GED Science: Matter and Chemical Reactions Concepts
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Overview
This chapter covers what matter is made of, how it changes, and how chemical reactions are written. You do not need to memorize the periodic table, but you should recognize common element names and symbols.
Atoms contain protons (positive) and neutrons (no charge) in the nucleus, with electrons (negative) around it. A physical change, such as melting, boiling, or cutting, changes form but not the substance. A chemical change makes a new substance; signs include gas bubbles, a color change, heat or light, or a new solid.
Density is mass divided by volume. An object floats in a liquid if it is less dense than the liquid, and liquids that do not mix stack with the least dense on top. The volume of an odd-shaped solid can be found by how much it raises the water level.
A balanced chemical equation has the same number of each kind of atom on both sides, because mass is conserved: atoms are rearranged, not created or destroyed. The limiting reactant is the one that runs out first and sets how much product can form. In a solution, the solvent dissolves the solute. Most solids dissolve better in hotter solvents, and a saturated solution holds all the solute it can at that temperature.
Key concepts
Atom
Protons (+) and neutrons (no charge) in the nucleus; electrons (−) around the nucleus.
Memory tipElectron = negative; proton = positive.
Physical and chemical change
Physical: same substance, new form (melting, boiling). Chemical: new substance (rusting, burning).
Memory tipAsk: is it still the same substance?
Density
Density = mass ÷ volume. Less dense than water (1.0 g/cm³) floats.
Memory tipLess dense floats; more dense sinks.
Conservation of mass
In a closed system, total mass before a reaction equals total mass after.
Memory tipGas has mass too.
Balancing equations
Same number of each atom on both sides. Example: 2H₂ + O₂ → 2H₂O.
Memory tipChange only the big numbers in front, never the small ones.
Limiting reactant
The reactant that runs out first; it decides how much product forms.
Memory tipUse the equation's ratio to see which one runs out.
Solutions
Solvent dissolves; solute is dissolved. Saturated means no more solute can dissolve at that temperature.
Memory tipIn salt water, water is the solvent and salt is the solute.
Often tested
- Common symbols: H hydrogen, O oxygen, C carbon, N nitrogen, Na sodium, Fe iron.
- Volume of a rock = rise in water level in a measuring cylinder.
- Gas made in a sealed bag still has mass, so the total mass does not change.
- Being able to burn is a chemical property; melting point and shine are physical properties.
Easy to confuse: Physical and chemical changes
| Feature | Physical change | Chemical change |
|---|---|---|
| New substance? | No | Yes |
| Examples | Melting, freezing, boiling, cutting | Rusting, burning, cooking an egg |
| Signs | Change in shape, size, or state | Gas, color change, heat or light, new solid |
3 sample questions
Verified practice questions from this unit.
Question 1. Which of these is a chemical change?
- ① Paper being cut in half
- ② Water boiling
- ③ An iron nail rusting
- ④ Ice melting
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Answer: ③ An iron nail rusting
Key point: Chemical change makes a new substance
In a chemical change, a new substance forms. Rust (iron oxide) is a new substance made when iron reacts with oxygen and water. Melting, boiling, and cutting are physical changes because the substance stays the same.
Wrong choices
- Paper being cut in half: Cutting paper changes only its size and shape. It is still paper.
- Water boiling: Boiling is a change of state from liquid to gas. It is still water.
- Ice melting: Melting is a change of state. The ice is still water, so it is a physical change.
Question 2. A block has a mass of 60 grams and a volume of 20 cubic centimeters. Using density = mass ÷ volume, what is its density?
- ① 3 g/cm³
- ② 0.33 g/cm³
- ③ 40 g/cm³
- ④ 1,200 g/cm³
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Answer: ① 3 g/cm³
Key point: Density = mass ÷ volume
- Density = mass ÷ volume.
- 60 ÷ 20 = 3 g/cm³.
Check: 3 × 20 = 60 grams.
Wrong choices
- 0.33 g/cm³: 0.33 comes from dividing the volume by the mass. Divide mass by volume.
- 40 g/cm³: 40 comes from subtracting 20 from 60.
- 1,200 g/cm³: 1,200 comes from multiplying 60 by 20.
Question 3. Water vapor in the air touches a cold glass and turns into liquid droplets. What is this change called?
- ① Melting
- ② Evaporation
- ③ Condensation
- ④ Freezing
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Answer: ③ Condensation
Key point: Condensation: gas → liquid
Condensation happens when a gas cools and turns into a liquid. The cold glass cools the water vapor, and droplets form on the outside of the glass.
Wrong choices
- Melting: Melting is a solid turning into a liquid.
- Evaporation: Evaporation is the opposite: liquid turning into gas.
- Freezing: Freezing is a liquid turning into a solid.
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