Properties of Matter
Matter has unique traits that define its behavior. Physical properties, like color and melting point, can be observed without changing what the substance is made of. Chemical properties, like flammability, describe how a substance reacts with other substances or transforms into something new.
Some properties change depending on how much material you have, while others stay constant regardless of quantity. Knowing the difference helps you predict how matter will behave and lets you identify unknown substances.
Physical vs. Chemical Properties
A physical property is anything you can observe or measure without changing the substance's chemical identity. When you measure the boiling point of water, it's still water afterward.
- Color (copper is reddish-orange, sulfur is yellow)
- Melting point (water at 0 °C, iron at 1538 °C)
- Boiling point (water at 100 °C, ethanol at 78.4 °C)
- Density (water at 1 g/mL, gold at 19.3 g/mL)
- Hardness (talc is 1 on the Mohs scale, diamond is 10)
- Malleability describes whether a substance can be hammered or pressed into a new shape without breaking. Gold and silver are highly malleable.
- Conductivity describes how well a substance transfers heat or electricity. Copper and aluminum are excellent conductors.
- State of matter (solid, liquid, gas) is itself a physical property, determined by temperature and pressure.
A chemical property describes how a substance changes into a different substance through a chemical reaction. You can only observe a chemical property by actually carrying out (or attempting) that reaction.
- Flammability describes how easily a substance ignites. Gasoline is highly flammable; water is not.
- Reactivity with acids describes whether a substance reacts when exposed to an acid. Many metals, like zinc, react with hydrochloric acid to produce hydrogen gas.
- Oxidation potential describes a substance's tendency to lose electrons. Iron rusts when exposed to oxygen and moisture; gold resists oxidation, which is why it doesn't tarnish.
- Corrosion resistance describes how well a substance withstands degradation from chemical reactions. Stainless steel and gold both have high corrosion resistance.
- Combustibility describes how readily a substance undergoes combustion (burning in the presence of oxygen). Wood and natural gas are both combustible.
Quick test: Ask yourself, "Did the substance become a different substance?" If yes, you're looking at a chemical property. If the substance is still the same thing, it's a physical property.
Extensive vs. Intensive Properties
Extensive properties change when you change the amount of matter. If you double the sample, the value doubles.
- Mass measures the quantity of matter (1 kg of water vs. 10 kg of water)
- Volume measures the space a substance occupies (1 L vs. 10 L)
- Energy measures the capacity to do work or transfer heat
- Heat capacity measures the total energy needed to raise the temperature of a given sample
Extensive properties are additive: the total mass of a mixture equals the sum of the masses of each component.
Intensive properties do not change with the amount of matter. A drop of water and a swimming pool of water share the same intensive properties.
- Density relates mass to volume. Water is 1 g/mL whether you have a teaspoon or a bathtub.
- Melting point is the temperature at which a solid becomes a liquid. Ice melts at 0 °C regardless of how much ice you have.
- Boiling point is the temperature at which a liquid becomes a gas. Water boils at 100 °C (at standard pressure) for any volume.
- Specific heat capacity is the energy required to raise 1 gram of a substance by 1 °C. For water, that's 4.18 J/g·°C no matter the sample size.
- Refractive index measures how much light bends passing through a substance (1.33 for water, regardless of amount).
Intensive properties are especially useful for identifying unknown substances, because they don't depend on how big your sample is.
Periodic Table and Properties
Organization of the Periodic Table
The periodic table arranges elements by increasing atomic number (the number of protons). This arrangement reveals repeating patterns in element properties.
- Periods are the horizontal rows. Period 1 has 2 elements; period 2 has 8.
- Groups are the vertical columns. Elements in the same group share similar chemical behavior. For example, Group 1 contains the alkali metals, and Group 18 contains the noble gases.
Three major periodic trends show up as you move across a period or down a group:
- Atomic radius decreases across a period (the growing nuclear charge pulls electrons closer) and increases down a group (each new period adds another electron shell).
- Ionization energy increases across a period (electrons are held more tightly by the stronger nuclear charge) and decreases down a group (outer electrons are farther from the nucleus and easier to remove).
- Electronegativity increases across a period (atoms attract shared electrons more strongly) and decreases down a group (the larger atomic radius weakens the pull on bonding electrons).
The periodic table also groups elements by their conductivity:
- Metals (left and center) conduct heat and electricity well because their valence electrons are delocalized and free to move. Examples: copper wiring, aluminum foil.
- Nonmetals (upper right) are poor conductors because their valence electrons are tightly held. Examples: sulfur, oxygen gas.
- Metalloids (along the zigzag staircase separating metals from nonmetals) have intermediate properties. Silicon and germanium, for instance, are semiconductors used in electronics.
Atomic Structure and Chemical Bonding
A substance's chemical properties trace back to its atomic structure, specifically its electron configuration. The electrons in the outermost energy level, called valence electrons, are the ones that participate in chemical reactions and bonding.
Different types of bonds produce different physical and chemical properties:
- Ionic bonds form when electrons transfer from a metal to a nonmetal. Ionic compounds tend to have high melting points and conduct electricity when dissolved in water (because the ions are free to move).
- Covalent bonds form when nonmetals share electrons. The way atoms share electrons determines molecular shape and reactivity.
- Metallic bonds occur in pure metals, where valence electrons are shared across a "sea" of atoms. This explains why metals are good conductors and can be bent without breaking.
Energy Changes in Chemical Reactions
Chemical reactions involve energy changes. Two categories to know:
- Exothermic reactions release energy (usually as heat) to the surroundings. Combustion and acid-base neutralization are common examples. The surroundings get warmer.
- Endothermic reactions absorb energy from the surroundings. Melting ice and photosynthesis both require energy input. The surroundings get cooler.
During any chemical reaction, reactants transform into products with different compositions and properties. Factors like temperature, concentration, and the presence of catalysts all influence how fast a reaction proceeds.