Question

Identify each of the following as a physical or chemical change: a. A medication tablet is broken in two. b. Carrots are grated for use in a salad. c. Malt undergoes fermentation to make beer. d. A copper pipe reacts with air and turns green.

Short answer

a. Physical change. b. Physical change. c. Chemical change. d. Chemical change.

Step-by-step solution

- Define Physical and Chemical Changes

Identify the key characteristics of physical and chemical changes. A physical change alters the form of a substance but not its chemical composition. A chemical change, on the other hand, results in the formation of one or more new substances with different properties.

- Analyze the Medication Tablet

Breaking a medication tablet into two pieces is a physical change because the composition of the tablet remains the same, only its form changes.

- Consider the Grated Carrots

Grating carrots for a salad is a physical change because it affects only the size and shape of the carrots, not their chemical composition.

- Evaluate the Fermentation Process

Malt undergoing fermentation to make beer is a chemical change because it involves the conversion of sugars into alcohol and carbon dioxide, creating new substances.

- Examine the Copper Pipe Reaction

When a copper pipe reacts with air and turns green, it undergoes a chemical change. The green substance is formed due to the reaction between copper and oxygen in the air, creating a new compound called copper carbonate.

Key concepts

Physical change

Physical changes alter the form or appearance of a substance, but they do not change the chemical makeup of that substance. Common examples include:

• Breaking a medication tablet in two
• Grating carrots for a salad

In both examples, the physical form is altered—for instance, a whole carrot becomes grated pieces and a whole tablet becomes two pieces. However, the internal chemical structure remains unchanged. Physical changes are easily reversible and typically involve changes in state, shape, or size rather than the substance’s molecular identity.

Chemical change

A chemical change results in the formation of one or more new substances with properties different from the original substance. Chemical changes often require a chemical reaction and involve the breaking and forming of chemical bonds. For example:

• Malt undergoing fermentation
• A copper pipe reacting with air

During fermentation, sugars in the malt are converted into alcohol and carbon dioxide, forming new chemical substances. Similarly, when copper reacts with air to form a green layer, it undergoes a chemical change to form copper carbonate. Chemical changes are usually not reversible and signify a transformation at the molecular level.

Fermentation process

Fermentation is a type of chemical change where microorganisms like yeast convert sugars into alcohol and carbon dioxide. In the context of brewing beer, malt (a form of processed barley) undergoes fermentation:

• Sugar from malt is broken down by yeast.
• Alcohol and carbon dioxide are produced.

This process not only creates new substances but also results in the characteristic flavors and effervescence of beer. The chemical equation for fermentation is:
\[ C_6H_{12}O_6 \rightarrow 2 C_2H_5OH + 2 CO_2 \] Here, glucose (\( C_6H_{12}O_6 \)) is converted into ethanol (\( C_2H_5OH \)) and carbon dioxide (\( CO_2 \)).

Oxidation reactions

Oxidation reactions are chemical processes where a substance combines with oxygen, often resulting in a noticeable change such as rust or tarnish. When a copper pipe turns green upon exposure to air, it is undergoing an oxidation reaction:

• Copper reacts with oxygen and moisture.
• A new substance, copper carbonate, is formed.

The green layer on copper structures is a classic example of this type of chemical change. The overall chemical reaction can be represented as:
\[ 2 Cu + H_2O + CO_2 + O_2 \rightarrow Cu_2(OH)_2CO_3 \] This shows copper (\( Cu \)) reacting with water (\( H_2O \)), carbon dioxide (\( CO_2 \)), and oxygen (\( O_2 \)) to form copper carbonate hydroxide (\( Cu_2(OH)_2CO_3 \)). The formation of this green compound indicates the material underwent a significant chemical transformation.