Compound Examples in Chemistry

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Compound.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Chemistry.

Concept Recap

A pure substance composed of two or more different elements chemically bonded together in a fixed ratio, whose properties differ entirely from those of its component elements.

Elements joined together to make something new with different properties.

Read the full concept explanation →

How to Use These Examples

  • Read the first worked example with the solution open so the structure is clear.
  • Try the practice problems before revealing each solution.
  • Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: Compound asks what the sample is, what property is being used, and whether a new substance is formed.

Common stuck point: Students often know a formula related to compound but skip the recognition step: Am I classifying matter or using properties, state, particle behavior, or mixture evidence to describe a sample? That leads to a correct-looking substitution attached to the wrong chemical model.

Sense of Study hint: Ask: Am I classifying matter or using properties, state, particle behavior, or mixture evidence to describe a sample?

Worked Examples

Example 1

easy
Define a compound and explain how it differs from both an element and a mixture.

Answer

Compound: fixed ratio, chemically bonded, two or more elements\text{Compound: fixed ratio, chemically bonded, two or more elements}

First step

1
A compound is a pure substance composed of two or more different elements chemically bonded in a fixed ratio.

Full solution

  1. 2
    Unlike an element, a compound contains more than one type of atom. Unlike a mixture, a compound has a definite composition that cannot vary.
  2. 3
    Compounds can only be separated into their elements by chemical means (e.g., electrolysis), not by physical means like filtering or distilling.
The law of definite proportions states that a compound always contains the same elements in the same proportions by mass, regardless of the source or method of preparation.

Example 2

medium
Water (H2O\text{H}_2\text{O}) is a compound that can be decomposed by electrolysis. Write the equation for this decomposition and explain why the products have different properties from water.

Example 3

hard
Suggest a single experimental test that distinguishes a compound from a mixture.

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

medium
Classify each as an element, compound, or mixture: (a) table salt (NaCl), (b) oxygen gas (O2\text{O}_2), (c) bronze (Cu/Sn alloy), (d) carbon dioxide (CO2\text{CO}_2).

Example 2

hard
Hydrogen peroxide (H2O2\text{H}_2\text{O}_2) and water (H2O\text{H}_2\text{O}) both contain hydrogen and oxygen. Explain why they are different compounds with different properties, even though they contain the same elements.

Example 3

easy
Is water (H2O\text{H}_2\text{O}) an element, a compound, or a mixture?

Example 4

easy
How does a compound differ from a mixture in how it is separated?

Example 5

easy
Table salt (NaCl\text{NaCl}) is safe to eat, though made from a reactive metal and a toxic gas. What does this show about compounds?

Example 6

easy
Does a compound have a fixed or variable ratio of elements?

Example 7

easy
Is CO2\text{CO}_2 an element or a compound?

Example 8

easy
Is O2\text{O}_2 a compound? Explain in one line.

Example 9

easy
Name the compound formed from sodium and chlorine.

Example 10

easy
Is rust (iron oxide, Fe2O3\text{Fe}_2\text{O}_3) a compound?

Example 11

medium
Air is mostly nitrogen and oxygen. Is air a compound or a mixture? Justify.

Example 12

medium
Hydrogen and oxygen gases are mixed but not reacted. Compound or mixture? Then they are ignited. Now what?

Example 13

medium
Why does H2O\text{H}_2\text{O} behave nothing like hydrogen gas or oxygen gas?

Example 14

medium
A sample is 11% hydrogen and 89% oxygen by mass, always, no matter the source. Compound or mixture?

Example 15

medium
Classify each: gold (Au), brass, CO2\text{CO}_2. (element, mixture, compound)

Example 16

medium
Carbon and oxygen can form CO\text{CO} or CO2\text{CO}_2. What principle explains two different compounds from the same elements?

Example 17

medium
Why is steel (iron + carbon) called a mixture rather than a compound?

Example 18

medium
Sand mixed with iron filings versus iron(II) sulfide (FeS): which is a mixture and which a compound?

Example 19

medium
Calcium oxide (CaO) always contains the same mass ratio of calcium to oxygen. What law does this illustrate, and what does it confirm about CaO?

Example 20

challenge
A 100 g compound is decomposed into 40 g of element X and 60 g of element Y. Another sample of the same compound is 25 g total. Predict the mass of X.

Example 21

challenge
You have a uniform shiny solid. Tests show fixed melting point and it cannot be separated physically, but electrolysis splits it into two elements. Element, compound, or mixture?

Example 22

challenge
Decompose 18 g of water fully. Given it is 2 g H to 16 g O ratio per 18 g, find the mass of oxygen produced and explain why the ratio is fixed.

Example 23

easy
Is methane (CH4_4) an element, compound, or mixture?

Example 24

easy
Classify: ammonia gas (NH3_3).

Example 25

easy
Classify: a glass of seawater.

Example 26

easy
True or false: a compound can be separated into its elements by simple filtering.

Example 27

easy
How many elements are in glucose (C6_6H12_{12}O6_6)?

Example 28

medium
Hydrogen and oxygen react to form water in a fixed mass ratio of 1:8 (H:O). What law does this support?

Example 29

medium
Classify each: (a) diamond, (b) carbon monoxide, (c) saltwater, (d) sugar.

Example 30

medium
What evidence shows that water has different properties from hydrogen and oxygen separately?

Example 31

medium
Bronze is an alloy of copper and tin. Is bronze a compound? Justify.

Example 32

medium
How is CO different from CO2_2, and what does the existence of both show?

Example 33

medium
Is brass (copper + zinc) a compound? Why or why not?

Example 34

medium
What is the formula for the compound made of one calcium ion (Ca2+^{2+}) and chloride ions (Cl−^-)?

Example 35

medium
Sugar dissolved in water and table salt dissolved in water look the same. Are both mixtures, both compounds, or different?

Example 36

medium
Write the formula of the compound containing Al3+^{3+} and O2−^{2-}.

Example 37

hard
A 50 g sample of a binary compound contains 40 g of element A and 10 g of element B. Another 100 g sample of the same compound must contain how much of A?

Example 38

hard
A pure white solid melts at exactly 801°C and is broken by electrolysis into a silvery metal and a green-yellow gas. Classify the solid.

Example 39

hard
A 36 g sample of water is electrolyzed. How many grams of hydrogen and how many of oxygen do you get? (Use the 1:8 H:O mass ratio.)

Example 40

hard
Glucose (C6_6H12_{12}O6_6) and formaldehyde (CH2_2O) have the same C:H:O atom ratio (1:2:1). Are they the same compound? Justify.

Example 41

hard
Two compounds of nitrogen and oxygen are NO and NO2_2. For a fixed mass of N (14 g), the masses of O are 16 g and 32 g. What ratio do these illustrate?

Example 42

hard
A compound is 75% C and 25% H by mass. Find its empirical formula (use atomic masses C = 12, H = 1).

Example 43

hard
Why is the formula H2_2O2_2 different from H2_2O even though both have only H and O?

Example 44

challenge
A 7.0 g sample of a metal M reacts with oxygen to form 9.0 g of an oxide MO. What is the atomic mass of M?
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Background Knowledge

These ideas may be useful before you work through the harder examples.

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