Question

Classify each of the following as a mixture or a pure substance. a. water b. blood c. the oceans d. iron e. brass \(\mathbf{f}\). uranium g. wine h. leather i. table salt Of the pure substances, which are elements and which are compounds?

Short answer

a. Water: pure substance (compound) b. Blood: mixture c. The oceans: mixture d. Iron: pure substance (element) e. Brass: mixture f. Uranium: pure substance (element) g. Wine: mixture h. Leather: mixture i. Table salt: pure substance (compound) Pure substances: water (compound), iron (element), uranium (element).

Step-by-step solution

a. water

Water is a pure substance, as it has a fixed composition (H2O) and cannot be separated by physical means. It is also a compound, as it is made up of two elements – hydrogen and oxygen – combined in a fixed proportion.

b. blood

Blood is a mixture because it contains various cells, proteins, and other substances that can be separated by physical means like filtration or centrifugation.

c. the oceans

The oceans are mixtures, as they consist of water, dissolved salts, minerals, and various living organisms that can be separated by physical means.

d. iron

Iron is a pure substance, as it has a fixed composition (Fe) and cannot be separated by physical means. It is also an element, as it cannot be broken down into simpler substances by chemical means.

e. brass

Brass is a mixture, specifically an alloy made up of copper and zinc. The proportion of copper and zinc in brass can vary, and they can be separated by physical means like melting and filtration.

f. uranium

Uranium is a pure substance, as it has a fixed composition (U) and cannot be separated by physical means. It is also an element, as it cannot be broken down into simpler substances by chemical means.

g. wine

Wine is a mixture, as it is composed of water, alcohol, sugars, and other compounds that can be separated by physical means like distillation.

h. leather

Leather is a mixture, as it is composed of collagen fibers, water, oils, and other substances that can be separated by physical means.

i. table salt

Table salt (sodium chloride) is a pure substance, as it has a fixed composition (NaCl) and cannot be separated by physical means. It is also a compound, as it is made up of two elements – sodium and chlorine – combined in a fixed proportion.

Summary

The pure substances are water, iron, and uranium. Water is a compound, while iron and uranium are elements. The mixtures include blood, the oceans, brass, wine, and leather.

Key concepts

Elements

Elements are pure substances that consist of only one type of atom. They have a fixed composition and cannot be decomposed into simpler substances by ordinary chemical means. This means you cannot separate them into different substances by physical processes like filtration, distillation, or evaporation.

For example, iron (Fe) is an element. It is made up entirely of iron atoms and has a fixed structure. Similarly, uranium (U) is another example of an element. Because of the uniqueness of each type of atom, elements have specific properties that do not change regardless of the amount present.

Elements can exist in various states - solid, liquid, or gas - depending on their physical and chemical properties. In the periodic table, elements are organized based on their atomic number, which effectively helps in identifying and classifying them systematically.

Compounds

Compounds are also pure substances, but unlike elements, they consist of two or more types of atoms that are chemically bonded together. The chemical bonds mean that the atoms are combined in fixed ratios, creating a substance with unique properties that are distinct from the individual elements that form them.

Water (\( H_2O \)) is a classic example of a compound. In water, two hydrogen atoms and one oxygen atom combine to form a molecule of water. This fixed proportion does not change; otherwise, it would not be water anymore. Another example is table salt, or sodium chloride (\( NaCl \)), which forms when sodium and chlorine atoms bond together chemically.

Compounds have unique properties not seen in their constituent elements. For example, water is a liquid at room temperature, even though hydrogen and oxygen are gaseous elements. Despite being made up of elements, compounds require chemical means to be broken down into simpler substances.

Classification of Substances

Understanding the classification of substances helps in identifying and separating them based on their compositions.

• Pure Substances: These have a fixed composition and distinct properties. They can be elements like iron and uranium or compounds like water and table salt. Pure substances are homogeneous and cannot be separated into other materials by physical means.
• Mixtures: Mixtures consist of two or more substances that are physically combined. Unlike pure substances, the composition of mixtures can vary, and they retain the properties of their individual components. Examples include blood, the ocean, brass, and wine. Mixtures can be homogeneous, where the different substances are evenly distributed, or heterogeneous, where the components can be distinguished from one another.

When classifying substances, it’s vital to understand whether a material is a pure substance (either an element or a compound) or a mixture, as this influences how it can be used and processed in various scientific and industrial applications.