【Science】Mastering "Matter Around Us" (Grade 7)

Hello! Let’s learn all about the "Matter Around Us" together.
We are surrounded by many things: water, air, metal, plastic, and so much more. By understanding the properties of these materials, the world of science will feel much more familiar and exciting.
If you feel like "science is a bit tricky," don't worry! We'll take it one step at a time, starting with examples from your everyday life.

1. Distinguishing Materials and Their Properties

First, let's start by distinguishing between the two terms: "objects" and "materials."

Difference between "Objects" and "Materials"

  • Object: Referring to something based on its shape or purpose (e.g., a cup, a spoon, a desk).
  • Material: Referring to the "stuff" or substance that makes up the object (e.g., glass, plastic, wood).

Organic and Inorganic Substances

Materials are broadly divided into two groups.

① Organic Substances
Substances that contain carbon (C). When burned, they produce carbon dioxide. A key characteristic is that they turn black and charred like charcoal.
Examples: Sugar, potato starch, plastic, paper, ethanol.

② Inorganic Substances
Substances that are not organic. They do not produce carbon dioxide when burned, nor do they turn into charcoal.
Examples: Salt, iron, copper, glass, water.

★ Pro-tip: How to tell them apart

Try imagining if the substance "turns black and charred" when burned. Sugar will char, but no matter how much you heat salt, it will never turn black.

Metals and Non-metals

Among inorganic substances, metals have four common characteristics (in addition to "metallic luster"):

  • Metallic Luster: Shines brightly when polished.
  • Electrical Conductivity: Electricity passes through them easily.
  • Thermal Conductivity: Heat transfers through them easily.
  • Malleability and Ductility: Can be hammered into thin sheets or stretched into wires.

Watch out!: "Sticking to a magnet" is NOT a common trait for all metals (Iron does, but copper and aluminum do not).

【Chapter 1 Summary】
・Materials that char and produce carbon dioxide when burned are organic.
・Metals share common traits: they shine, conduct electricity/heat, and are flexible (malleable/ductile).


2. Density

Even if two things have the same size (volume), iron and wood have different weights. This "compactness" is called density.

Density Formula

Density is the mass (weight) per \(1\text{cm}^3\) of a substance.
\(Density = \frac{Mass \text{ of the substance} [\text{g}]}{Volume \text{ of the substance} [\text{cm}^3]}\)

★ Memorization Hack: The "Ladybug" Diagram

Use the "M-D-V" triangle method!
Put "M (Mass)" on top, and "D (Density)" and "V (Volume)" on the bottom.
・To find Density: Mass ÷ Volume
・To find Mass: Density × Volume

Common Mistake: Forgetting Units

The unit for density is \(\text{g/cm}^3\). You read it as "grams per cubic centimeter." It’s easy to remember if you think of it as the fraction form written out!

【Chapter 2 Summary】
・Density is constant for each type of substance.
・Objects with a lower density than water (\(1.0\text{g/cm}^3\)) float, and those with a higher density sink.


3. Properties and Collection of Gases

Since gases are invisible, the secret to mastering them is to learn their "personalities" (properties) and "capture methods" (how to collect them) as a set.

How to Collect Gases (3 Methods)

① Water Displacement Method
Used for gases that don't dissolve easily in water. Since you get pure gas this way, use this method whenever possible.
Examples: Oxygen, Hydrogen, Nitrogen.

② Upward Displacement Method
Used for gases that are soluble in water and lighter than air.
Example: Ammonia (just remember this one!).

③ Downward Displacement Method
Used for gases that are soluble in water and heavier than air.
Examples: Carbon dioxide, Hydrogen chloride.

Key Characteristics of Common Gases

  • Oxygen: Helps other things burn.
  • Carbon Dioxide: Turns limewater cloudy. Dissolves slightly in water to create carbonated water.
  • Hydrogen: The lightest gas. When a flame is brought near, it burns with a "pop" sound and produces water.
  • Ammonia: Has a distinct pungent odor. It is extremely soluble in water, and the solution is alkaline.
Fun Fact: The Ammonia Fountain Experiment

Because ammonia is so incredibly soluble in water, you can create a fountain inside a flask. If you add phenolphthalein solution, it reacts with the alkaline property to create a beautiful red fountain!

【Chapter 3 Summary】
・If it doesn't dissolve in water, use the Water Displacement Method.
・Remember the 4-part combo for Ammonia: "Soluble in water, light, alkaline, and smelly!"


4. Properties of Aqueous Solutions

A state where something is dissolved in a liquid is called an "aqueous solution."

Terminology

  • Solute: The substance being dissolved (e.g., salt).
  • Solvent: The liquid that does the dissolving (e.g., water).
  • Solution: The entire mixture of solute dissolved in solvent (e.g., saltwater).

Calculating Concentration

The "how strong is it" percentage is called the mass percentage concentration.
\(Concentration [\%] = \frac{Mass \text{ of solute} [\text{g}]}{Total \text{ mass of solution} [\text{g}]} \times 100\)

Watch out!: The denominator is not just the "weight of the water," but "water + the dissolved solute"! Many people make mistakes here, so double-check your math!

Saturation and Recrystallization

A solution where the maximum amount of a substance has been dissolved is called a saturated solution.
The process of taking a dissolved substance and turning it back into a solid by lowering the temperature or evaporating the water is called recrystallization.

【Chapter 4 Summary】
・In concentration calculations, the denominator is the "Total (water + solute)"!
・Recrystallization uses the difference in how much a substance dissolves at different temperatures to recover a solid.


5. Changes in State

When a substance changes its form between "solid, liquid, and gas" based on temperature, it is called a change of state.

Key Points on Changes of State

  • Mass: Does not change (because the number of particles remains the same).
  • Volume: Changes significantly (Generally increases in order: Solid < Liquid <<< Gas).
  • Exception: Water is special! The solid (ice) has a larger volume than the liquid. That’s why ice floats in water.

Boiling Point and Melting Point

  • Melting Point: The temperature at which a solid turns into a liquid.
  • Boiling Point: The temperature at which a liquid boils and turns into a gas.

A super important characteristic is that for pure substances, the temperature remains constant while changing state. The flat part of the graph represents the temperature during that "transformation."

Distillation

A method to separate and recover components from a liquid mixture by utilizing the difference in boiling points.
Example: When heating a mixture of water and ethanol, ethanol (which has a lower boiling point of about 78°C) turns into a gas first. If you cool this gas back down into a liquid, you can extract concentrated ethanol.

【Chapter 5 Summary】
・Even when the state changes, the weight (mass) stays the same.
・Distillation is used to separate liquids using their different boiling points.


Final Words: Study Advice

You might feel like there’s a lot to memorize at first, but don't worry. Focus on mastering these three areas first: "Organic vs. Inorganic," "How to Collect Gases," and "Density Calculations."
If you keep asking yourself, "Why does this happen?" as you study, science starts to feel as fun as solving a puzzle. I'm rooting for you!