Elements, Compounds, Mixtures
What do we call a pure substance made of only one type of atom?
An element (for example, oxygen, iron, or gold).
Name the three subatomic particles and give the charge of each.
Proton (positive, +), neutron (no charge, 0), electron (negative, −).
What does an element’s atomic number tell you?
The number of protons in an atom of that element. It identifies the element.
Name two signs that a chemical reaction has taken place.
Any two: color change, gas produced (bubbles), precipitate (solid forms in a liquid), energy change (heat, light, or temperature change).
State the Law of Conservation of Mass.
Matter is not created or destroyed in a chemical reaction, so the total mass of the reactants equals the total mass of the products.
Classify each as an element, compound, or mixture: (a) oxygen gas (O₂),
(b) salad dressing,
(c) table salt (NaCl),
(a) Element: only oxygen atoms. (b) Mixture: different substances physically combined, not bonded. (c) Compound: two kinds of atoms chemically bonded in a fixed ratio.
Where are protons, neutrons, and electrons located in an atom?
Protons and neutrons are in the nucleus at the center. Electrons are in the space (electron cloud) outside the nucleus.
Sort these elements into metal, nonmetal, or metalloid: Na, Cl, Si, Fe.
Na: metal. Cl: nonmetal. Si: metalloid. Fe: metal.
Classify each change as physical or chemical: (a) crumpling paper, (b) burning wood, (c) ice melting, (d) iron rusting.
(a) Physical (shape). (b) Chemical. (c) Physical (state). (d) Chemical.
Is H₂ + O₂ → H₂O balanced? Explain, then write a balanced equation.
No. Left: 2 H and 2 O. Right: 2 H and 1 O. Balanced: 2H₂ + O₂ → 2H₂O (4 H and 2 O on each side).
Model A shows identical particles, each made of 1 red atom bonded to 2 white atoms.
Model B shows separate red atoms and white atoms scattered together, not bonded. Classify each model and explain how you know.
Model A is a compound: atoms of different elements are bonded in a fixed ratio and every particle is identical. Model B is a mixture (of two elements): the atoms are not bonded, and the amounts could vary.
An atom has 6 protons, 6 neutrons, and 6 electrons. Identify the element, give its atomic number, and explain why the atom is neutral.
Carbon. Atomic number = 6 (the number of protons). It is neutral because the 6 positive protons and 6 negative electrons cancel each other.
Data: Lithium reacts with water quickly, sodium reacts more vigorously, and potassium reacts explosively. Where do these elements sit on the periodic table, and what pattern does the data show?
They are in the same column (Group 1, the alkali metals). Elements in a group have similar properties, and reactivity increases going down the group. This shows the table’s columns organize elements by repeating patterns.
Baking soda is mixed with vinegar. Bubbles form and the mixture gets colder. Is this a physical or chemical change? Use two pieces of evidence.
Chemical change. Evidence: a gas (carbon dioxide) forms, and the temperature changes (an energy change). New substances are produced.
Vinegar and baking soda are sealed in a bag with a total mass of 120 g. After reacting, what is the mass? If the same reaction is done in an open beaker, the mass drops to 116 g. Explain the difference.
Closed bag: still 120 g (closed system, nothing escapes). Open beaker: carbon dioxide gas escaped into the air, so the beaker lost mass. The mass was not destroyed; it left the system.
A student claims, “Air is a compound because it contains oxygen and nitrogen.” Evaluate this claim and give a corrected explanation.
The claim is incorrect. Air is a mixture: its gases are not chemically bonded to each other, the proportions can vary, and the gases can be separated physically. Oxygen (O₂) and nitrogen (N₂) are themselves elements; carbon dioxide in air is a compound.
A student draws an atom with the electrons inside the nucleus and says most of the atom’s mass comes from the electrons. Identify two errors and explain how to fix the model.
Error 1: electrons are outside the nucleus in the electron cloud, so move them out. Error 2: nearly all the mass is in the nucleus because protons and neutrons are about 1,800 times more massive than electrons. The nucleus should be drawn as tiny and dense, with the atom mostly empty space.
Elements in the same group have similar chemical properties and tend to bond with similar elements. Elements in groups 1 and 17 tend to bond to each other, as do those is groups 2 and 16.
Boiling water and splitting water with electricity (electrolysis) both make bubbles. Explain how you can tell which is physical and which is chemical.
Boiling is physical: the bubbles are water vapor, still H₂O, only the state changed, and cooling returns liquid water. Electrolysis is chemical: the bubbles are hydrogen and oxygen gas, new substances with different properties, because atoms were rearranged into new particles.
Balance CH₄ + O₂ → CO₂ + H₂O and show the atom count. If 16 g of methane reacts completely with 64 g of oxygen and makes 44 g of carbon dioxide, how much water forms? Explain how this supports conservation of mass.
CH₄ + 2O₂ → CO₂ + 2H₂O. Each side has 1 C, 4 H, 4 O. Reactants: 16 + 64 = 80 g, so water = 80 − 44 = 36 g. Atoms are only rearranged, never lost, so mass in equals mass out.