Problem 8 - Chemistry and Chemical Reactivity

Question

In each case, decide if the change is a chemical or physical change. (a) A cup of household bleach changes the color of your favorite T-shirt from purple to pink. (b) Water vapor in your exhaled breath condenses in the air on a cold day. (c) Plants use carbon dioxide from the air to make sugar. (d) Butter melts when placed in the sun.

Step-by-Step Solution

Answer

(a) Chemical, (b) Physical, (c) Chemical, (d) Physical changes.

Step 1: Identifying the Type of Change in Part (a)

When household bleach changes the color of a T-shirt from purple to pink, it is due to a chemical reaction between the bleach and the dye in the fabric. The bleach oxidizes the dye, causing it to change color, which indicates a chemical change.

Step 2: Identifying the Type of Change in Part (b)

When water vapor from your breath condenses in the air, it changes from a gas to a liquid. This process is called condensation, and it is a physical change because the chemical structure of the water (H₂O) remains unchanged.

Step 3: Identifying the Type of Change in Part (c)

Plants use carbon dioxide and water to perform photosynthesis, a process that transforms these substances into glucose and oxygen. This transformation involves breaking and forming of chemical bonds, thus it is a chemical change.

Step 4: Identifying the Type of Change in Part (d)

When butter melts in the sun, it changes from a solid to a liquid. This is known as melting, which is a physical change because the composition of butter, at the molecular level, remains the same.

Key Concepts

Chemical Reaction

A chemical reaction occurs when substances interact to form new substances with different properties. This process involves the breaking and formation of chemical bonds, leading to a change in composition. One common example of a chemical reaction is when household bleach changes the color of fabric. When bleach is applied to a T-shirt, it reacts with the dye molecules, resulting in a new color change. This indicates that a chemical change has occurred.
The characteristics of a chemical reaction include:

• Change in color, as seen in the bleach example.
• Formation of a gas or solid (e.g., bubbles).
• A temperature change even without heating or cooling.
• Emission or absorption of light.

Remember that in a chemical reaction, the fundamental identity of the original substances alters.

Physical Change

A physical change involves a transformation in the state or appearance of a substance, but the chemical composition remains unchanged. No new substances are created during a physical change. A clear example of a physical change is when butter melts in the sun. Here, the butter changes from a solid to a liquid, yet its chemical structure stays the same.
Some key points to recognize a physical change include:

• Change in state, such as melting, freezing, or vaporization.
• Alteration in shape or size.
• No formation of new chemical products.

Physical changes are generally reversible, meaning the original form can be restored.

Photosynthesis

Photosynthesis is a vital chemical process performed by plants to convert light energy into chemical energy, in the form of glucose. This process utilizes carbon dioxide and water, along with sunlight, to produce glucose and oxygen. Essentially, it is a chemical reaction because new substances are formed, involving breaking and forming chemical bonds.
The equation for photosynthesis can be expressed as:\[ 6CO_2 + 6H_2O + light ightarrow C_6H_{12}O_6 + 6O_2 \]During photosynthesis:

• Plants absorb carbon dioxide from the air through tiny openings known as stomata.
• Water is taken up by roots from the soil.
• Chlorophyll in the leaves captures sunlight.

This energy transformation process not only sustains plant life but also produces oxygen, essential for the survival of most living organisms on Earth.

Condensation

Condensation is a type of physical change where water vapor in the air transforms into liquid water. This process generally occurs when warm, moist air encounters a cooler surface or environment. A good everyday example is seeing your breath on a cold day; water vapor from your breath condenses as it cools, becoming visible. Despite the change in state from gas to liquid, the chemical composition of water ( H₂O ) remains unchanged.
Essential aspects of condensation include:

• It involves a change of state from gas to liquid.
• No new substances are formed during the process.
• The process is often reversible through temperature change.

Condensation contributes to the water cycle, playing a significant role in cloud formation and precipitation.