Reactants & Products
What do we call the substances that are present before a chemical reaction starts?
Reactants.
Name one visible sign that a chemical reaction may be occurring.
Example: gas bubbles.
When balancing chemical equations, what must be equal on both sides of the arrow?
The number and type of atoms.
Which type of reaction joins two or more substances to make a single product? Give the general form (use A, B, AB).
Synthesis. General: A + B >> AB
What do we call a reaction that releases heat to its surroundings? (one word)
Exothermic.
In a chemical reaction, are the properties of reactants the same as the properties of products? Answer "Yes" or "No" and explain briefly.
No. Reactants and products have different properties because atoms are rearranged to make new substances.
Besides gas bubbles, name two other signs that a chemical reaction is taking place.
Examples: release of energy (light/thermal/electricity), formation of a precipitate, color change.
If you start with 2 hydrogen atoms in the reactants, how many hydrogen atoms must appear in the products?
2 hydrogen atoms.
Which reaction type splits a compound into two or more simpler substances? Give the general form.
Decomposition. General: AB >> A + B
What do we call a reaction that absorbs heat from its surroundings? (one word)
Endothermic.
Write the word that describes substances produced by a chemical reaction.
Products.
A reaction gives off light and heat. Which type of energy change is this: endothermic or exothermic? Explain in one sentence.
Exothermic — it releases heat and light to the surroundings.
Balance this equation: H2 + O2 → H2O. (Write the balanced equation.)
Balanced: 2H2 + O2 >> 2H2O
What type of reaction is this: Zn + CuCl2 → ZnCl2 + Cu? Name the type and give the general form.
Single replacement (single displacement). General: AB + C >> AC + B
Give one real-life example of a chemical reaction that gives off heat and one that absorbs heat (from the unit text).
Exothermic example: burning wood (releases heat). Endothermic example: cold packs that absorb heat.
True or false: Every atom in the reactants becomes part of the products. Explain your answer in one sentence.
True. Atoms are rearranged, not created or destroyed.
A reaction makes the area around it colder. Is this endothermic or exothermic? Give one real-life example from the unit.
Endothermic — example: cold pack absorbs heat from surroundings.
Balance this equation: Al + O2 → Al2O3. (Write the balanced equation.)
Balanced: 4Al + 3O2 >> 2Al2O3
Identify the reaction type: BaCl2 + Na2SO4 → 2NaCl + BaSO4. Explain how you recognized it.
Double replacement (double displacement). I recognized two compounds swapping partners to form new products,
Explain briefly why breaking and forming chemical bonds involves energy changes.
Because breaking bonds requires energy input and forming bonds releases energy; the net difference appears as heat absorbed or released.
Give a short description of the Law of Conservation of Mass as it applies to chemical reactions.
Mass is neither created nor destroyed; the total mass of reactants equals the total mass of products.
Describe two different observable signs that would let you know a chemical reaction occurred in an experiment and explain why each indicates a reaction.
Example answers: gas bubbles show a new gas forms; temperature change shows energy transfer as bonds break/form.
Explain, in one or two sentences, why balancing equations follows the Law of Conservation of Mass.
Because atoms are neither created nor destroyed, equations must show equal counts of each element on both sides.
For each of these equations, name the reaction type: a) 2 HgO → 2 Hg + O2
b) C3H6O + 4 O2 → 3 CO2 + 3 H2O
c) P4 + 5 O2 → P4O10
a) Decomposition.
b) Combustion (products CO2 and H2O).
c) Synthesis.
Design challenge question: Describe a simple classroom-safe design (one paragraph) for a project that either releases or absorbs thermal energy using a chemical reaction (name which type—hot pack or cold pack—and explain how students could show energy change safely).
Look for clear identification of hot pack (exothermic) or cold pack (endothermic), safe materials listed, steps to mix components safely, and a way to measure temperature change.