Question

Classify each of the following as a pure substance or a mixture. If a mixture, indicate whether it is homogeneous or heterogeneous: (a) rice pudding, (b) seawater, (c) magnesium, (d) crushed ice.

Short answer

(a) Rice pudding is a heterogeneous mixture, (b) seawater is a homogeneous mixture, (c) magnesium is a pure substance, and (d) crushed ice is a pure substance.

Step-by-step solution

Definitions

- Pure substances: A pure substance is a form of matter that has a constant composition and properties that are constant throughout the sample. It contains only one kind of particle (atoms, ions or molecules). - Mixtures: A mixture is a combination of two or more pure substances in variable proportions such that each substance retains its own chemical identity. - Homogeneous mixture: Also known as a solution, is a mixture in which the components are uniformly distributed throughout the sample on a molecular level. - Heterogeneous mixture: A mixture in which the components are not uniformly distributed throughout the sample, resulting in regions with different properties. Step 2: Analyze and classify each example

(a) Rice pudding

Rice pudding is a mixture of different ingredients such as rice, milk, and sugar. The components of the rice pudding are not uniformly distributed, making it a heterogeneous mixture.

(b) Seawater

Seawater is a mixture of water, salts, and other dissolved substances. These components are evenly distributed at a molecular level, making seawater a homogeneous mixture (solution).

(c) Magnesium

Magnesium is a metallic chemical element and is composed of only magnesium atoms, making it a pure substance.

(d) Crushed ice

Crushed ice is a solid form of water, which is composed of water molecules and has a consistent structure and properties throughout the sample, making it a pure substance.

Key concepts

Pure Substances

In chemistry, understanding pure substances is essential when distinguishing forms of matter. A pure substance is characterized by having a constant composition and consistent properties throughout the entire sample. This means that it contains only one kind of particle, be it atoms, ions, or molecules.

For example, magnesium is a pure substance because it consists solely of magnesium atoms. Pure substances can come in two main forms: elements and compounds.

• Elements are the simplest form of pure substances, like oxygen or carbon, which cannot be broken down into simpler substances by chemical means.
• Compounds are substances that contain two or more elements chemically combined in fixed ratios, such as water (H₂O). The combination is so complete that the compound has properties different from the individual elements.

Pure substances are crucial in chemistry because they provide a benchmark for determining the composition of mixtures, and their consistent nature makes them ideal for studying and predicting chemical reactions.

Mixtures

Mixtures are combinations of two or more pure substances where each retains its own chemical identity. Unlike pure substances, mixtures do not have a fixed composition. This means you can find variable ratios of component substances within them.

A few examples can help to clarify this concept:

• Rice pudding is a mixture of ingredients like rice, milk, and sugar, each maintaining its unique properties within the dish.
• Seawater is a mixture containing water, salts, and other dissolved elements.

Notably, mixtures can be separated into their component substances through physical methods such as filtration or evaporation. The substances in a mixture can be combined in any ratio, illustrating the flexibility of mixtures compared to the rigidity of pure substances. This property makes mixtures very common in natural and artificial contexts.

Homogeneous and Heterogeneous Mixtures

Mixtures can be further classified into homogeneous and heterogeneous types, depending on the uniformity of their composition.

Homogeneous mixtures, also known as solutions, have components that are uniformly distributed at a molecular level, so you cannot distinguish one part of the mixture from another. A prime example is seawater, where the salt is evenly distributed such that any sample from the mixture will have the same composition.

• These mixtures appear uniform and consistent, meaning you cannot see individual components, even under a microscope.

On the other hand, heterogeneous mixtures have components that are not uniformly distributed. They include mixtures like rice pudding, where distinct parts can be seen with the naked eye, such as clumps of rice separate from the milk and sugar.

These two types of mixtures showcase the diverse ways in which substances can combine, from thoroughly blended solutions to mixtures with noticeable differences across samples.