Problem 42 - Chemistry The Molecular Science

Question

Classify each of these as an element, a compound, a heterogeneous mixture, or a homogeneous mixture. Explain your choice in each case. (a) Table salt (sodium chloride) (b) Methane (which burns in pure oxygen to form only carbon dioxide and water) (c) Chocolate chip cookie (d) Silicon

Step-by-Step Solution

Answer

(a) Compound, (b) Compound, (c) Heterogeneous mixture, (d) Element.

Step 1: Understanding Table Salt (sodium chloride)

Table salt, also known as sodium chloride, is a chemical compound composed of two elements: sodium (Na) and chlorine (Cl). These elements are chemically bonded in a fixed proportion, making it a compound. In chemical terms, a compound is a pure substance consisting of two or more different elements bonded together in a specific ratio. Therefore, table salt is a compound.

Step 2: Understanding Methane

Methane is a chemical compound with the formula CH₄, consisting of one carbon atom and four hydrogen atoms. It is a pure substance as it has a consistent composition of elements combined in a fixed manner. When methane burns in oxygen, it results in carbon dioxide and water. Hence, methane is classified as a compound due to its specific elemental composition and chemical bonding.

Step 3: Understanding Chocolate Chip Cookie

A chocolate chip cookie is a mixture containing different ingredients such as dough and chocolate chips that retain their individual properties and can be separated physically. These ingredients do not mix uniformly to form a single substance, leading to variations throughout the cookie. Because of these visible differences in composition, a chocolate chip cookie is classified as a heterogeneous mixture.

Step 4: Understanding Silicon

Silicon is a chemical element with the symbol Si and atomic number 14. It is a pure substance made up of just one type of atom. Elements are substances that cannot be broken down into simpler substances by chemical methods. Since silicon consists solely of silicon atoms, it is classified as an element.

Key Concepts

Element

Elements are the simplest form of matter and consist of only one type of atom. They are pure substances that cannot be broken down into simpler substances using chemical reactions. Each element is unique and has distinct properties.
Atomic symbols, like "Si" for silicon, represent these elements on the periodic table. Silicon itself is a perfect example of an element because it contains only silicon atoms, making it pure and consistent throughout.

• Cannot be decomposed further by chemical means
• Consist of one type of atom
• Represented by a unique chemical symbol

Compound

Compounds are substances formed when two or more different kinds of elements chemically bond in fixed proportions. These chemical bonds hold the compound together to display properties distinct from the individual elements.
Table salt, or sodium chloride (NaCl), demonstrates this as it consists of sodium and chlorine in a 1:1 ratio. Similarly, methane (CH₄) is made up of one carbon atom bonded to four hydrogen atoms resulting in a gas with specific flammable properties.

• Composed of different types of atoms in a fixed ratio
• Have distinct chemical properties different from individual elements
• Can be broken into simpler substances through chemical reactions

Heterogeneous Mixture

Heterogeneous mixtures are combinations of substances where the individual components remain distinct and visible. These mixtures are non-uniform, meaning that they do not have the same composition throughout and the components can often be physically separated.
Think of a chocolate chip cookie. It consists of distinguishable parts like chunks of chocolate and dough, each maintaining its own set of properties within the mix.

• Components can be observed separately
• Composition varies in different parts of the mixture
• Can usually be separated physically

Homogeneous Mixture

Unlike heterogeneous mixtures, homogeneous mixtures are uniform in composition throughout. The substances involved are mixed at the molecular level, making it difficult to distinguish between them or separate them physically.
An example of a homogeneous mixture is a simple solution like saltwater, where salt is completely dissolved in water to form a single phase.

• Uniform composition throughout
• Components are not visually distinguishable
• Cannot be separated easily by physical means