibit different properties from the reactants.

Burning sulfur is a great example where sulfur reacts with oxygen, forming sulfur dioxide. This transformation is irreversible, showing how chemical properties have completely changed.

Substance Properties

Every substance possesses unique physical and chemical properties that help identify and differentiate it from others.
Physical properties include color, texture, density, and melting point. These properties can be observed or measured without changing the substance's chemical identity. For instance, the sweet taste of sugar or the crystalline structure of salt are physical properties.
Chemical properties, on the other hand, describe a substance's potential to undergo chemical changes. These can only be observed during a chemical reaction. For example, the flammability of sulfur is a chemical property, as it illustrates sulfur's ability to burn and transform into sulfur dioxide. Understanding these properties is crucial for predicting how substances will react in different situations.

Chemical Reactions

Chemical reactions are processes where reactants transform into products through the breaking and forming of chemical bonds. These reactions are central to understanding chemical changes.
Common signs of a chemical reaction include:

• Change in color or temperature.
• Formation of a gas or precipitate.
• Distinct odor or light emission.

Digesting food, like a chili dog, involves numerous chemical reactions. Enzymes break down complex molecules into simpler ones, allowing the body to absorb nutrients. The alteration of these molecules and the formation of new ones mark the essential nature of chemical reactions in digestion.