Question

Classify each as a physical change or a chemical change. a. breaking a pencil in two b. water freezing and forming ice c. frying an egg d. burning wood e. leaves changing colors in the fall

Short answer

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

Step-by-step solution

a. Breaking a pencil in two

Breaking a pencil in two only changes the size and shape of the pencil, but the pencil remains the same material. No new substance is formed. Thus, this is a physical change.

b. Water freezing and forming ice

When water freezes and forms ice, the state of the water changes from liquid to solid. However, the water molecules remain the same, and no new substance is formed. This is a physical change.

c. Frying an egg

When an egg is fried, heat causes the proteins in the egg to denature and solidify, creating a new texture and appearance. The chemical composition of the egg is changed, and it cannot be reverted to its original state. Therefore, this is a chemical change.

d. Burning wood

When wood is burned, it undergoes a chemical reaction with oxygen, producing new substances like ash, carbon dioxide, and water vapor. The original wood cannot be recovered. This is a chemical change.

e. Leaves changing colors in the fall

During fall, leaves change color due to the breakdown of chlorophyll and the production of other pigments like anthocyanins, which produce the vibrant colors we see. The change in color is an indication of a change in the chemical composition of the leaves, making this a chemical change.

Key concepts

Physical Change

Physical changes are alterations in the size, shape, or state of a substance that do not involve a change in its chemical identity. For instance, when you break a pencil in two, the pencil is only divided into smaller pieces. Its material, wood, and graphite remain the same.
Similarly, when water freezes to form ice, it transitions from a liquid to a solid state. However, the molecular structure of H2O doesn't change.

• No new substances are formed during a physical change.
• The process can often be reversed.
• Examples include ripping paper, melting ice, or dissolving sugar in water.

Chemical Change

Chemical changes result in the formation of one or more new substances with different properties. This process is usually irreversible. For example, frying an egg causes the proteins to denature, permanently altering the egg's structure. You cannot change it back to its original form.
When wood burns, it reacts with oxygen to form ash, carbon dioxide, and water vapor, substances that didn't exist before.

• Chemical changes often involve energy changes, such as heat being absorbed or released.
• Signs of a chemical change include color change, gas production, or the formation of a precipitate.
• Other examples include digesting food or souring milk.

Phase Change

Phase changes are special types of physical changes where a substance changes from one state of matter to another. Common phase changes include melting, freezing, boiling, and condensing. For instance, water freezing into ice is a transition from a liquid phase to a solid phase, but the chemical makeup of water stays H2O.
Phase changes are typically reversible processes, like melting ice back to water.

• The energy involved in phase changes generally affects the state without altering the chemical identity.
• Examples include boiling water turning to steam or dry ice sublimating into carbon dioxide gas.

Chemical Reaction

A chemical reaction involves the transformation of reactants into new products through the breaking and forming of chemical bonds. For example, when wood burns, the cellulose reacts with oxygen to yield carbon dioxide, water vapor, and ash. This process is a chemical reaction.
Leaves changing color in fall are due to chemical reactions where chemical pigments change or new ones form.

• Chemical reactions usually require activation energy to start.
• They are often accompanied by energy changes like heat, light, or sound.
• Reactions can be fast, like burning a match, or slow, like rusting iron.