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.
- Balanced chemical equations show conservation of mass because atoms are rearranged, not created or destroyed.
- Solution questions often ask whether a solution is dilute, concentrated, saturated, or affected by temperature.
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.
Matter and Properties
Matter has mass and takes up space. Atoms combine to form molecules and larger substances. A physical property can be observed without changing what the substance is. A chemical property describes how a substance can form new substances.
| Observation | Physical or Chemical? | Reason |
|---|---|---|
| Ice melts into liquid water | Physical change | State changes, but the substance is still water |
| Wood burns and forms ash and gases | Chemical change | New substances form |
| Salt dissolves in water | Physical change | Salt particles spread through water and can often be recovered |
| Iron rusts | Chemical change | Iron reacts with oxygen to form a new substance |
GED items often ask you to choose evidence for a chemical reaction. Strong evidence includes gas formation, temperature change not caused only by heating or cooling, light production, color change with a new substance, or formation of a solid precipitate when two liquids are mixed.
Density From Data
Density is mass divided by volume. It helps identify substances and predict floating or sinking. If a formula is needed, GED questions usually provide it, but you still need to choose 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 with density about 1.0 g/mL, Sample C would float if it does not dissolve, while Samples A and B would sink. Notice that Sample B is not densest because it has the greatest mass alone. Density compares mass to volume.
Chemical Equations and Conservation
In a chemical reaction, atoms rearrange. A balanced equation has the same number of each type of atom on both sides. This matches the law of conservation of mass: in a closed system, total mass before and after a reaction stays the same.
For example, a sealed container has 10 g of reactant X and 16 g of reactant Y. After they react, the container holds 26 g of products. The mass is conserved because 10 + 16 = 26. If an open container appears to lose mass during a reaction that produces gas, the gas may have escaped into the air rather than disappeared.
Solutions and Solubility
A solution has a solute dissolved in a solvent. Salt water has salt as the solute and water as the solvent. A concentrated solution has more solute per amount of solvent than a dilute solution. A saturated solution holds as much dissolved solute as it can under those conditions.
Solubility often changes with temperature. If a table shows that 100 g of water dissolves 36 g of a salt at 20 degrees C but 50 g at 60 degrees C, the supported conclusion is that this salt is more soluble in hotter water. Do not assume every substance behaves exactly the same unless the data say so.
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?