WCAS - Washington Comprehensive Assessment of Science Matter and Its Interactions Questions and Answers — Questions and Answers
Question 1: A student conducts an experiment by mixing 50 grams of vinegar with 5 grams of baking soda in a sealed plastic bag. The bag inflates due to gas production, and the substances turn into a new liquid. According to the law of conservation of mass, what should be the total mass of the contents inside the bag after the reaction?
- Less than 55 grams, because gas is lighter than liquid.
- Exactly 55 grams. (Correct answer)
- More than 55 grams, because the bag inflated.
- It is impossible to determine without knowing the exact gas produced.
Correct answer: Exactly 55 grams.
The law of conservation of mass states that matter cannot be created or destroyed in a chemical reaction. [4, 19] Therefore, the total mass of the reactants (vinegar and baking soda) must equal the total mass of the products (the new liquid and the gas). Since the initial mass was 50g + 5g = 55g, the final mass will also be 55g, as long as the system is closed (sealed bag) and no mass escapes. [17, 30]
Question 2: Which of the following is an example of a chemical change?
- Melting an ice cube into liquid water.
- Shredding a piece of paper into smaller pieces.
- A bicycle frame rusting over time. (Correct answer)
- Dissolving sugar in a cup of hot tea.
Correct answer: A bicycle frame rusting over time.
A chemical change results in the formation of a new substance with different properties. [1] Rusting is a chemical reaction where iron reacts with oxygen to form iron oxide (rust), a new substance. [7] Melting ice, shredding paper, and dissolving sugar are all physical changes because the chemical identity of the substance (Hâ‚‚O, cellulose, sucrose) does not change. [3]
Question 3: A scientist is studying an unknown element. She observes that it has 12 protons and 12 neutrons in its nucleus. Based on the periodic table, what can be inferred about this element?
- The element's atomic mass is 12.
- The element is a noble gas.
- The element has similar properties to the element with 11 protons.
- The element's atomic number is 12. (Correct answer)
Correct answer: The element's atomic number is 12.
The atomic number of an element is defined by the number of protons in its nucleus. [6, 8] Since the unknown element has 12 protons, its atomic number is 12, which identifies it as Magnesium (Mg). The atomic mass is the sum of protons and neutrons (approximately 24 in this case). Elements are grouped by similar properties in columns, not by adjacent atomic numbers. [2, 15] Atomic number 12 is an alkaline earth metal, not a noble gas.
Question 4: When thermal energy is added to a pure substance in its solid state, the particles within the substance begin to vibrate more rapidly. If enough thermal energy is added, what change will most likely occur?
- The substance will undergo a chemical reaction.
- The substance will condense into a liquid.
- The particles will break apart into individual atoms.
- The substance will melt into a liquid state. (Correct answer)
Correct answer: The substance will melt into a liquid state.
Adding thermal energy increases the kinetic energy of particles in a substance. In a solid, this causes increased vibration. When the vibrations become strong enough to overcome the forces holding the particles in a fixed lattice, the solid melts and transitions into the liquid state. This is a physical change, not a chemical reaction. Condensation is the process of a gas turning into a liquid. Breaking particles into individual atoms would require a chemical reaction, not just a phase change.
Question 5: A student observes several phenomena in a lab. Which of the following observations most strongly indicates that a chemical reaction has occurred?
- A solid substance dissolves in a liquid.
- The temperature of a mixture decreases as two liquids are combined. (Correct answer)
- A liquid boils when heated over a burner.
- A block of wood is cut into smaller pieces.
Correct answer: The temperature of a mixture decreases as two liquids are combined.
While not all chemical reactions involve a temperature change, an unexpected change in temperature (either getting hotter or colder) when substances are mixed is a strong indicator of a chemical reaction, as it signifies a change in chemical energy. [7, 12, 20] Dissolving, boiling, and cutting are typically physical changes where the substance's chemical identity remains the same. [1]
Question 6: A model of a water molecule (Hâ‚‚O) is shown to a student. Which statement accurately describes the atomic composition of this molecule?
- It is composed of one hydrogen atom and two oxygen atoms.
- It is composed of two hydrogen atoms and one oxygen atom. (Correct answer)
- It is an extended structure with repeating subunits of hydrogen and oxygen.
- It is a single atom, not a molecule.
Correct answer: It is composed of two hydrogen atoms and one oxygen atom.
The chemical formula Hâ‚‚O indicates the atomic composition of a water molecule. The subscript '2' applies to the element symbol preceding it (H for Hydrogen), and the absence of a subscript for Oxygen (O) implies '1'. Therefore, a water molecule consists of two hydrogen atoms and one oxygen atom bonded together. [25] It is a simple molecule, not an extended structure like a crystal. [25]
WCAS - Washington Comprehensive Assessment of Science Matter and Its Interactions Questions and Answers
A student conducts an experiment by mixing 50 grams of vinegar with 5 grams of baking soda in a sealed plastic bag.
The bag inflates due to gas production, and the substances turn into a new liquid.
According to the law of conservation of mass, what should be the total mass of the contents inside the bag after the reaction?