7.4 Matter, Phase Changes, and Atomic Structure

7.4 Matter, Phase Changes, and Atomic Structure

The Science portion of the AdUCET (Adamson University College Entrance Test) draws on the K-12 senior-high-school chemistry you met in Grades 11 and 12. Adamson University publishes no content outline, so treat any item counts or per-topic weightings you read online as candidate-reported rather than official. What is reliable is the level: general chemistry, answerable by reasoning rather than by calculator work. This section builds the foundation — what matter is, how chemists sort and separate it, how it changes state, and what an atom is made of. Everything about the periodic table, bonding, equations and acids is handled separately in section 7.5.

What matter is and how it is classified

Matter is anything that has mass and occupies space. A bar of soap, the air in a jeepney, and the seawater off Dasol all qualify; heat, light and sound do not, because they have no mass.

Chemists sort matter with a single branching question: is the composition fixed?

                              MATTER
                    (has mass, occupies space)
                                 |
              +------------------+------------------+
              |                                     |
       PURE SUBSTANCE                            MIXTURE
   fixed composition, one              two or more substances
   fixed set of properties             physically combined
              |                                     |
      +-------+-------+                   +---------+---------+
      |               |                   |                   |
   ELEMENT        COMPOUND          HOMOGENEOUS         HETEROGENEOUS
 one kind of    two or more        uniform throughout   visibly different
 atom only      elements chemi-    (a solution)         parts
 Fe, Au, O2     cally bonded in    salt water, air,     sand in water,
                a fixed ratio      brass, 70% alcohol   halo-halo, buko salad
                H2O, NaCl, CO2

The line that examiners probe is compound versus mixture. A compound has a fixed ratio of elements and properties that belong to neither ingredient — sodium is a metal that explodes in water, chlorine is a poisonous green gas, and sodium chloride is the salt on your table. A mixture keeps its components' properties, comes in any proportion, and can be pulled apart by purely physical means.

Separation techniques

None of these six changes the chemical identity of anything. That is the whole point: if a separation is possible without a reaction, you were looking at a mixture.

The states of matter

Kinetic molecular theory explains all of them with two competing quantities: the kinetic energy of the particles (which temperature measures) and the strength of the attractions between them.

Plasma is the fourth state and the one candidates forget. Because its charges are free, plasma conducts electricity and bends in a magnetic field — you see it in a lightning stroke over Manila Bay, inside a fluorescent tube, and in the Sun.

The six phase transitions

One rule replaces the whole table: every step towards the gas state absorbs energy, and every step towards the solid state releases it. Melting, vaporisation and sublimation are endothermic; freezing, condensation and deposition are exothermic.

Heating curves and latent heat

Heat a block of ice steadily and plot temperature against energy supplied:

 T (°C)
  120 |                                          ___ steam warms
      |                                         /
  100 |                    +===================+        <- boiling plateau
      |                   /
   50 |                  /   liquid water warms
      |                 /
    0 |      +=========+                                <- melting plateau
      |     /
  -20 |____/  ice warms
      +--------------------------------------------------> heat added

The two flat stretches are the exam's target. During a plateau the burner is still supplying energy, yet the thermometer refuses to move. The reason is that the incoming energy is being spent breaking the attractions that hold particles in place, not on raising their average kinetic energy — and it is average kinetic energy that a thermometer reads. That hidden energy is latent heat: the latent heat of fusion of water is 334 J per gram, and the latent heat of vaporisation is about 2,260 J per gram.

Worked example 1 — the cost of melting. How much energy turns 50 g of ice at 0 °C into 50 g of water at 0 °C?

1. Identify the process: a phase change at constant temperature, so use $Q = m \times L_f$, not a temperature formula.
2. Substitute: $Q = 50 \text{ g} \times 334 \text{ J/g}$.
3. Compute: $Q = 16{,}700 \text{ J} = 16.7 \text{ kJ}$.
4. Interpret: the sample begins at 0 °C and ends at 0 °C. Every joule bought a change of state.

For contrast, warming that same 50 g of liquid water by one degree needs only $50 \times 4.18 = 209$ J. Melting therefore costs roughly eighty times as much energy as a one-degree warm-up — which is why an ice block keeps a cooler cold for hours.

Properties and changes

• A physical property can be measured without destroying the substance: colour, odour, density, hardness, melting and boiling point, solubility, malleability, conductivity.
• A chemical property describes only what the substance would turn into: flammability, toxicity, ability to rust, reactivity with acid. You cannot observe one without consuming part of the sample.

A physical change alters form, state or appearance while the substance stays chemically the same — melting, dissolving asukal in coffee, tearing paper, slicing mango. A chemical change produces one or more genuinely new substances, as when iron rusts or lechon char forms.

Five observable signs point to a chemical change:

1. Gas is produced where nothing is boiling — the fizz when suka meets baking soda.
2. A precipitate forms — an insoluble solid appears on mixing two clear solutions.
3. A permanent colour change occurs that is not simple mixing of pigments.
4. Energy is exchanged — the container becomes hot or cold on its own, or light is emitted.
5. A new odour appears, as in spoiled food.

Each sign can be counterfeited by a physical process, which is exactly where items are set. Bubbles in a pot of boiling water are water vapour, not a new substance. Powdered juice colouring a pitcher is mixing. The decisive question is always: is a new substance present, with properties neither starting material had, and is it hard to reverse?

The law of conservation of mass (Lavoisier) governs every chemical change: in a closed system, the total mass of the products equals the total mass of the reactants. Burn 4 g of hydrogen with 32 g of oxygen and you get exactly 36 g of water. A candle appears to lose mass only because its carbon dioxide and water vapour escape; steel wool burned in air appears to gain mass because oxygen from the air joins the iron.

Inside the atom

The nucleus carries virtually all of the mass in a vanishingly small share of the volume; the electrons occupy nearly all the space and do all the chemistry.

• Atomic number $Z$ = the number of protons. It defines the element — change $Z$ and you have a different element entirely.
• Mass number $A$ = protons + neutrons, always a whole number for a single atom.
• Neutrons $N = A - Z$.
• In a neutral atom, electrons = protons.

Isotope notation places the mass number above and the atomic number below the symbol: $^{A}_{Z}\text{X}$.

Isotopes are atoms of one element with different neutron counts, so the same $Z$ but different $A$. Carbon-12 and carbon-14 both have 6 protons; the second simply carries 8 neutrons instead of 6. They react identically, because reactions are decided by electrons. Since a real sample is a blend of isotopes, the periodic table lists a weighted average atomic mass, which is why chlorine reads 35.45 rather than a whole number.

Worked example 2 — reading isotope symbols and averaging them. Chlorine occurs as $^{35}{17}\text{Cl}$ (about 75%) and $^{37}{17}\text{Cl}$ (about 25%).

1. Read $Z$: both show 17, so both have 17 protons, and 17 electrons when neutral.
2. Neutrons from $N = A - Z$: $35 - 17 = 18$ neutrons in the lighter isotope, $37 - 17 = 20$ in the heavier.
3. Weight each mass by its abundance: $(35 \times 0.75) + (37 \times 0.25) = 26.25 + 9.25$.
4. Add: 35.5 amu, matching the tabulated 35.45.
5. Sanity check: a weighted average must fall between the two isotope masses and closer to the more abundant one. 35.5 sits nearer 35, as it should.

Ions

An ion is an atom that has gained or lost electrons. Charge = protons − electrons.

• A cation is positive because electrons were lost. Metals do this: $\text{Na} \rightarrow \text{Na}^+ + e^-$; magnesium loses two to give $\text{Mg}^{2+}$.
• An anion is negative because electrons were gained. Non-metals do this: $\text{Cl} + e^- \rightarrow \text{Cl}^-$; oxygen gains two to give $\text{O}^{2-}$.

So a chloride ion written $^{35}_{17}\text{Cl}^-$ has 17 protons, 18 neutrons and 18 electrons. Note carefully that the proton count never moves. Forming an ion is an electron transaction only.

Electron shells and valence electrons

Electrons occupy shells at increasing distance from the nucleus, and inner shells fill first. For the first twenty elements the working capacities are 2, then 8, then 8, then 2 — the third shell can eventually expand to 18, but only once the transition metals begin.

Valence electrons are the electrons in the outermost occupied shell. They alone determine how an atom reacts, which is why the periodic table's columns behave alike.

Lewis dot structures

A Lewis dot structure draws the symbol with one dot per valence electron. Place a single dot on each of the four sides first; only from the fifth electron onward do dots pair up.

        top side
            *
  left  * [ X ] *  right side      dots 1-4: one on each side
            *                      dots 5-8: pair up with an existing dot
       bottom side

The count of unpaired dots is the number of bonds the atom normally forms — four for carbon, three for nitrogen, two for oxygen, one for a halogen, none for a noble gas. How those bonds actually form is section 7.5's business.

What examiners test

• Reversed energy direction. Freezing releases energy; it does not absorb it, even though a freezer feels cold.
• Temperature must rise when heat is added. False during any phase change.
• Compound treated as a mixture. Air is a mixture; water is a compound. Only one of them has a fixed formula.
• Neutrons read straight off the symbol. Neutrons are $A - Z$, never $A$.
• Ions gaining protons. Ions change electron count only.
• Dissolving classified as chemical. Sugar dissolved in water is recoverable by evaporation, so the change is physical.
• Evaporation equated with boiling. Evaporation happens at the surface at any temperature; boiling happens throughout the liquid, only at the boiling point.