Question

Which of the following substances is(are) homogeneous mixture(s)? a. chocolate chip cookie b. iodine crystals \(\left(\mathrm{I}_{2}\right)\) c. air (in this room) d. gasoline (for a car) e. sucrose, \(\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}\) (commonly found in table sugar)

Short answer

The homogeneous mixtures among the given substances are c. air (in this room) and d. gasoline (for a car).

Step-by-step solution

Analyze each option

For each substance, check if it has a uniform composition throughout its mass. Let's analyze each option one by one. a. Chocolate chip cookie - It has chocolate chips distributed unevenly, and it does not have a uniform composition. b. Iodine crystals \(\left(\mathrm{I}_{2}\right)\) - This is a pure element consisting only of iodine atoms, forming a crystal lattice. This substance has a uniform composition. c. Air (in this room) - It consists of various components such as oxygen, nitrogen, and traces of other gases. Despite the presence of different components, the composition of air is usually considered uniform. d. Gasoline (for a car) - Gasoline is a combination of various hydrocarbons. It is assumed to have a uniform composition as the components are mixed evenly. e. Sucrose, \(\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}\) (commonly found in table sugar) - Sucrose is a pure compound with a uniform and specific composition.

Identify the homogeneous mixtures

We'll now identify the homogeneous mixtures based on the uniform composition criterion: a. Chocolate chip cookie - Not a homogeneous mixture b. Iodine crystals \(\left(\mathrm{I}_{2}\right)\) - Not a homogeneous mixture (it's a pure element) c. Air (in this room) - Homogeneous mixture d. Gasoline (for a car) - Homogeneous mixture e. Sucrose, \(\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}\) (commonly found in table sugar) - Not a homogeneous mixture (it's a pure compound) Thus, the answer is c. air (in this room) and d. gasoline (for a car).

Key concepts

Pure Substances

Pure substances are materials that have a definite and constant composition. They are made up of only one type of particle, which can either be a single element or a compound. This means they have uniform properties and composition throughout. For instance, iodine crystals (I2) are a pure substance, as they consist merely of iodine atoms arranged in a crystal lattice. Similarly, table sugar, chemically known as sucrose (C12H22O11), is a chemical compound that qualifies as a pure substance because it features a specific chemical composition and set of properties.
Understanding pure substances is crucial because they exhibit consistent behavior, which is predictable and identical no matter where you sample it from in the material. This characteristic is what differentiates them from mixtures, where the composition can vary.

Uniform Composition

Uniform composition refers to a material having consistent proportions of its components throughout its entire mass. In chemistry, this often applies to solutions or mixtures where the substance looks and behaves the same everywhere.
Homogeneous mixtures, like air and gasoline, possess uniform composition. Even though air consists of different gases like nitrogen, oxygen, and others in minor amounts, these gases are evenly distributed, making it uniform. The same goes for gasoline, which is made up of various hydrocarbons that are mixed completely, giving it uniformity.
This property is essential because if a substance has a uniform composition, it typically means that any part of it will have the same chemical properties and makeup as any other part, making them practically indistinguishable at a microscopic level.

Mixtures vs Pure Substances

Understanding the difference between mixtures and pure substances is a key concept in chemistry. A mixture is composed of two or more substances that are physically combined, which means they can usually be separated by physical means. Mixtures can be either homogeneous or heterogeneous.

• Homogeneous mixtures: These mixtures have a uniform composition, such as air or gasoline, where the different components are mixed at a molecular level.
• Heterogeneous mixtures: These do not have a uniform composition, like a chocolate chip cookie, where the chocolate chips are not uniformly distributed.

Pure substances, in contrast, consist of a single type of element or compound, and they have consistent chemical characteristics. Their compositions are unchanging, no matter the sample size or where the sample is taken from.

Chemical Compounds

Chemical compounds are pure substances formed by the chemical combination of two or more different elements in a certain ratio and structure. They are characterized by having fixed proportions and physical and chemical properties that are different from their constituent elements.
For example, sucrose (C12H22O11) is a chemical compound composed of carbon, hydrogen, and oxygen in a specific ratio and structure. Its properties are distinct from those of carbon, hydrogen, and oxygen individually.
Chemical compounds play a critical role in numerous applications, from everyday use in food and medicine to more specialized functions in industrial processes. Recognizing chemical compounds helps in understanding reactivity, compatibility, and functionalities in various chemical interactions.