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The Chemistry of Water

How Hydrogen and Oxygen Combine to Form Water

            📚 Science
                            🎓 Grade 6, 7, 8
                                        ⏱️ 35 minutes
                    

Learning Objectives

Describe the molecular structure of hydrogen gas ( $\ce{H2}$ ), oxygen gas ( $\ce{O2}$ ), and water ( $\ce{H2O}$ )
Explain how atoms rearrange during a chemical reaction to form new substances
Balance the chemical equation for water formation and demonstrate conservation of matter
Identify evidence that a chemical reaction has occurred

            Progress
            8 sections
        

1
Introduction
                            ~2 minutes                        

The Chemistry of Water

Water is essential for all life on Earth. But have you ever wondered what water is actually made of? In this lesson, we'll explore how two invisible gases,hydrogen and oxygen,combine to create the water we drink, swim in, and depend on every day.

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                Essential Question            

How do atoms from different elements combine to form new substances with different properties?

2
Understanding Molecules
                            ~6 minutes                        

Building Blocks: Atoms and Molecules

All matter is made of atoms,tiny particles far too small to see. An element is a substance made of only one type of atom. Hydrogen and oxygen are both elements.

When atoms bond together, they form molecules. Some elements exist naturally as molecules made of two identical atoms bonded together. We call these diatomic molecules.

Molecular Hydrogen ( $\ce{H2}$ )

Hydrogen is the lightest and most abundant element in the universe. On Earth, hydrogen gas doesn't exist as single atoms,instead, two hydrogen atoms bond together to form a hydrogen molecule, written as $\ce{H2}$ .

The small "2" (subscript) tells us there are 2 hydrogen atoms in each molecule.

Molecular Hydrogen (H₂)

                HHbondH₂Hydrogen Molecule (2 atoms)            
Diagram showing a hydrogen molecule (H2) consisting of two hydrogen atoms bonded together. Each hydrogen atom is represented as a light blue circle with the letter H inside. A horizontal line between ...

Molecular Oxygen ( $\ce{O2}$ )

Oxygen makes up about 21% of Earth's atmosphere. Like hydrogen, oxygen gas exists as diatomic molecules. Two oxygen atoms bond together to form $\ce{O2}$ ,the oxygen we breathe.

Oxygen atoms are larger than hydrogen atoms and form a stronger double bond with each other.

Molecular Oxygen (O₂)

                Molecular Oxygen (O₂)Two oxygen atoms bonded together with a double bond forming a diatomic moleculeOOO₂Oxygen Molecule (2 atoms, double bond)            
Diagram showing an oxygen molecule (O2) consisting of two oxygen atoms bonded together. Each oxygen atom is represented as a red circle, larger than the hydrogen atoms shown previously, with the lette...

3
The Water Molecule
                            ~4 minutes                        

The Water Molecule ( $\ce{H2O}$ )

Water is a compound,a substance made of two or more different elements chemically bonded together. Each water molecule contains:
• 2 hydrogen atoms
• 1 oxygen atom

This is why we write water as $\ce{H2O}$ . The oxygen atom is in the center, with two hydrogen atoms attached at an angle.

The Water Molecule (H₂O)

                Water Molecule (H₂O)A water molecule showing one oxygen atom bonded to two hydrogen atoms in a bent shape at approximately 104.5 degreesOHH≈104.5°H₂OWater Molecule (2 H atoms + 1 O atom = bent shape)            
Diagram showing a water molecule (H2O) with its characteristic bent shape. A larger red circle in the center represents the oxygen atom, labeled with O. Two smaller light blue circles represent hydrog...

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                Why Is Water Bent?            

Water molecules aren't straight,they have a bent shape (about 104.5°). This bent shape is what gives water many of its special properties, like being able to dissolve so many substances and having a high surface tension.

4
Check Understanding: Molecules
                            ~5 minutes                        

Question 1

What does the "2" in $\ce{H2}$ tell us about a hydrogen molecule?

Hydrogen weighs 2 grams
There are 2 hydrogen atoms bonded together
Hydrogen has 2 protons
There are 2 hydrogen molecules

Question 2

Both hydrogen gas ( $\ce{H2}$ ) and oxygen gas ( $\ce{O2}$ ) are examples of:

Compounds
Diatomic molecules
Single atoms
Mixtures

Question 3

A water molecule ( $\ce{H2O}$ ) contains ______ hydrogen atom(s) and ______ oxygen atom(s).

Question 4

Water molecules have a straight, linear shape.

Question 5

Match each chemical formula with its description:

$\ce{H2}$

$\ce{O2}$

$\ce{H2O}$

5
The Chemical Reaction
                            ~8 minutes                        

The Chemical Reaction

When hydrogen gas and oxygen gas are combined and energy is added (like a spark), they react to form water. This is a chemical reaction,the atoms rearrange to form new substances.

But here's the key: we can't just combine one $\ce{H2}$ with one $\ce{O2}$ . Let's see why.

The Unbalanced Equation Problem

                Unbalanced EquationShows why H₂ plus O₂ cannot simply equal H₂O - the oxygen atoms don't balance✗ This Doesn't Work!REACTANTSHHH₂+OOO₂→PRODUCTSOHHH₂OAtom Count CheckReactants:2 H atoms2 O atoms≠Products:2 H atoms ✓1 O atom ✗            
Diagram illustrating why the simple equation H2 + O2 → H2O doesn't work. On the left side (reactants), one H2 molecule shows 2 hydrogen atoms, and one O2 molecule shows 2 oxygen atoms. An arrow points...

Conservation of Matter

In a chemical reaction, atoms are never created or destroyed,they only rearrange. This is the Law of Conservation of Matter.

In the unbalanced equation above, we start with 2 oxygen atoms but end with only 1. That oxygen atom can't just disappear! We need to balance the equation.

Balancing the Equation

To balance the equation, we need the same number of each type of atom on both sides. Here's how:

1. Start with what we have: $\ce{H2 + O2 -> H2O}$ 2. We have 2 oxygen atoms on the left, but only 1 on the right 3. If we make 2 water molecules ( $\ce{2H2O}$ ), we use both oxygen atoms 4. But now we need 4 hydrogen atoms on the left ( $\ce{2H2}$ )

The balanced equation is: $\ce{2H2 + O2 -> 2H2O}$

The Balanced Chemical Equation

                Balanced Chemical EquationThe balanced equation 2H₂ + O₂ → 2H₂O showing conservation of matter with 4 hydrogen and 2 oxygen atoms on each side✓ Balanced EquationREACTANTSHHHH2H₂+OOO₂→PRODUCTSOHHOHH2H₂O2H₂ + O₂ → 2H₂O✓ Atom Count,Perfectly Balanced!Reactants (Left)H atoms: 2 + 2 = 4O atoms: 2Products (Right)H atoms: 2 + 2 = 4 ✓O atoms: 1 + 1 = 2 ✓            
Diagram showing the balanced equation for water formation: 2H2 + O2 → 2H2O. On the left side (reactants), two H2 molecules are shown (4 hydrogen atoms total as light blue circles) plus one O2 molecule...

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                Reading the Equation            

$\ce{2H2 + O2 -> 2H2O}$ reads as: "Two molecules of hydrogen gas plus one molecule of oxygen gas yields two molecules of water." The arrow means "yields" or "produces."

6
Check Understanding: Balancing
                            ~6 minutes                        

Question 6

In a chemical reaction, what happens to the atoms?

They are destroyed and new ones are created
They rearrange to form new substances
They disappear completely
They change into different elements

Question 7

Water ( $\ce{H2O}$ ) is classified as a:

Element
Atom
Compound
Diatomic molecule

Question 8

The arrow (→) in a chemical equation means "______" or "produces."

Question 9

Put these events in order for the formation of water:

                        ⋮⋮
                        Hydrogen and oxygen gases are present
                    
                        ⋮⋮
                        Water molecules are produced and energy is released
                    
                        ⋮⋮
                        Atoms rearrange and bonds form
                    
                        ⋮⋮
                        Energy (spark) is added to start the reaction

Drag items to reorder, then confirm

Question 10

What is the balanced equation for water formation?

$\ce{H2 + O2 -> H2O}$
$\ce{H2 + O -> H2O}$
$\ce{2H2 + O2 -> 2H2O}$
$\ce{H2 + O2 -> 2H2O}$

Question 11

In the equation $\ce{2H2 + O2 -> 2H2O}$ , there are ______ hydrogen atoms and ______ oxygen atoms on each side.

Question 12

In a balanced chemical equation, the number of each type of atom must be the same on both sides.

Question 13

Why can't we write the equation as $\ce{H2 + O2 -> H2O}$ (without the 2s in front)?

It would be too simple
The oxygen atoms would not balance,we'd lose an oxygen atom
Hydrogen doesn't react with oxygen
Water must always be written as 2H₂O

7
Energy and Evidence
                            ~5 minutes                        

Energy and the Reaction

The reaction between hydrogen and oxygen is exothermic,it releases energy in the form of heat and light. This is why hydrogen is being explored as a clean fuel source!

However, the reaction needs a small amount of energy to get started (activation energy). Once started, it releases much more energy than it took to begin.

Energy in the Reaction

                Energy DiagramShows that water formation is exothermic, releasing energy as the products are at a lower energy level than the reactantsEnergy in the ReactionEnergy →Reaction Progress →2H₂ + O₂2H₂OActivation Energy(spark needed)↓Energy Released(heat + light)EXOTHERMICProducts lower energy= energy released            
Energy diagram showing the exothermic nature of water formation. A curved line shows energy level starting at the reactants (2H2 + O2) at a medium height, rising to a peak labeled 'Activation Energy (...

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                Hydrogen Safety            

Because hydrogen burns so readily with oxygen, hydrogen gas must be handled carefully. The famous Hindenburg airship disaster in 1937 occurred when the hydrogen gas that kept it afloat ignited.

Evidence of Chemical Reaction

How do we know a chemical reaction has occurred? Look for these signs:

- Energy change: Heat or light is released (or absorbed)
• New substance formed: Water has completely different properties than hydrogen or oxygen gases
• Difficult to reverse: You can't easily separate water back into hydrogen and oxygen
• Definite proportions: Water always contains hydrogen and oxygen in a 2:1 ratio

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                Different Properties            

Hydrogen is a flammable gas. Oxygen supports combustion. But water,made from both,puts out fires! This dramatic change in properties is clear evidence that a new substance has been created through a chemical reaction.

Summary

- Hydrogen gas ( $\ce{H2}$ ) and oxygen gas ( $\ce{O2}$ ) are both diatomic molecules
• They combine in a chemical reaction to form water ( $\ce{H2O}$ )
• The balanced equation is: $\ce{2H2 + O2 -> 2H2O}$
• Matter is conserved: 4 H atoms and 2 O atoms on both sides
• The reaction is exothermic,it releases energy
• Water has different properties than the elements that form it

8
Final Assessment
                            ~5 minutes                        

Question 14

The reaction between hydrogen and oxygen releases energy as heat and light. This type of reaction is called:

Endothermic
Exothermic
Isothermic
Hypothermic

Question 15

Which of the following are evidence that a chemical reaction has occurred when hydrogen and oxygen form water? (Select ALL that apply)

Energy is released as heat and light
A new substance with different properties is formed
The atoms change into different elements
The reaction is difficult to reverse
The elements combine in definite proportions

Select all that apply.

Question 16

Explain why water can put out fires even though it's made from hydrogen (which is flammable) and oxygen (which supports burning).

Expected length: 30-100 words

Your Score

Learning Objectives

Introduction

The Chemistry of Water

Understanding Molecules

Building Blocks: Atoms and Molecules

Molecular Hydrogen ($\ce{H2}$)

Molecular Oxygen ($\ce{O2}$)

The Water Molecule

The Water Molecule ($\ce{H2O}$)

Check Understanding: Molecules

The Chemical Reaction

The Chemical Reaction

Conservation of Matter

Balancing the Equation

Check Understanding: Balancing

Energy and Evidence

Energy and the Reaction

Evidence of Chemical Reaction

Summary

Final Assessment

Molecular Hydrogen ( $\ce{H2}$ )

Molecular Oxygen ( $\ce{O2}$ )

The Water Molecule ( $\ce{H2O}$ )