7.4 Matter, Phase Changes, and Atomic Structure

Adamson University College Entrance Test AdUCET

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1 Introduction & Exam Overview

1.1 AdUCET Overview, Format, and Admission Process 1.2 Test-Taking and Time-Management Strategies

2 Chapter 2: Mathematics I — Numbers, Algebra, and Functions

2.1 Number Sense, Operations, and Fraction–Decimal–Percent Conversions 2.2 Algebra, Polynomials, and Linear/Quadratic Equations 2.3 Inequalities, Absolute Value, and Systems of Equations 2.4 Radicals, Rational Expressions, and Scientific Notation 2.5 Coordinate Geometry, Lines, and Functions

3 Chapter 3: Mathematics II — Geometry, Measurement, and Trigonometry

3.1 Plane Geometry: Angles, Triangles, Polygons, and Circles 3.2 Solid Mensuration, Measurement, and Unit Conversion 3.3 Right-Triangle Trigonometry and Applications

4 Chapter 4: Mathematics III — Sequences, Proportion, and Data

4.1 Arithmetic and Geometric Sequences and Series 4.2 Ratio, Proportion, Percentage, Work, and Mixture Problems 4.3 Descriptive Statistics and Data Interpretation 4.4 Probability: Rules, Compound Events, and Conditional Probability 4.5 Counting Principles: Permutations and Combinations

5 Chapter 5: English I — Reading Comprehension and Vocabulary

5.1 Reading Comprehension: Main Idea, Purpose, and Passage Structure 5.2 Inference, Supporting Detail, and Author's Tone 5.3 Word Roots, Prefixes, and Suffixes 5.4 Vocabulary in Context, Synonyms, Antonyms, and Confusables

6 Chapter 6: English II — Grammar, Usage, and Written Expression

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7 Chapter 7: General Science I — Biology and Chemistry

7.1 Cell Biology, Transport, and Cellular Energetics 7.2 Genetics, Heredity, and Molecular Biology 7.3 Human Body Systems, Ecology, and Evolution 7.4 Matter, Phase Changes, and Atomic Structure 7.5 Periodic Trends, Chemical Bonding, Reactions, Acids and Bases

8 Chapter 8: General Science II — Physics and Earth Science

8.1 Mechanics: Motion, Newton's Laws, Work, and Energy 8.2 Waves, Sound, Light, Heat, Electricity, and Magnetism 8.3 Earth's Structure, Plate Tectonics, and the Rock Cycle 8.4 The Atmosphere, Weather, the Water Cycle, and Astronomy

9 Chapter 9: Aptitude and Abstract Reasoning

9.1 Figure Series and Pattern Progression 9.2 Matrix Reasoning and Figural Analogies 9.3 Spatial Reasoning: Rotation, Reflection, and Chirality 9.4 Paper Folding, Cube Nets, and Orthographic Views 9.5 Deductive Reasoning, Syllogisms, and Venn Diagrams 9.6 Number, Letter, and Alphanumeric Series 9.7 Odd-One-Out, Classification, and Logic Puzzles

7.4 Matter, Phase Changes, and Atomic Structure

Key Takeaways Explain This Topic AI

Every phase change towards the gas state — melting, vaporisation, sublimation — absorbs energy, and every change towards the solid state — freezing, condensation, deposition — releases it.

Temperature stalls during a phase change because the energy supplied breaks particle attractions instead of raising average kinetic energy: 334 J per gram to melt water, about 2,260 J per gram to vaporise it.

Neutron count is mass number minus atomic number, so chlorine-35 written with Z = 17 has 18 neutrons, never 35.

An ion's charge equals protons minus electrons, and forming an ion changes only the electron count — the proton number is fixed for the life of the atom.

For the first twenty elements the electron shells fill 2, 8, 8, 2, so chlorine at Z = 17 is arranged 2, 8, 7 and carries seven valence electrons.

Last updated: August 2026

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

Technique Property it exploits Everyday Philippine example

Filtration one component is an insoluble solid, the other passes through pores straining coffee grounds out of brewed kapeng barako

Distillation difference in boiling point producing lambanog by distilling fermented coconut tuba

Evaporation / crystallisation the solvent evaporates, the dissolved solid does not harvesting sea salt from the banigan salt beds of Dasol, Pangasinan

Chromatography components travel at different speeds along a stationary phase separating the dyes inside one pentel-pen ink blot on filter paper

Magnetic separation one component is magnetic lifting magnetite grains out of Ilocos black sand

Decantation density difference; the heavier solid settles first pouring off the clear water above settled sediment in hugas-bigas

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.

State Shape Volume Compressible? Particle behaviour
Solid definite definite almost none packed in fixed positions, only vibrating
Liquid takes the container definite very little touching but free to slide past one another
Gas fills the container fills the container highly far apart, fast, effectively independent
Plasma none none highly a hot ionised gas of free electrons and positive ions

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

Change Direction Energy Filipino example
Melting (fusion) solid → \rightarrow → liquid absorbed (endothermic) ice softening in a glass of halo-halo

Freezing (solidification) liquid → \rightarrow → solid released (exothermic) ice candy setting overnight in the freezer

Vaporisation (evaporation, boiling) liquid → \rightarrow → gas absorbed seawater drying in a salt bed under the April sun

Condensation gas → \rightarrow → liquid released droplets beading on a cold bottle of softdrinks
Sublimation solid → \rightarrow → gas absorbed naphthalene moth balls shrinking away inside an aparador
Deposition gas → \rightarrow → solid released andap , the frost that coats cabbage leaves in Atok, Benguet

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?

Identify the process: a phase change at constant temperature, so use Q = m × L f Q = m \times L_f Q = m × L f , not a temperature formula.

Substitute: Q = 50 g × 334 J/g Q = 50 \text{ g} \times 334 \text{ J/g} Q = 50 g × 334 J/g .

Compute: Q = 16,700 J = 16.7 kJ Q = 16{,}700 \text{ J} = 16.7 \text{ kJ} Q = 16 , 700 J = 16.7 kJ .

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 × 4.18 = 209 50 \times 4.18 = 209 50 × 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:

Gas is produced where nothing is boiling — the fizz when suka meets baking soda.

A precipitate forms — an insoluble solid appears on mixing two clear solutions.

A permanent colour change occurs that is not simple mixing of pigments.

Energy is exchanged — the container becomes hot or cold on its own, or light is emitted.

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

Particle Symbol Relative charge Relative mass (amu) Location
Proton p + p^+ p + + 1 +1 + 1 1 nucleus
Neutron n 0 n^0 n 0 0 1 nucleus

Electron e − e^- e − − 1 -1 − 1 ≈ 1 1836 \approx \tfrac{1}{1836} ≈ 1836 1 (0.00055) shells surrounding the nucleus

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 Z Z = the number of protons. It defines the element — change Z Z Z and you have a different element entirely.

Mass number A A A = protons + neutrons, always a whole number for a single atom.

Neutrons N = A − Z N = A - Z N = A − Z .

In a neutral atom, electrons = protons.

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

Isotopes are atoms of one element with different neutron counts, so the same Z Z Z but different A A 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 17 35 Cl ^{35}_{17}\text{Cl} 17 35 Cl (about 75%) and 17 37 Cl ^{37}_{17}\text{Cl} 17 37 Cl (about 25%).

Read Z Z Z : both show 17, so both have 17 protons , and 17 electrons when neutral.

Neutrons from N = A − Z N = A - Z N = A − Z : 35 − 17 = 18 35 - 17 = 18 35 − 17 = 18 neutrons in the lighter isotope, 37 − 17 = 20 37 - 17 = 20 37 − 17 = 20 in the heavier.

Weight each mass by its abundance: ( 35 × 0.75 ) + ( 37 × 0.25 ) = 26.25 + 9.25 (35 \times 0.75) + (37 \times 0.25) = 26.25 + 9.25 ( 35 × 0.75 ) + ( 37 × 0.25 ) = 26.25 + 9.25 .

Add: 35.5 amu , matching the tabulated 35.45.

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: Na → Na + + e − \text{Na} \rightarrow \text{Na}^+ + e^- Na → Na + + e − ; magnesium loses two to give Mg 2 + \text{Mg}^{2+} Mg 2 + .

An anion is negative because electrons were gained. Non-metals do this: Cl + e − → Cl − \text{Cl} + e^- \rightarrow \text{Cl}^- Cl + e − → Cl − ; oxygen gains two to give O 2 − \text{O}^{2-} O 2 − .

So a chloride ion written 17 35 Cl − ^{35}_{17}\text{Cl}^- 17 35 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.

Element Z Z Z Shell arrangement Valence electrons
Helium 2 2 2 (full)
Carbon 6 2, 4 4
Sodium 11 2, 8, 1 1
Chlorine 17 2, 8, 7 7
Argon 18 2, 8, 8 8 (full)
Calcium 20 2, 8, 8, 2 2

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

Group Valence electrons Dot arrangement Example
1 1 one single dot Na
2 2 two single dots Mg
13 3 three single dots B
14 4 four single dots, none paired C
15 5 one pair plus three singles N
16 6 two pairs plus two singles O
17 7 three pairs plus one single Cl
18 8 four pairs, no singles Ne (helium shows just one pair)

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 A - Z A − Z , never A A 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.