Question

Classify each of the following as a physical or chemical change or property. a. A fireplace poker glows red when you heat it in the fire. b. A marshmallow turns black when toasted too long in a campfire. c. Hydrogen peroxide dental strips will make your teeth whiter. d. If you wash your jeans with chlorine bleach, they will fade. e. If you spill some nail polish remover on your skin, it will evaporate quickly. f. When making ice cream at home, salt is added to lower the temperature of the ice being used to freeze the mixture. g. A hair clog in your bathroom sink drain can be cleared with drain cleaner. h. The perfume your boyfriend gave you for your birthday smells like flowers. i. Mothballs pass directly into the gaseous state in your closet without first melting. j. A log of wood is chopped up with an axe into smaller pieces of wood. k. A log of wood is burned in a fireplace.

Short answer

a. Physical Change b. Chemical Change c. Chemical Change d. Chemical Change e. Physical Change f. Physical Property g. Chemical Change h. Physical Property i. Physical Change j. Physical Change k. Chemical Change

Step-by-step solution

a. Fireplace poker glows red when heated

The glowing red color when heating a fireplace poker is due to the increase in temperature and is not related to any new substance formation. So, this is a physical change.

b. Marshmallow turns black when toasted

When a marshmallow turns black, it means that the sugars in it have undergone a chemical reaction and have been broken down to form new substances, such as carbon. This is a chemical change.

c. Whitening of teeth with hydrogen peroxide dental strips

The hydrogen peroxide dental strips react chemically to remove stains on teeth, and form new products as a result. This is a chemical change.

d. Jeans fade when washed with chlorine bleach

Chlorine bleach reacts chemically with the dyes in jeans to break down the colored molecules, resulting in faded jeans. This is a chemical change.

e. Evaporation of nail polish remover when spilled on skin

Nail polish remover evaporates without forming any new substances due to a change in its physical state (from liquid to gas), and so this is a physical change.

f. Adding salt to lower ice temperature when making ice cream

When making ice cream, salt is added to lower the temperature of the ice, which is a property related to the solution's concentration and freezing point. This is a physical property.

g. Clearing a clogged drain with drain cleaner

The drain cleaner chemically reacts with the substances clogging the drain and breaks them down, forming new substances in the process. This is a chemical change.

h. Smell of perfume

The smell of perfume is due to the physical presence of perfume molecules that enter your nose, without any chemical reaction involved. This is a physical property.

i. Mothballs directly passing into the gaseous state

Mothballs go from the solid state directly to the gaseous state via a process called sublimation, without forming any new substances. This is a physical change.

j. Chopping wood with an axe

When chopping wood, the log is physically split into smaller pieces, without producing any new substances. This is a physical change.

k. Burning wood in a fireplace

When wood is burned, it reacts with oxygen from the air and produces new substances such as carbon dioxide and water vapor. This is a chemical change.

Key concepts

Physical Properties

Physical properties are characteristics of a substance that can be observed or measured without changing its identity. This means that the substance remains the same even though its form might appear different.
Examples of physical properties include:

• Color: This is simply the way a substance reflects light, like the red color of a fireplace poker when heated.
• Odor: How a substance smells, such as the floral scent of perfume.
• Melting and boiling points: These determine the temperatures at which a substance changes state, like the sublimation of mothballs.
• Density: A measure of how much mass is contained in a given volume.
• Solubility: Describes the ability of a substance to dissolve in a solvent.

Identifying these properties requires no chemical reaction and provides vital clues about the nature of the material.

Chemical Properties

Chemical properties refer to the ability of a substance to undergo chemical change or react with other substances. Observing these properties usually involves a chemical reaction with evidence of new substance formation.
Key chemical properties include:

• Reactivity with other chemicals: For instance, how bleach reacts with fabric dyes, leading to fading of jeans.
• Flammability: The capacity to catch fire and burn, such as wood burning in a fireplace.
• Oxidation states: How a substance combines with oxygen, like the oxidative properties of hydrogen peroxide used in dental strips.
• Acidity or basicity: pH levels that indicate how a substance might react when dissolved in water.

In observing chemical properties, one knows that changes are occurring beyond what is just visible, often producing new substances.

Chemical Reactions

A chemical reaction involves the rearrangement of atoms in one or more substances to form one or more different substances. This process is marked by chemical changes and is often irreversible. During these reactions, bonds are broken and new bonds are formed.
Cues that a chemical reaction has occurred include:

• Change in color: Like the marshmallow turning black when its sugar caramelizes and burns.
• Formation of a gas: As seen when drain cleaner reacts with gunk in the pipes.
• Emission of heat or light: Burning a log in a fireplace involves such heat emission.
• Formation of a precipitate: A solid that forms out of a solution.

Chemical reactions may require specific conditions such as temperature, pressure, or the presence of a catalyst to occur.

Phase Changes

Phase changes are transitions between different states of matter such as solid, liquid, and gas. These changes are usually physical and involve no chemical modification of the substance.
Examples of phase changes include:

• Melting: Transition from solid to liquid state, like ice cream melting.
• Freezing: Liquid turning into a solid, such as when ice cream solidifies.
• Evaporation: Liquid to gas transformation, like nail polish remover evaporating.
• Sublimation: Direct change from solid to gas, observed with mothballs in a closet.

Phase changes depend on factors like temperature and pressure but do not involve any chemical reactions or the formation of new substances. Understanding these changes is essential in everyday applications, from culinary tasks to industrial processes.