Evidence vs Change
Give one observable sign that a chemical reaction has occurred (name any one)
formation of a gas (bubbling with odor), formation of a precipitate (solid forms), color change accompanied by other evidence, temperature change not caused by external heating/cooling, light emission.
In the reaction: H2+Cl2→HCl count the number of hydrogen atoms on the reactant side and the product side (before balancing).
Reactants: hydrogen atoms = 2 (in H2); Products: hydrogen atoms = 1 (in unbalanced HCl).
Does an exothermic reaction release or absorb heat? State which and one classroom example (e.g., combustion, hand warmers)
Exothermic releases heat. Example: combustion of a candle, many neutralization reactions, some hand warmers.
What is a prototype in engineering or design?
Prototype: an early model of a product built to test ideas and functions.
What element has the symbol H?
Hydrogen
Describe one observation that indicates a physical change rather than a chemical change.
change of state (melting, freezing), change in shape or size, phase change without new substance formation, reversible physical separation (dissolving that can be reversed).
Balance the equation: H2+Cl2→HCl Then state the number of chlorine atoms on each side after balancing.
Balanced: H2+Cl2→2HCl. Chlorine atoms: 2 on reactant side, 2 on product side.
Does an endothermic reaction release or absorb heat? State which and one classroom example (e.g., ice pack melting).
Endothermic absorbs heat. Example: instant cold packs (when ammonium nitrate dissolves), photosynthesis (requires sunlight).
Define "criteria" in the context of a design challenge.
Criteria: measurable goals the design must achieve
What element has the symbol N?
Nitrogen
True or false: A change in color alone always proves a chemical reaction has occurred. Explain briefly.
False. Color change can indicate a physical change (e.g., concentration change, dilution) or chemical change; additional evidence is needed.
A reaction shows 4 nitrogen atoms on the reactant side. After balancing, how many nitrogen atoms must appear on the product side? Explain the conservation principle briefly.
By conservation of atoms, there must be 4 nitrogen atoms on the product side as well; atoms are neither created nor destroyed in chemical reactions.
Describe one quick way to test whether a reaction is exothermic or endothermic using temperature measurements.
Measure temperature of the reaction mixture before and during reaction with a thermometer; an increase indicates exothermic, a decrease indicates endothermic.
Define "constraints" in a design task and give one example.
Constraints: limits or restrictions (e.g., budget, time, available materials, safety). Example: must cost under $20.
What element has the symbol Cu?
Copper
List two pieces of evidence you would look for to decide whether bubbling indicates a chemical reaction or just a physical change (like boiling).
Check for temperature change (not caused by heating), presence of new substance (precipitate), odor change, permanence (irreversible), or confirm gas identity (e.g., test for CO2). If boiling, temperature will be at boiling point and reversible.
For the balanced reaction 2Al+3Cl2→2AlCl3, how many chlorine atoms are present in the products altogether?
6
A chemical reaction container’s temperature increases during the reaction. Explain in terms of energy flow whether the reaction is endothermic or exothermic and why.
Temperature increase — exothermic — because energy is released from chemical bonds into the surroundings raising temperature.
When evaluating two prototype designs that meet the same criteria, name two reasons you might prefer one over the other that relate to constraints.
Examples: one design may be cheaper (cost constraint) while the other is more durable (performance criterion); you might prefer durability if safety is critical, or cost if budget is tight. You might prefer the smaller size if space is limited.
What element has the symbol Na?
Sodium
A substance decomposes into a solid residue and a gas with a strong odor. Explain why this is stronger evidence of a chemical reaction than simply a change in shape.
Formation of a new solid and a gas with odor usually indicates chemical bond rearrangement producing new substances — stronger evidence than physical distortion.
Given the unbalanced equation C3H8+O2→CO2+H2O, what is the coefficient for CO2 when the equation is balanced?
Balanced equation is C3H8+5O2→3CO2+4H2O so coefficient for CO2 is 3.
Explain how bond breaking and bond making relate to whether a reaction is endothermic or exothermic (which step absorbs energy, which releases).
Breaking bonds requires energy input (endothermic); forming new bonds releases energy (exothermic).
Describe a short example of a hand warmer design problem related to safely containing an exothermic chemical reaction. Include one criterion and one constraint.
Must cause exothermic reaction and keep maximum temperature below a safe limit. Constraint: use only materials available in the school budget, activate on command and be safe for the user.
What element has the symbol Mg?
Magnesium