Question

What type of change, physical or chemical, takes place in each of the following? a. Gold is hammered into thin sheets. b. A silver pin tarnishes in the air. c. A tree is cut into boards at a saw mill. d. Food is digested. e. A chocolate bar melts.

Short answer

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

Step-by-step solution

Gold is hammered into thin sheets

Hammering gold into thin sheets is a physical change because the gold's shape is altered, but its chemical composition remains the same.

A silver pin tarnishes in the air

Tarnishing of a silver pin is a chemical change since it results from a reaction with substances in the air, forming a new compound on the surface.

A tree is cut into boards at a saw mill

Cutting a tree into boards is a physical change because the form of the tree is changed, but its chemical structure remains unchanged.

Food is digested

Digestion of food is a chemical change as it involves breaking down food into simpler substances through chemical reactions.

A chocolate bar melts

Melting a chocolate bar is a physical change since the chocolate changes from solid to liquid without altering its chemical makeup.

Key concepts

Physical Changes

A physical change occurs when the form or appearance of a substance changes, but its chemical identity remains the same. Common examples include:

🔸Changing state (solid to liquid)
🔸Breaking or cutting a material
🔸Dissolving a substance in a solvent

In the exercise, hammering gold into thin sheets and cutting a tree into boards are physical changes. The gold and wood retain their original chemical properties even though their shapes have been altered.

An easy way to identify a physical change is if you can reverse the process and get back the original substance.

Chemical Changes

Chemical changes happen when substances react and form new substances with different properties. Some indicators of chemical changes include:

🔸Change in color
🔸Formation of a gas or odor
🔸Change in temperature
🔸Formation of a precipitate (solid)

Examples from the exercise include the tarnishing of a silver pin and the digestion of food. In both cases, new chemical compounds are formed. Tarnished silver is the result of a reaction with sulfur compounds in the air, and food digestion breaks down food into nutrients, forming different chemicals.

Chemical Reactions

Chemical reactions are processes in which substances interact to form new substances. They are characterized by a rearrangement of atoms and can be understood through balanced chemical equations.

Reactants (starting materials) combine to form products (new substances). For example:

Reactants: A + B → Products: C + D

Consider the tarnishing of a silver pin from the exercise. The reaction can be simplified as:

Ag (silver) + H₂S (hydrogen sulfide) → Ag₂S (silver sulfide) + H₂ (hydrogen)

This shows how the silver reacts with sulfur compounds to create a new substance, silver sulfide.

States of Matter

Matter exists in different states: solid, liquid, gas, and plasma. Each state is defined by its distinct properties.

🔸Solids have a definite shape and volume.
🔸Liquids have a definite volume but take the shape of their container.
🔸Gases have neither a definite shape nor volume, expanding to fill their container.

In the exercise, the melting of a chocolate bar is an example of changing states of matter. The chocolate changes from solid to liquid due to heat. This is a physical change as the composition of the chocolate remains the same, only its state changes.

Understanding these states helps in identifying physical changes, as substances can shift between states without altering their chemical identity.