Matter & Evidence
Name one observable sign (other than temperature change) that might suggest a chemical reaction has occurred.
Examples: color change, smell, bubbling, formation of a precipitate, temperature change
What does it mean for a chemical equation to be balanced?
Balanced means same number of each type of atom on both sides of the arrow.
What is a synthesis reaction? Give a short example in words
Synthesis: two or more reactants form one product (e.g., metal + oxygen → metal oxide).
What does combustion mean?
Combustion means burning — a reaction that usually produces heat and light and uses oxygen.
Boiling water is an example of a chemical change.
False — boiling is a physical change (phase change).
Explain why bubbling (formation of gas) can be evidence of a chemical change.
Gas formation (bubbles) indicates new substances (a product gas) formed.
Balance this word-style equation in simple number form: Hydrogen gas + Oxygen gas → Water.
Balanced coefficients are 2,1,2
2H2 + 1O2 → 2H2O
What is a decomposition reaction? Give a short classroom-friendly example in words.
Decomposition: one reactant breaks into two or more products (e.g., a compound heated into simpler substances).
True or false? Combustion always needs oxygen.
True — combustion requires oxygen
Substances produced as a result of a chemical reaction are called reactants.
False — products are the substances formed; reactants are the starting substances.
A mixture of two clear liquids turns cloudy and gives off a smell. Which two pieces of evidence from the board support that a chemical change happened? (Give two.
Cloudiness (precipitate) and smell (new substance); could also mention gas or temperature
If the unbalanced equation is Fe+O2→Fe2O3, what smallest whole-number coefficients balance it?
Coefficients: 4,3,2 for Fe+O2→Fe2O3 gives 4Fe+3O2→2Fe2O3
Explain what happens in a single-replacement reaction and give a simple word example.
Single-replacement: one element replaces another in a compound (e.g., element + compound → new element + new compound).
Oxidation _________ electrons
Losing
Synthesis reactions always have a triple product.
False — synthesis reactions combine reactants into one product.
Describe how you could tell the difference between a physical change and a chemical change when water boils versus when metal rusts.
Boiling water = physical change (phase change); rusting = chemical change (new substance iron oxide forms).
A student writes H2+Cl2→HCl. Is this balanced? If not, give balanced coefficients.
Not balanced as written; balanced is 1,1,2 for H2+Cl2→2HCl
Describe a double-replacement reaction in words and name one situation where one might see such a reaction
Double-replacement: ions in two compounds swap to make two new compounds (e.g., precipitation reactions in solutions).
Give an example of a redox situation you might observe
Example: Rusting — iron reacts with oxygen; iron is oxidized (loses electrons or gains oxygen
A color change always guarantees a chemical reaction has taken place.
False — color change can be caused by physical changes or mixtures; not guaranteed but is evidence to consider.
Example: food coloring
Give a classroom-safe example of a chemical reaction, list the reactants and products in simple words, and name two pieces of evidence that a chemical change occurred.
Example: vinegar + baking soda → carbon dioxide gas + salt + water; reactants = vinegar (acid) and baking soda; products = salt, water, carbon dioxide; evidence: bubbling (gas), temperature change, or new substance.
Explain, in one or two sentences, why you cannot change the chemical formulas of substances to balance an equation and what you are allowed to change instead.
You may only change coefficients (the numbers in front of formulas) to balance; you cannot change subscripts in formulas because that changes the substances.
Compare and contrast combustion and synthesis reactions — describe one key difference and one similarity in simple terms.
Combustion involves burning with oxygen and releases energy; synthesis joins substances to form one product — similarity: both make new substances.
Explain why combustion reactions often give off hea
Combustion releases energy because bonds in fuel and oxygen rearrange to form stronger bonds in products, releasing energy
Endothermic reactions create energy.
False— endothermic reactions absorb energy (not create it); exothermic reactions release energy.