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3.5: Pure Substances and Mixtures

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Learning Objective

• Distinguish betwixt pure substances and mixtures
• Identify mixtures every bit homogeneous or heterogeneous

Pure Substances

When we speak of a pure substance, nosotros are speaking of something that contains only one kind of matter. This can either be one single chemical element or one unmarried compound, but every sample of this substance that you examine must contain exactly the same affair with a fixed, definite set of backdrop.

Pure Substance	Element or Chemical compound?	Consists of:
Lead (Pb)	chemical element	lead atoms
Oxygen gas (O2)	chemical element	oxygen molecules*
Water (H2O)	chemical compound	h2o molecules
Ammonia (NH3)	compound	ammonia molecules

*Note: pure oxygen gas consists of molecules simply it is yet considered an element, rather than a compound, as the molecules are made up of a unmarried type of element. Compounds are fabricated up of one or more element.

Mixtures

If nosotros take 2 or more pure substances and mix them together, we refer to this as a mixture. Mixtures tin always be separated again into component pure substances, considering bonding among the atoms of the constituent substances does not occur in a mixture. Whereas a compound may have very different properties from the elements that etch it, in mixtures the substances keep their individual properties. For instance sodium is a soft shiny metal and chlorine is a pungent greenish gas. These two elements can combine to form the compound, sodium chloride (tabular array salt) which is a white, crystalline solid having none of the properties of either sodium or chlorine. If, however, you mixed tabular array salt with ground pepper, you lot would still exist able to see the individual grains of each of them and, if yous were patient, you could take tweezers and advisedly separate them back into pure common salt and pure pepper.

Heterogeneous mixture

A heterogeneous mixture is a mixture in which the limerick is not uniform throughout the mixture. Vegetable soup is a heterogeneous mixture. Any given spoonful of soup volition contain varying amounts of the different vegetables and other components of the soup.

Homogeneous mixture/ Solution

A homogeneous mixture is combination of two or more substances that are so intimately mixed that the mixture behaves as a unmarried substance. Another word for a homogeneous mixture is solution. Thus, a combination of salt and steel wool is a heterogeneous mixture because it is easy to see which particles of the matter are salt crystals and which are steel wool. On the other paw, if you lot have salt crystals and deliquesce them in water, information technology is very hard to tell that yous take more than one substance present only by looking—fifty-fifty if y'all use a powerful microscope. The salt dissolved in h2o is a homogeneous mixture, or a solution (Figure \(\PageIndex{3}\)).

Figure \(\PageIndex{three}\): Types of Mixtures © Thinkstock On the left, the combination of 2 substances is a heterogeneous mixture because the particles of the 2 components expect different. On the right, the salt crystals have dissolved in the water and so finely that you cannot tell that common salt is nowadays. The homogeneous mixture appears like a single substance.

Example \(\PageIndex{3}\)

Place the following combinations every bit heterogeneous mixtures or homogenous mixtures.

• soda water (Carbon dioxide is dissolved in water.)
• a mixture of iron metallic filings and sulfur pulverization (Both fe and sulfur are elements.)

Effigy \(\PageIndex{4}\): A mixture of iron filings and sulfur pulverisation  (Asoult, Fe-S mixture 03, CC BY 4.0)

Solution

1. Because carbon dioxide is dissolved in water, we tin can infer from the behavior of common salt crystals dissolved in water that carbon dioxide dissolved in water is (also) a homogeneous mixture.
2. Assuming that the iron and sulfur are simply mixed together, it should be easy to see what is iron and what is sulfur, so this is a heterogeneous mixture.

Practice \(\PageIndex{3}\)

Are the following combinations homogeneous mixtures or heterogeneous mixtures?

1. the homo body
2. an amalgam, a combination of some other metals dissolved in a modest amount of mercury

Answers

1. heterogeneous mixture
2. homogeneous mixture

Categorizing Matter

Figure \(\PageIndex{1}\): Relationships betwixt the Types of Matter and the Methods Used to Carve up Mixtures

Ordinary common salt is called sodium chloride. It is considered a substance because it has a compatible and definite composition. All samples of sodium chloride are chemically identical. Water is also a pure substance. Salt hands dissolves in water, merely table salt h2o cannot be classified as a substance because its composition can vary. You may dissolve a small amount of common salt or a big amount into a given corporeality of water. A mixture is a physical blend of 2 or more components, each of which retains its own identity and backdrop in the mixture. Only the form of the table salt is inverse when it is dissolved into water. Information technology retains its composition and properties.

Phase

A phase is any part of a sample that has a uniform composition and properties. By definition, a pure substance or a homogeneous mixture consists of a unmarried stage. A heterogeneous mixture consists of two or more phases. When oil and water are combined, they exercise not mix evenly, but instead form 2 split up layers. Each of the layers is chosen a phase.

Example \(\PageIndex{1}\)

Identify each substance as a chemical compound, an element, a heterogeneous mixture, or a homogeneous mixture (solution).

1. filtered tea
2. freshly squeezed orange juice
3. a compact disc
4. aluminum oxide, a white pulverization that contains a 2:3 ratio of aluminum and oxygen atoms
5. selenium

Given: a chemical substance

Asked for: its classification

Strategy:

1. Decide whether a substance is chemically pure. If it is pure, the substance is either an chemical element or a compound. If a substance tin be separated into its elements, it is a compound.
2. If a substance is not chemically pure, it is either a heterogeneous mixture or a homogeneous mixture. If its composition is uniform throughout, it is a homogeneous mixture.

Solution:

1. A) Tea is a solution of compounds in h2o, so it is not chemically pure. It is usually separated from tea leaves by filtration.
   B) Because the composition of the solution is uniform throughout, it is a homogeneous mixture .
2. A) Orangish juice contains particles of solid (pulp) besides equally liquid; it is not chemically pure.
   B) Considering its composition is not uniform throughout, orange juice is a heterogeneous mixture .
3. A) A compact disc is a solid fabric that contains more than one element, with regions of unlike compositions visible along its edge. Hence a compact disc is not chemically pure.
   B) The regions of different limerick indicate that a compact disc is a heterogeneous mixture.
4. A) Aluminum oxide is a single, chemically pure compound .
5. A) Selenium is ane of the known elements .

Exercise \(\PageIndex{ane}\)

Identify each substance as a chemical compound, an element, a heterogeneous mixture, or a homogeneous mixture (solution).

1. white wine
2. mercury
3. ranch-style salad dressing
4. table sugar (sucrose)

Answer a:

homogeneous mixture (solution)

Answer b:

element

Answer c:

heterogeneous mixture

Respond d:

compound

Example \(\PageIndex{2}\)

How would a pharmacist categorize each example of matter?

1. saltwater
2. soil
3. water
4. oxygen

SOLUTION

1. Saltwater acts as if it were a single substance fifty-fifty though it contains two substances—salt and h2o. Saltwater is a homogeneous mixture, or a solution.
2. Soil is composed of small pieces of a multifariousness of materials, so it is a heterogeneous mixture.
3. H2o is a substance; more specifically, considering water is composed of hydrogen and oxygen, information technology is a compound.
4. Oxygen, a substance, is an element.

Exercise \(\PageIndex{2}\)

How would a chemist categorize each case of matter?

1. coffee
2. hydrogen
3. an egg

Answer a:

a homogeneous mixture (solution), assume it's filtered coffee

Answer b:

element

Respond c:

heterogeneous mixture.

Summary

Matter tin can exist classified into two broad categories: pure substances and mixtures. A pure substance is a form of affair that has a constant limerick and backdrop that are constant throughout the sample. Mixtures are physical combinations of two or more elements and/or compounds. Mixtures can exist classified as homogeneous or heterogeneous. Elements and compounds are both examples of pure substances. Compounds are substances that are fabricated up of more than one type of atom. Elements are the simplest substances made up of only 1 type of atom.

Fundamental Takeaways

• Pure substances are composed of a single element or compounds.
• Combinations of different substances are called mixtures.
• Homogeneous mixtures are mixtures of two or more than compounds (or elements) that are not visually distinguishable from each other.
• Heterogeneous mixtures are mixtures of two or more than compounds (or elements) that are visually distinguishable from one another.

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Source: https://chem.libretexts.org/Courses/Los_Angeles_Trade_Technical_College/Foundations_of_Introductory_College_Chemistry/04:_Matter/3.5:_Pure_Substances_and_Mixtures

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