【Science】Grade 8: Chemical Changes and Atoms/Molecules 〜Let’s peek into the invisible world〜
Hello! Let’s start our journey into "Chemical Changes and Atoms/Molecules."
When you hear the word "chemistry," you might think of difficult calculations or complicated symbols and feel like, "Oh, I'm not good at this." But don't worry!
In this chapter, we will learn that everything around us is actually made of "tiny particles." From why baking powder makes a cake rise to what happens when you pass electricity through water, there are so many exciting discoveries to be made through experiments.
Let’s take it one step at a time, just like solving a puzzle!
1. Decomposition: Breaking things down!
The process where one substance splits into two or more different substances is called decomposition. You can decompose substances by applying heat or passing an electric current through them.
① Thermal Decomposition
This is decomposition caused by applying heat.
These are the two experiments that often appear on tests:
- Thermal decomposition of sodium bicarbonate (baking soda):
This is the "baking soda" that makes pancakes fluffy. When heated, it breaks down into three things: sodium carbonate, water, and carbon dioxide.
Key point: "We confirm carbon dioxide was produced because limewater turns cloudy," and "We confirm water was produced because blue cobalt chloride paper turns pink." - Thermal decomposition of silver oxide:
When black silver oxide is heated, it breaks down into shiny silver and oxygen.
Key point: "We confirm oxygen was produced because a glowing splint will burn brightly when inserted."
② Electrolysis
This is decomposition caused by passing an electric current through a substance.
The most famous example is the electrolysis of water. When electricity is passed through water, it splits into hydrogen and oxygen.
(*Since water doesn't conduct electricity very well on its own, we add a little sodium hydroxide to the water for the experiment.)
【Pro Tip!】
In the electrolysis of water, hydrogen and oxygen are produced in a volume ratio of "hydrogen : oxygen = 2 : 1."
Instead of trying to memorize it with wordplay, think of it as "Water = \( H_2O \)", so there are two H (hydrogen) atoms and one O (oxygen) atom. If you visualize it that way, you won’t forget it!
Summary: Decomposition means splitting a substance into substances with different properties!
2. Atoms and Molecules: The tiny particles that make up matter!
A particle that cannot be divided any further is called an atom. This was proposed by a scientist named John Dalton in the 19th century.
① Three Rules of Atoms
- They cannot be created, destroyed, or changed into different types of atoms through chemical changes.
- They cannot be divided into smaller parts.
- Each type of atom has a specific mass and size.
② Molecules
When several atoms combine to form the smallest unit that shows the properties of a substance, it is called a molecule.
For example, a single oxygen atom (O) does not have the properties of "oxygen gas," but when two atoms combine (\( O_2 \)), it becomes the "oxygen" that we breathe.
【Did you know?】
An atom is about 1/100,000,000th of a centimeter in size! If a 1-yen coin were the size of the Earth, an atom would be about the size of a marble. They are incredibly small!
3. Chemical Formulas and Equations: Let's use symbols!
This is a point where many people struggle, but it's actually easy once you learn the rules!
① Element Symbols
These represent atoms using alphabet letters. Let’s start by memorizing these!
・Hydrogen: H ・Oxygen: O ・Carbon: C ・Nitrogen: N
・Silver: Ag ・Copper: Cu ・Iron: Fe ・Magnesium: Mg
② Chemical Formulas
These use element symbols to represent a substance.
・Water: \( H_2O \)
・Carbon dioxide: \( CO_2 \)
・Oxygen: \( O_2 \)
③ Chemical Equations
An equation that uses chemical formulas to represent a chemical change. The golden rule is to "balance the number of atoms" on both the left and right sides.
【Steps: Let’s write the equation for the synthesis of water】
1. Write it in words: Hydrogen + Oxygen → Water
2. Convert to chemical formulas: \( H_2 + O_2 \rightarrow H_2O \)
3. Check the numbers: There are two O atoms on the left, but only one on the right!
4. Double the \( H_2O \) on the right: \( H_2 + O_2 \rightarrow 2H_2O \)
5. Now there are four H atoms on the right, so balance the H on the left: \( 2H_2 + O_2 \rightarrow 2H_2O \) Done!
【Common Mistake!】
You cannot change \( H_2O \) to \( H_2O_2 \) to balance the numbers! \( H_2O_2 \) is a different substance called "hydrogen peroxide." The only thing you are allowed to change is the large number in front of the formula (the coefficient).
4. Oxidation and Reduction: Trading oxygen
① Oxidation
This is when a substance reacts and combines with oxygen. The resulting substance is called an oxide.
・Rapid oxidation: Combustion. It reacts violently, releasing light and heat. (e.g., wood burning, magnesium glowing)
・Slow oxidation: Such as iron rusting.
② Reduction
This is the removal of oxygen from an oxide.
For example, if you mix copper oxide (black) with carbon and heat it, the oxygen moves to the carbon, turning the copper back into its shiny, metallic form.
\( 2CuO + C \rightarrow 2Cu + CO_2 \)
*Think of the carbon "stealing" the oxygen from the copper!
Summary: Adding oxygen is "Oxidation," removing oxygen is "Reduction"!
5. Chemical Changes and Mass: What happens to the weight?
The total mass remains the same before and after a chemical change. This is called the Law of Conservation of Mass.
"Why doesn't the weight change?"
Because even when a chemical change occurs, only the types and combinations of atoms change; the number of atoms themselves stays exactly the same. It’s the same as taking apart a Lego castle to build a car—the total weight of the blocks used remains the same!
【Caution!】
If you heat sodium bicarbonate in an open container, the carbon dioxide escapes, so the mass of what's left behind will be lighter. However, if you add the weight of the gas that escaped, the total mass is the same as when you started!
How was that?
The world of chemistry might look complicated at first, but it actually runs on very simple "atom-shuffling rules."
Start by memorizing a few element symbols. Once you do that, you'll be able to solve chemical equations like puzzles! I'm rooting for you!