Description
Students learn how molecular hydrogen ($\ce{H2}$) and molecular oxygen ($\ce{O2}$) combine in a chemical reaction to form water ($\ce{H2O}$), exploring molecular structure, chemical equations, and conservation of matter.
Learning Objectives
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Describe the molecular structure of hydrogen gas ($\ce{H2}$), oxygen gas ($\ce{O2}$), and water ($\ce{H2O}$)
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Explain how atoms rearrange during a chemical reaction to form new substances
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Balance the chemical equation for water formation and demonstrate conservation of matter
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Identify evidence that a chemical reaction has occurred
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# 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.
How do atoms from different elements combine to form new substances with different properties?
## 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 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.
## 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.
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.
## 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.
## 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: $\ce{2H2 + O2 -> 2H2O}$
$\ce{2H2 + O2 -> 2H2O}$ reads as: "Two molecules of hydrogen gas plus one molecule of oxygen gas yields two molecules of water." The arrow means "yields" or "produces."
## 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.
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
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
Assessment Questions
16 questionsWhat does the "2" in $\ce{H2}$ tell us about a hydrogen molecule?
Both hydrogen gas ($\ce{H2}$) and oxygen gas ($\ce{O2}$) are examples of:
A water molecule ($\ce{H2O}$) contains ______ hydrogen atom(s) and ______ oxygen atom(s).
Water molecules have a straight, linear shape.
Match each chemical formula with its description:
Standards Alignment
Resource Details
- Subject
- Science
- Language
- EN-US
- Author
- East Voyager Academy
- License
- CC-BY-4.0
- PRISM ID
- water-formation-molecular