Question

Classify each of the following as a physical or chemical change or property. a. Oven cleaners contain sodium hydroxide, which converts the grease/oil spatters inside the oven to water-soluble materials, which can be washed away. b. A rubber band stretches when you pull on it. c. A cast-iron frying pan will rust if it is not dried after washing. d. Concentrated hydrochloric acid has a choking, pungent odor. e. Concentrated hydrochloric acid will burn a hole in cotton jeans because the acid breaks down the cellulose fibers in cotton. f. Copper compounds often form beautiful blue crystals when a solution of a given copper compound is evaporated slowly. g. Copper metal combines with substances in the air to form a green "patina" that protects the copper from further reaction. h. Bread turns brown when you heat it in a toaster. i. When you use the perfume your boyfriend gave you for your birthday, the liquid of the perfume evaporates quickly from your skin. j. If you leave your steak on the gas grill too long, the steak will turn black and char. k. Hydrogen peroxide fizzes when it is applied to a cut or scrape.

Short answer

a. Chemical change b. Physical change c. Chemical change d. Physical property e. Chemical change f. Physical change g. Chemical change h. Chemical change i. Physical change j. Chemical change k. Chemical change

Step-by-step solution

a. Oven cleaners with sodium hydroxide

This statement describes a chemical change, as sodium hydroxide reacts with grease and oil to transform them into water-soluble materials which can be washed away.

b. A rubber band stretches

Stretching a rubber band is a physical change, as it only affects the shape and size of the material but does not create a new substance.

c. Rusting of a cast-iron frying pan

Rusting of a cast iron frying pan is a chemical change, as it involves the reaction between iron and oxygen to form a new substance, iron oxide.

d. Odor of concentrated hydrochloric acid

The odor of concentrated hydrochloric acid is a physical property, as it relates to the sensory perception of the substance but doesn't involve a chemical reaction.

e. Burning holes in cotton jeans

Concentrated hydrochloric acid burning a hole in cotton jeans is a chemical change, as it breaks down the cellulose fibers in cotton into new materials.

f. Formation of blue crystals

The formation of blue crystals by the evaporation of a copper compound solution is a physical change, as it only involves a change in phase from liquid to solid.

g. Green patina formation on copper

The formation of a green patina on copper is a chemical change, as the copper reacts with substances in the air to form a new material, such as copper carbonate.

h. Bread turning brown in a toaster

Bread turning brown when heated in a toaster is a chemical change called the Maillard reaction, which occurs between amino acids and reducing sugars, causing new compounds to form and the bread to turn brown.

i. Perfume evaporation

Perfume evaporation from the skin is a physical change, as it only involves the change of phase from liquid to gas and does not generate any new substances.

j. Charring of steak on a gas grill

A steak charring on a gas grill is a chemical change, as the burning of organic compounds in the steak causes new substances to form, such as carbon and other compounds.

k. Hydrogen peroxide fizzing

Hydrogen peroxide fizzing when applied to a cut or scrape is a chemical change, as it reacts with enzymes and other chemicals in the wound, breaking down into oxygen gas and water, causing the fizzing.

Key concepts

Chemical Reactions

Chemical reactions occur when substances combine or break apart to form new substances. A classic hallmark of a chemical reaction is a change that produces one or more new substances, such as a color change, gas production, or temperature shift.
One example is when oven cleaners, which contain sodium hydroxide, react with grease and oil. This process converts the greasy deposits into water-soluble materials that can be washed away, exhibiting a chemical change due to the formation of new substances.
Rusting, such as in a cast-iron frying pan, is another prime example of a chemical reaction. The iron reacts with oxygen in the air to form iron oxide, a new compound. Similarly, burning, whether of cotton fibers or organic material in steak, represents a chemical reaction because new substances emerge during the process.
Chemical reactions are fundamental to many processes, from cooking and cleaning to the natural rusting of metals.

Physical Properties

Physical properties are characteristics of substances that can be observed without changing the substance's chemical identity. These qualities include properties such as color, odor, melting point, boiling point, density, and physical state.
For instance, the strong, pungent odor of concentrated hydrochloric acid is a physical property. Although it can be overpowering, the scent reflects nothing of a chemical transformation taking place.
When analyzing physical properties, it's important to note that they include both quantitative measures such as melting point and qualitative observations like texture and appearance. The evaporation of perfume from skin or the stretching of a rubber band also involve physical properties, as only the state or shape changes respectively, with no new substance formed.
Understanding physical properties helps in identifying substances and predicting their behavior in various scenarios.

Chemical Properties

Chemical properties describe a substance's ability to undergo specific chemical changes. They indicate how substances interact with other chemicals and reflect the transformations a substance can undergo and the types of new substances that can form.
For example, the ability of copper metal to form a green patina when exposed to air is a chemical property. This characteristic involves copper reacting with elements in the environment, such as oxygen, moisture, and pollutants, and is an indicator of the material's reactive potential.
Similarly, the tendency of hydrochloric acid to burn through cotton by degrading its cellulose fibers shows a strong chemical property. It signifies the acid's potential for causing chemical changes through reactions with materials it contacts.
Recognizing chemical properties allows for predicting a substance's chemical reactivity and potential changes during applications or processes.

Chemical Changes

Chemical changes, also known as chemical reactions, result in the production of one or more new materials with distinct properties. These changes are irreversible under normal circumstances, unlike physical changes.
The transformation of bread to a brown hue in a toaster is a chemical change known as the Maillard reaction. This change not only affects the color but also produces new flavor and aroma compounds, illustrating the creation of entirely new substances.
Charring steak on a grill embodies a chemical change where the organic compounds in steak undergo combustion, leading to blackened crust formation due to new substances such as carbon developing.
Lastly, the fizzing of hydrogen peroxide on a cut is another example of a chemical change. It reacts with substances in the wound to break down into water and oxygen gas, manifesting as bubbles.
Chemical changes are pivotal in cooking, cleaning, and biological processes, underscoring their importance in everyday life.