Problem 16 - Chemistry and Chemical Reactivity

Question

In each case, decide if the change is a chemical or physical change. (a) \(\mathrm{A}\) 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.

Step 1: Analyze Change (a)

In example (a), the color change from purple to pink when bleach is applied to the T-shirt indicates that the bleach chemically reacts with the dye molecules in the fabric. This alteration of the chemical structure of the dye represents a chemical change.

Step 2: Analyze Change (b)

In example (b), water vapor condensing upon cooling in the cold air represents a change of state from gas to liquid without altering the chemical identity of the substance. Thus, this is a physical change.

Step 3: Analyze Change (c)

In example (c), when plants utilize carbon dioxide and convert it into sugar through photosynthesis, this process involves a complicated chemical reaction that transforms carbon dioxide and water into glucose and oxygen. Thus, this is a chemical change.

Step 4: Analyze Change (d)

In example (d), when butter melts (changes from solid to liquid) under the Sun's heat, it involves only a change in physical state, as the chemical composition of butter remains unchanged. Therefore, this is a physical change.

Key Concepts

Chemical Reactions

A chemical reaction is like a transformation spell that changes substances into something completely different. When bleach changes the color of a T-shirt from purple to pink, it's a perfect example of a chemical reaction. Here's what happens: the bleach reacts with the fabric dye. This interaction alters the chemical structure of the dye molecules, making them reflect light differently, and thus changing the color we see.
The main idea behind a chemical reaction is the rearrangement of atoms. New substances form with different properties from the initial materials.

• The original substances in a chemical reaction are called reactants.
• The new substances produced are known as products.
• During the reaction, atom bonds break and new ones form.
• Energy changes often occur, like heat or light release.

Chemical reactions are all around us, from digesting food to burning gasoline, making them essential to everyday life.

Physical Properties

Physical properties are attributes of substances that you can observe or measure without changing the substance's chemical identity. When thinking about physical properties, consider characteristics like:

• Color
• Hardness
• Density
• Melting and boiling points

These properties help us identify materials and predict how they behave in different situations.
For example, when butter melts under the sun, it undergoes a physical change. This process involves the transformation from solid to liquid state without altering its chemical makeup. Only physical properties, such as state and texture, are changed.
Understanding physical properties is crucial because it allows us to classify materials and understand their potential uses, as well as how they might interact with other substances.

Photosynthesis

Photosynthesis is like nature's magic trick, transforming sunlight into energy. It's the process plants use to convert carbon dioxide and water into sugar and oxygen, using sunlight as the energy source. This is a chemical process, as it creates a substantial change in the structure of the participating molecules.
Here's a simple breakdown of what occurs:

• Plants take in carbon dioxide from the air and water from the soil.
• Using sunlight, they transform these ingredients into glucose (a type of sugar) and oxygen.
• The oxygen is released into the air, which is great news for us and other living creatures because it provides the oxygen we need to breathe.

The equation for photosynthesis can be summed up as follows:
\[6 ext{CO}_2 + 6 ext{H}_2 ext{O} + ext{light energy} \rightarrow ext{C}_6 ext{H}_{12} ext{O}_6 + 6 ext{O}_2\]Photosynthesis doesn't just give plants the energy to grow, but it also supports life on Earth by producing oxygen and serving as the foundation of the food chain.

State Change

State change refers to the transition of a substance from one state of matter to another: solid, liquid, or gas. Unlike chemical reactions, state changes do not alter the chemical identity of the substance.
A simple example is water vapor condensing in cold air, a process you can see when your breath forms mist in chilly weather. This is a physical change where gaseous water (water vapor) cools down and turns into liquid. The molecules themselves remain H₂O, thus no new substance is formed.
Other common state changes include:

• Melting: solid to liquid (like ice turning into water).
• Freezing: liquid to solid (like water freezing into ice).
• Evaporation: liquid to gas (like water turning to vapor when boiled).
• Sublimation: solid to gas (like dry ice going directly to vapor).

Understanding state changes is essential in many fields, from cooking to engineering, as it helps us predict and manage how substances behave under different conditions.