Question

Classify each of the following changes as physical or chemical. a. the crushing of salt b. the rusting of iron c. the burning of natural gas in a stove d. the vaporization of gasoline

Short answer

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

Step-by-step solution

Identify Characteristics of Physical and Chemical Changes

Physical changes alter the form or appearance of a material but do not make any substance with a new chemical composition; such changes are usually reversible. Chemical changes result in new substances with new chemical properties; these changes typically involve chemical reactions and are not easily reversible.

Classify the Crushing of Salt

The crushing of salt is a physical change because it changes the size of the salt crystals but does not change the chemical identity of the salt (sodium chloride).

Classify the Rusting of Iron

The rusting of iron is a chemical change because it results in a new substance, iron oxide, with different chemical properties than iron.

Classify the Burning of Natural Gas in a Stove

The burning of natural gas (primarily methane) in a stove is a chemical change. It is a combustion reaction that produces carbon dioxide and water, which are different substances from the original methane and oxygen.

Classify the Vaporization of Gasoline

The vaporization of gasoline is a physical change because it changes from liquid to gas without changing its chemical composition. The gasoline molecules are the same in both liquid and gaseous states.

Key concepts

Chemical Reactions

When we talk about chemical reactions, we're examining the process by which substances interact to form new substances with different chemical properties. For instance, when natural gas burns in a stove, it reacts with oxygen from the air to create water and carbon dioxide. This is a classic example of a combustion reaction.

In our everyday lives, chemical reactions are happening all around us, often hidden in plain sight. The burning of natural gas is a transformative process where chemical bonds are broken and new ones are formed, showcasing the permanent nature of chemical changes. It's like baking a cake; once you've mixed the ingredients and baked them, you can't retrieve the individual eggs or flour – they've become something new.

Chemical Properties

Understanding Chemical Properties

Chemical properties describe a substance's potential to undergo a specific chemical change. For example, the ability of iron to rust when exposed to moisture and oxygen is one of its chemical properties. This property is intrinsic and can only be observed during a chemical reaction, unlike physical properties like melting point or density, which you can measure without changing the substance's identity.

Teaching students about chemical properties means helping them recognize the underlying characteristics that dictate how a substance reacts. The rusting of iron, turning to iron oxide, is evidence of a fundamental identity shift that can't be reversed by simply drying the iron or physically removing the rust.

Reversible Changes

In contrast to the firm finality of chemical changes, reversible changes are typically associated with physical changes. Taking the vaporization of gasoline as an example: it is a physical change because the process can be reversed. If the gasoline vapor is cooled down, it condenses back into liquid gasoline without altering its chemical structure. It's akin to water turning into ice or steam and then back to water – the substance remains H₂O throughout.

These reversible changes are essential for understanding the fluidity of physical states and should be highlighted as an important aspect of physical properties. They play a significant role in various applications, from industrial processes to everyday phenomena like the condensation on a cold beverage glass.