Pure substance or mixture?
Twenty single-part examples

20 single-part examples, questions only

11 multi-part examples

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Example #1: (a) In one lump, you have five grams of silver and, in another lump, five grams of gold. Pure substances or mixtures? (b) You melt the two substances and mix them throughly and allow it to become a solid lump. Pure substance or mixture?

Solution to (a):

Both are pure substances.

Solution to (b):

It is a mixture. When a phrase like 'mixed thoroughly' is used, the resulting mixture is taken to be homogeneous.

Discussion:

But, someone may ask, if silver is a pure substance and gold is a pure substance, isn't the result of the mixing also a pure substance?

The answer, of course, lies in what, exactly, a pure substance is. Pure substances are a sample composed of either one element or one compound. When you mix the silver and gold, you have gold atoms and silver atoms spread throughout the entire sample. Since there are two different atoms (not chemically joined together) spread throughout, it is called a mixture.


Example #2: If you have ever had difficulty swallowing aspirin, you know it has an unpleasant taste. To help a child swallow aspirin, a parent may crush the tablet and add it to apple sauce. Is the aspirin-apple-sauce combination classified as a compound or a mixture? Is it homogeneous or heterogeneous? Explain the reason for your choices.

Solution:

It's a heterogeneous mixture.

The applesauce is, itself, heterogeneous. Anything with visible lumps of stuff in it is heterogeneous.

Now, mix the crushed aspirin into the applesauce. It's still heterogeneous. If you take samples in different places of the mixture, there will be differing amounts of aspirin and applesauce in each sample.

That's heterogeneous.


Example #3: Which of the following would best be described as a heterogeneous mixture?

(a) strawberry milk shake   (d) granite stone
(b) motor oil   (e) liquid nitrogen
(c) diamond   (f) cough syrup

Solution:

The first correct answer is granite stone. The component materials are spread unevenly throughout a sample of the granite. That makes it heterogeneous.

(b) and (f) are homogeneous mixtures.

Strawberry milk shake is the second correct answer. There are small clump of frozen milk shake spread unevenly through the shake. Also, air bubbles are dispersed through the shake.

(e) is a pure substance.

The answer for diamond varies. Some teachers accept the presence of impurities to be sufficient so as to make the diamond be a heterogeneous mixture. Other teachers ignore the impurities (with are always very tiny amounts compared to the carbon) and consider diamond to be a pure substance, with the hetero-/homo- issue rendered moot.


Example #4: Which of the following is not a characteristic of a compound?

(a) It has different properties from the elements that formed it.
(b) It is a pure substance made of two or more different elements.
(c) Samples taken from different parts of a large amount of the compound will have different properties.
(d) It can be represented by a chemical formula.

Solution:

Answer choice (c) is the correct answer.

Example #5: An example of a mixture is:

(a) the air in a room.
(b) all of these.
(c) gold.
(d) hydrogen fluoride.
(e) purified water.

Solution:

Answer choice (a) is the correct answer.

Example #6: An example of a pure substance is:

(a) compound.
(b) carbon dioxide.
(c) elements.
(d) all of these above.
(e) pure water.

Solution:

Answer choice (d) is the correct answer.

Example #7: Water is an example of:

(a) a heterogenous mixture.
(b) a compound.
(c) a homogeneous mixture.
(d) an element.

Solution:

Answer choice (b) is the correct answer.

Example #8: Which of the following statement(s) is false? Why?

(a) Solutions are always homogeneous mixtures.
(b) The terms “atom” and “element” can have different meanings.
(c) Elements can exist as atoms or molecules.
(d) Compounds can exist as atoms or molecules.
(e) At least two of the four above statements are false.

Solution:

Answer choice (d) is false. Compounds only exist as molecules.

Example #9: Why does the dissolved component not settle out of a solution?

Solution:

In a solution, the dissolved component is at the ionic or molecular level. Each particle (ion or molecule) is being constantly (and randomly) hit by molecules of the solvent (the dissolving component) from all sides. The dissolved components do feel the force of gravity, but the force of the collisions from below is sufficient to keep each dissolved component (remember, either an ion or a molecule) from settling out.

By the way, colloids do not settle out under the force of gravity either


Example #10: Which of the following statements is false?

(a) Two or more different substances combine physically to form a mixture.
(b) Two or more different atoms combine chemically to form a molecule.
(c) Two or more different atoms combine chemically to form an element.
(d) Two or more different elements combine chemically to form a compound.

Solution:

The false statement is (c). Two or more different atoms would chemically combine to form a molecule. The key word is 'different.' If two or more of the same element combined chemically, you would have an element. An example of this would be P4, where four phosphorus atoms are chemically combined. However, it is not considered a compound in the same way H2O, for example, is.

Example #11: Which of the following is a pure substance?

colloid
mixture
element
solution

Solution:

Element. There are two categories of pure chemical substances: elements and compounds.

Example #12: Which of the following is a mixture?

steel
water
oxygen
gold

Answer:

Steel. The components that make up steel are all mixed at very high temperatures, such that all the components are in the liquid state. The components are thoroughly mixed and the steel is allowed to cool to the solid state. Water, oxygen and gold are all pure chemical substances.

Example #13: A good definition of a chemical is any substance that:

causes corrosion
has definite composition
is hazardous
is used in industrial processes

Solution:

Has definite composition. Each chemical substance that exists has the same composition everywhere. Water has the formula H2O and it never has a different composition. A different composition (say H2O2) is not water. It is some other chemical substance (hydrogen peroxide in the example given).

Example #14: Matter that is free to move and fills its available volume is in the _____state.

liquid
solid
gaseous
Bose-Einstein

Solution:

gaseous

Example #15: The state of matter in which a material is most likely to resist compression is the _____ state?

solid
liquid
gaseous
plasma

Solution:

solid

Example #16: A chemical change occurs when

Dissolved minerals solidify to form a crystal
Ethanol is purified through distillation
Salt deposits form from evaporated seawater
A leaf changes color

Solution:

A leaf changes color.

The chemical structure of green chlorophyll molecules breaks down, revealing the yellow xanthophylls, orange carotenoids and red anthocyanins that are also present.


Example #17: Which of the following shows a physical change occurring?

A peach spoils
A sterling silver spoon tarnishes
A copper statue turns green
A hot-glue gun melts a glue stick

Solution:

A hot-glue gun melts a glue stick

Example #18: Which of the following is not a matter?

(a) sunlight
(b) copper wire
(c) alcohol
(d) dust

Solution:

sunlight

Example #19: Which has both a definite shape and definite volume?

(a) solid
(b) plasma
(c) gas
(d) liquid

Solution:

solid

Example #20: Which is a pure substance?

(a) element
(b) compound
(c) mixture
(d) both (a) and (b)

Solution:

both (a) and (b)

Bonus Example: An atom is to an element as a _____ is to a compound?

Solution:

The correct answer is molecule. The atom is the smallest unit of an element just as the molecule is the smallest unut of a compound.

11 multi-part examples

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