【Science】Chemical Changes and Mass of Matter 〜Understanding the Rules of Weight!〜
Hello everyone! In our previous lessons, we learned about "chemical changes," where substances transform into entirely new ones. Today, we’re going to tackle a mystery: "How does the weight (mass) of substances change when a chemical change occurs?"
Some of you might feel that "the weight seems to change during an experiment..." but, in fact, there is an absolute rule hidden behind it. While calculation problems might pop up, once you grasp the trick, solving them becomes as fun as a puzzle! Let’s do our best together.
1. The Law of Conservation of Mass
How does the total mass of the substances involved in a reaction change before and after a chemical change?
★ Rule: The total weight never changes before and after the reaction!
Even if a chemical change occurs, the total mass of the substances before the reaction is always equal to the total mass of the substances after the reaction. This is known as the Law of Conservation of Mass.
It was discovered in 1789 by the French scientist Antoine Lavoisier.
Why doesn't the weight change?
Because a chemical change is simply a rearrangement of the atoms that were already there. Atoms themselves never suddenly disappear or magically appear out of thin air.
Think of it like LEGO bricks: whether you build a castle or break it down to build a car, the number of bricks used (the total weight) remains exactly the same!
Warning! When it feels like "the weight decreased"
In some experiments, it may look like the weight decreased or increased after the reaction. There is a reason for this:
- When gas is produced: If you experiment in an open container, the gas produced escapes into the air, making it seem lighter by that amount.
- When substances combine with things in the air: When copper or magnesium is heated and combines with oxygen from the air, it looks heavier by the weight of the oxygen added.
【Point】 If you account for the escaping gas or the newly added oxygen in your calculations, the weight will always be perfectly "equal"!
Summary: The types and numbers of atoms do not change before and after a chemical change. Therefore, the total mass does not change either!
2. The Mass Ratio of Reacting Substances
The Law of Conservation of Mass is about the "total," but next, let's look at the "proportions" in which substances react. In fact, the ratio of weights at which substances react is always constant.
① Reaction between Copper and Oxygen
When copper is heated to create copper oxide, copper and oxygen combine in the following ratio:
Copper : Oxygen = \(4 : 1\)
(The resulting copper oxide has a ratio of \(4 + 1 = 5\))
② Reaction between Magnesium and Oxygen
When magnesium is heated to create magnesium oxide, the ratio is as follows:
Magnesium : Oxygen = \(3 : 2\)
(The resulting magnesium oxide has a ratio of \(3 + 2 = 5\))
💡 Memory Tricks (Trivia)
Some people use puns to remember them, like "Copper is \(4:1\) ('yoi' copper)" or "Magnesium is \(3:2\) ('sunny' magnesium)"!
【Common Mistake】
It is easy to mistakenly think that "magnesium reacts with more oxygen," but be sure to look at the ratios carefully. If you have equal weights of these metals, magnesium (at \(3:2\)) takes in more oxygen than copper does.
3. Steps for Mass Calculations
Here is how to solve common test problems like, "If you completely oxidize \(4.0g\) of copper, how many grams of oxygen will combine with it?"
Calculation Steps
- Check the ratio: For copper and oxygen, it is \(4 : 1\).
- Create a proportion: Let the unknown mass of oxygen be \(x\).
\(4 (\text{copper}) : 1 (\text{oxygen}) = 4.0g (\text{actual copper}) : x (\text{unknown oxygen})\) - Solve the equation: Multiply the inner terms and the outer terms.
\(4 \times x = 1 \times 4.0\)
\(4x = 4.0\)
\(x = 1.0g\)
The answer is \(1.0g\)!
Reading Graphs
In tests, you might see graphs where the horizontal axis is "mass of metal" and the vertical axis is "mass of oxygen combined" or "mass of oxide produced." When you see these, look for the spots where the line passes perfectly through the grid intersections, and use those to determine the ratio (like \(4:1\) or \(3:2\)).
Summary: The mass ratio of reacting substances is fixed. Use proportions to calculate them!
4. Final Note: Study Advice
You might feel like the calculations are difficult at first, but don't worry!
The basic foundation is just knowing the numbers: "Copper is \(4:1\), Magnesium is \(3:2\)." Once you have those memorized, the rest is just simple math using proportions.
Review of Today's Key Points:
1. The total weight before and after a chemical change does not change (Law of Conservation of Mass).
2. In open containers, the weight might appear to change due to gas moving in or out.
3. The ratio at which metals and oxygen combine is fixed (Law of Definite Proportions *in junior high, the ratio is more important than the name).
As you solve more problems, your hands will naturally learn what to do. Try starting with the example problems in your textbook! I'm rooting for you!