11.4 Chemistry, Matter, and Reactions
Key Takeaways
- Matter is described by particles, states, density, and properties that can be observed or measured.
- Physical changes alter form or state, while chemical changes produce new substances with different properties.
- Density, solubility, and phase-change data are common GED ways to test matter concepts through tables and graphs.
- A balanced chemical equation shows conservation of mass because atoms are rearranged, not created or destroyed.
- Solution questions often ask whether a solution is dilute, concentrated, or saturated, and how temperature changes solubility.
Matter and Reactions in GED Context
GED chemistry questions are usually practical. A prompt may describe a lab table, a cooking process, a cleaning solution, a metal sample, or a reaction diagram. The safest approach is to identify the property being measured, decide whether a new substance formed, and use the data to support the conclusion. Chemistry items appear inside the Physical Science band of the 40-item, 90-minute Reasoning Through Science test.
Matter and Properties
Matter has mass and takes up space. Atoms combine to form molecules and larger substances, which exist as solid, liquid, or gas depending on how tightly their particles are arranged and how fast they move. A physical property (color, density, melting point, state) can be observed without changing what the substance is. A chemical property describes how a substance forms new substances.
| Observation | Physical or Chemical? | Reason |
|---|---|---|
| Ice melts into liquid water | Physical change | State changes, but it is still water |
| Wood burns into ash and gases | Chemical change | New substances form |
| Salt dissolves in water | Physical change | Particles spread out and can be recovered |
| Iron rusts | Chemical change | Iron reacts with oxygen to form a new compound |
GED items often ask for evidence of a chemical reaction. Strong signs include: gas formation (bubbling), a temperature change not caused only by heating or cooling, light or flame production, a color change tied to a new substance, or a solid precipitate forming when two liquids mix.
Density From Data
Density is mass divided by volume (g/mL or g/cm cubed). It helps identify substances and predict floating or sinking. The GED usually supplies the formula, but you must pick the right numbers.
| Sample | Mass (g) | Volume (mL) | Density (g/mL) |
|---|---|---|---|
| A | 24 | 12 | 2.0 |
| B | 30 | 10 | 3.0 |
| C | 18 | 24 | 0.75 |
Sample C has the lowest density. In water (density about 1.0 g/mL), Sample C would float if it does not dissolve, while A and B would sink. Notice Sample B is densest even though Sample A is not, and the greatest mass alone (B at 30 g) does not decide the answer, density compares mass to volume.
Chemical Equations and Conservation
In a chemical reaction, atoms rearrange but are not created or destroyed. A balanced equation has the same number of each type of atom on both sides, matching the law of conservation of mass: in a closed system, total mass before equals total mass after.
Example: a sealed container holds 10 g of reactant X and 16 g of reactant Y. After they react, the container holds 26 g of products, because 10 + 16 = 26. If an open container seems to lose mass during a reaction that produces gas, the gas escaped into the air, the mass did not vanish.
Solutions and Solubility
A solution has a solute dissolved in a solvent. In salt water, salt is the solute and water is the solvent. A concentrated solution has more solute per amount of solvent than a dilute one. A saturated solution holds as much dissolved solute as it can under the given conditions; adding more leaves undissolved solid.
Solubility usually changes with temperature. If a table shows that 100 g of water dissolves 36 g of a salt at 20 degrees Celsius but 50 g at 60 degrees Celsius, the supported conclusion is that this salt is more soluble in hotter water. Do not assume every substance behaves the same, some gases, for instance, are less soluble as temperature rises. ### Reading a Solubility Curve
GED chemistry often shows how much solute dissolves in 100 g of water at different temperatures.
| Temperature (degrees C) | Salt Dissolved (g per 100 g water) |
|---|---|
| 20 | 36 |
| 40 | 38 |
| 60 | 41 |
| 80 | 44 |
The trend is a gradual rise, so this salt is slightly more soluble in hotter water. If a solution at 80 degrees holds 44 g and is then cooled to 20 degrees, only 36 g can stay dissolved, so about 8 g should crystallize out as solid. Questions that ask "how much will come out of solution" are really subtraction problems once you read the curve.
States of Matter and Phase Changes
Heating or cooling moves matter between states without changing its identity, a physical change.
| Change | Direction | Example |
|---|---|---|
| Melting | Solid to liquid | Ice to water |
| Freezing | Liquid to solid | Water to ice |
| Evaporation / Boiling | Liquid to gas | Water to steam |
| Condensation | Gas to liquid | Steam to dew |
During a phase change, temperature holds steady while energy goes into rearranging particles, which is why a heating graph shows a flat plateau at the melting and boiling points.
Common GED Chemistry Traps
- Picking the sample with the largest mass as "densest" without dividing by volume.
- Assuming an open container that releases gas "loses" mass; the gas escaped, mass is still conserved.
- Calling dissolving (a physical change) a chemical reaction.
- Reading a solubility curve backward, hotter water usually dissolves more solid solute.
For GED chemistry, keep the evidence chain short: identify the measured property, compare the numbers, and state only the conclusion the data support.
A solid sample has a mass of 45 g and a volume of 15 mL. Using density = mass / volume, what is its density?
In a sealed container, 12 g of one reactant combines with 7 g of another reactant. What mass of products should be present after the reaction is complete?