LET Elementary Chemistry Reviewer 2026

Answer: B. Anything that has mass and occupies space

Step 1: Recall the scientific definition of matter. Matter is defined as anything that has MASS and occupies SPACE (volume). Step 2: Check each option. Option A is wrong because many forms of matter (like air) are colorless and invisible. Option C is wrong because gases also are matter but do not have a fixed shape. Option D mentions energy, which is NOT matter — energy has no mass and takes up no space. Step 3: Option B correctly states BOTH defining properties of matter: mass and volume. This is the standard definition used in the Philippine K-12 Science curriculum and in LET review materials.

Answer: C. Physical change, because the sugar can be recovered by evaporating the water

Step 1: Identify what makes a change physical vs. chemical. A PHYSICAL change does not produce a new substance; the original substance can still be recovered. A CHEMICAL change produces new substances with new properties. Step 2: When sugar dissolves in water, the sugar molecules spread out but their identity does NOT change. Step 3: You can recover the sugar by evaporating the water — this confirms no new substance was made. Step 4: Option A and B are wrong because no new substance is formed. Option D is a poor reason because color change alone can occur in physical changes; moreover, sugar solution is mostly colorless. The ability to REVERSE the process by physical means is the key sign of a physical change.

Answer: C. Water (H₂O)

Step 1: Recall the definition of a compound. A compound is a PURE SUBSTANCE made of two or more ELEMENTS chemically combined in a FIXED RATIO. Step 2: Evaluate each option. Salt water (tubig-alat) is a MIXTURE because it has a variable composition and the salt and water are not chemically combined. Gold is an ELEMENT — it contains only one kind of atom. Air is a MIXTURE of gases (nitrogen, oxygen, argon, etc.). Step 3: Water (H₂O) is made of hydrogen and oxygen chemically bonded in a fixed 2:1 ratio, making it a COMPOUND. Step 4: The key test: can it be written with a fixed chemical formula? Yes — H₂O. This is the standard example used in Philippine elementary and secondary science.

Answer: B. Melting; heat is absorbed

Step 1: Identify the phase change. Ice (solid) is changing to water (liquid), which is called MELTING (also called fusion). Step 2: Recall the heat rule for phase changes. When matter changes from a MORE ordered state to a LESS ordered state (solid → liquid → gas), heat is ABSORBED. Step 3: Melting is solid → liquid, so heat is absorbed from the surroundings. That is why a glass of ice feels cold — it is absorbing heat from your hand. Step 4: Freezing is the reverse (liquid → solid) and releases heat. Evaporation is liquid → gas. Condensation is gas → liquid and releases heat. Step 5: The correct answer is Melting; heat is absorbed.

Answer: C. Sublimation

Step 1: The key detail is that the mothballs change from SOLID to GAS directly, skipping the liquid phase. Step 2: This process is called SUBLIMATION — when a solid changes directly into a gas without passing through the liquid state. Step 3: Evaporation is a liquid-to-gas change, so it is not correct here. Condensation is gas-to-liquid. Deposition is gas-to-solid (the reverse of sublimation), like frost forming on a cold surface. Step 4: Mothballs (naphthalene) and dry ice (solid CO₂) are classic examples of sublimation. This concept is commonly tested in the LET Science component. Step 5: Remember: Sublimation = Solid → Gas (directly). Deposition = Gas → Solid (directly).

Answer: B. Electrons

Step 1: Recall the three subatomic particles: protons (+), neutrons (0), and electrons (−). Step 2: A NEUTRAL atom has no overall electric charge. For the atom to be electrically neutral, the positive charges must balance the negative charges. Step 3: Protons carry a positive charge and electrons carry a negative charge. So in a neutral atom, number of protons = number of electrons. Step 4: Neutrons carry NO charge, so they do not need to balance protons for electrical neutrality. Nucleons is a general term for both protons and neutrons in the nucleus — it is not a specific particle. Isotopes are atoms of the same element with different neutron numbers, not a particle. Step 5: The correct answer is ELECTRONS.

Answer: C. 6

Step 1: Recall the formula for finding neutrons. Neutrons = Mass number − Atomic number. Step 2: Identify the given values. Atomic number (Z) = 6 (this is the number of protons). Mass number (A) = 12 (this is protons + neutrons). Step 3: Apply the formula. Neutrons = 12 − 6 = 6. Step 4: This is the element Carbon-12 (C-12), the most common isotope of carbon. It has 6 protons, 6 neutrons, and (since it is neutral) 6 electrons. Step 5: Option A (12) would be the mass number itself, not the neutron count. Option B (18) has no logical basis. Option D (3) would result from dividing, which is incorrect.

Answer: C. Ernest Rutherford

Step 1: Recall the history of atomic models in order: Dalton → Thomson → Rutherford → Bohr → Quantum model. Step 2: John Dalton proposed that atoms are tiny, solid, indivisible spheres (billiard-ball model). Step 3: J.J. Thomson discovered the electron and proposed the 'plum pudding' model — electrons embedded in a positive sphere. Step 4: Ernest Rutherford fired alpha particles at a thin gold foil. Most passed through, but some bounced back sharply. This proved that most of the atom is empty space and that there is a SMALL, DENSE, POSITIVELY CHARGED NUCLEUS at the center. Step 5: Niels Bohr came AFTER Rutherford and proposed that electrons travel in fixed orbits (energy levels). The gold-foil experiment is Rutherford's signature contribution.

Answer: C. Valence electrons in the outermost shell

Step 1: Recall the structure of the periodic table. PERIODS are horizontal rows; GROUPS (families) are vertical columns. Step 2: Elements in the same group have the same number of VALENCE ELECTRONS — the electrons in the outermost energy shell. Step 3: Valence electrons determine how an atom bonds and reacts with other atoms. Since all elements in a group have the same number of valence electrons, they show similar chemical behavior. For example, all Group 1 alkali metals have 1 valence electron and react vigorously with water. Step 4: Protons in the nucleus identify the specific element (atomic number), and this changes every step. Neutrons vary between isotopes. The number of energy levels (shells) INCREASES down a group, not stays the same. Step 5: The correct answer is VALENCE ELECTRONS.

Answer: C. It decreases because more protons pull electrons closer to the nucleus

Step 1: Moving left to right across a period, the atomic number (number of protons) INCREASES by one each step. Step 2: More protons in the nucleus means a STRONGER positive pull on the electrons. Step 3: Even though more electrons are also being added, they go into the SAME energy level (same shell), so no new shell is added. The increased nuclear attraction pulls the electron cloud INWARD. Step 4: The result is that atomic radius DECREASES across a period from left to right. For example, sodium (Na) in Group 1 is larger than chlorine (Cl) in Group 17, both in Period 3. Step 5: Option A is a common misconception — adding electrons to the same shell does not increase size because the stronger nuclear pull compensates. The correct trend is that atomic size DECREASES left to right across a period.

Answer: B. Ionic bond

Step 1: Identify the elements. Sodium (Na) is a metal and chlorine (Cl) is a nonmetal. Step 2: Recall the rule – ionic bonds form between a metal and a nonmetal through the TRANSFER of electrons. Step 3: Na loses one electron (becomes Na⁺) and Cl gains one electron (becomes Cl⁻). The opposite charges attract, forming an ionic bond. Step 4: Covalent bonds involve SHARING of electrons (usually nonmetal + nonmetal), so that choice is wrong. Metallic bonds occur among metal atoms only. Hydrogen bonds are weak intermolecular attractions, not the primary bond in NaCl.

Answer: C. Covalent bond

Step 1: Identify the elements. Hydrogen (H) and oxygen (O) are both nonmetals. Step 2: Recall the rule – covalent bonds form between nonmetals through SHARING of electrons. Step 3: In H₂O, each hydrogen atom shares one electron with oxygen, giving all atoms a full outer shell. Step 4: Ionic bonds involve transfer of electrons (metal + nonmetal), so that is incorrect. Metallic bonds involve a 'sea' of electrons among metal atoms only. 'Electrostatic bond' is not a standard bond classification.