Chemical Reactions, Acids and Bases Lesson Test

Answer: B

Correct answer: Water and oxygen gas. Products are written after the arrow. Here the arrow points to ${\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{O}}_2$, so water and oxygen gas are the products, and hydrogen peroxide is the reactant.

• A: This mixes the starting substance with one of the products; anything before the arrow is a reactant.
• B (correct): These formulas appear after the arrow, so they are the new substances formed.
• C: Hydrogen peroxide is written before the arrow, which makes it the reactant, not a product.
• D: No hydrogen gas appears in the equation; the hydrogen ends up inside the water molecules.

Answer: D

Correct answer: Freezing fruit juice into ice pops. A physical change alters only form or state. After freezing, the juice is still the same substance, just solid. Toasting, burning and rusting all produce new substances.

• A: Browning forms new compounds with new flavors and colors, which makes it a chemical change.
• B: Rust is iron oxide, a new compound, so rusting is a chemical change.
• C: Combustion turns propane and oxygen into carbon dioxide and water, which are new substances.
• D (correct): Frozen juice is still the same juice in solid form, so only its state changed.

Answer: C

Correct answer: It turns red. Litmus is an indicator dye that changes color with acidity: acids turn blue litmus red, and bases turn red litmus blue. Lemon juice contains citric acid and has a pH of about 2, so the blue paper turns red. A common memory aid is BRA: blue to red means acid. Other indicators use colors such as green or pink, but litmus shows only red or blue.

• A: Blue litmus stays blue only in a neutral or basic liquid, and lemon juice is acidic.
• B: Green is the neutral color on universal indicator, not a litmus result.
• C (correct): Lemon juice contains citric acid, and acids turn blue litmus red.
• D: Pink is the color phenolphthalein shows in a fairly basic solution, not a litmus color for acid.

Answer: A

Correct answer: It speeds up a reaction without being used up. A catalyst provides a reaction pathway with lower activation energy, so the reaction goes faster. It is regenerated by the end and does not change the final amount of product.

• A (correct): A catalyst opens a lower-energy pathway and is regenerated at the end, so it can be reused.
• B: A catalyst changes only how fast equilibrium or completion is reached, not the final yield.
• C: A catalyst may take part in intermediate steps but is regenerated, so it is not consumed overall.
• D: This is backward; a catalyst lowers the activation energy barrier.

Answer: A

Correct answer: Acidic. The pH scale tracks hydrogen-ion activity, and at about 25°C the value 7 marks neutral, like pure water. Values below 7 are acidic and values above 7 are basic, with each whole step representing a tenfold change. A pH of 3 is four units below neutral, roughly the acidity of vinegar or many soft drinks. The number describes acidity only; it says nothing about whether a solution is safe to touch or drink.

• A (correct): pH 3 is below the neutral reference near pH 7.
• B: A pH value does not guarantee safety.
• C: Neutral pure water is near pH 7 at 25°C.
• D: Basic solutions generally have pH above 7.

Answer: C

Correct answer: Endothermic, absorbing heat from its surroundings. When the pack feels cold, heat is moving from the surroundings, including the hand, into the reaction. A process that absorbs energy from its surroundings is endothermic.

• A: Cold is not a substance that flows out; the pack feels cold because heat flows into it.
• B: An exothermic process would make the pack feel warm, like a hand warmer.
• C (correct): Heat flows from her hand into the pack, which is why the pack feels cold.
• D: This mixes the terms; exothermic means heat is released, not absorbed.

Answer: B

Correct answer: Hydroxide ion, OH⁻. A base increases the concentration of hydroxide ions, $\mathrm{O}{\mathrm{H}}^{-}$, in water, or removes hydrogen ions. That is why bases have a pH above 7.

• A: Carbonate is only one specific ion; the general feature of a base is an increase in hydroxide.
• B (correct): Bases raise the hydroxide-ion level in water, which pushes the pH above 7.
• C: Chloride comes from compounds such as hydrochloric acid or table salt; it is not what defines a base.
• D: Raising hydronium is what acids do, and it lowers the pH.

Answer: B

Correct answer: A salt and water. In neutralization,

The remaining positive ion from the base and negative ion from the acid make up a salt, so the products are a salt and water.

• A: Neutralization does not release oxygen; the acid and base simply use each other up to form salt and water.
• B (correct): Hydrogen ions from the acid join hydroxide ions from the base to make water, and the leftover ions form a salt.
• C: Hydrogen gas forms when an acid reacts with a reactive metal, not with a base.
• D: Carbon dioxide forms when an acid reacts with a carbonate, not in an ordinary acid-base neutralization.

Answer: D

Correct answer: 2. On the pH scale, lower numbers mean more hydrogen ions and stronger acidity, because pH is the negative logarithm of hydrogen-ion activity. That inverse relationship is the common trap: a bigger number feels like more acid, but it means less. Each unit is a factor of ten, so pH 2 is about 1,000 times more acidic than pH 5. Values above 7, such as 10, are basic, and 7 is roughly neutral.

• A: pH 7 is approximately neutral.
• B: pH 5 is acidic but less so than pH 2.
• C: pH 10 is basic.
• D (correct): The lowest pH has the highest hydrogen-ion activity among these values.

Answer: D

Correct answer: Synthesis. The equation has two reactants and only one product. Several reactants joining into one product is the shape of a synthesis, or combination, reaction.

• A: No element replaces another element in a compound; two compounds simply join.
• B: Decomposition starts with one reactant and makes several products, the reverse of what this equation shows.
• C: Combustion is a fuel reacting with oxygen gas to release heat and usually light, and no oxygen gas is a reactant here.
• D (correct): Two reactants combine into a single product, which is the pattern A plus B gives AB.

Answer: B

Correct answer: 4Al + 3O_2 → 2Al_2O_3. Oxygen goes from 2 per molecule to 3 per formula unit, so the smallest common count is 6: use $3{\mathrm{O}}_2$ and $2A{\mathrm{l}}_2{\mathrm{O}}_3$. That gives 4 aluminum atoms on the right, so place 4 in front of Al. Each side then has 4 Al and 6 O.

• A: There are 2 oxygen atoms on the left but 3 on the right, so oxygen is not balanced.
• B (correct): Both sides have 4 aluminum atoms and 6 oxygen atoms, using only coefficients.
• C: Aluminum matches, but there are 6 oxygen atoms on the left and only 3 on the right.
• D: This changes the product's subscripts, describing a different substance instead of aluminum oxide.

Answer: D

Correct answer: 12. The coefficients give 2 ethane to 4 carbon dioxide. Six ethane molecules is 3 times the equation's 2, so multiply 4 by 3.

• A: This is the number of water molecules formed, using the 2 to 6 ratio instead of 2 to 4.
• B: This assumes a 1 to 1 ratio and ignores the coefficients 2 and 4.
• C: This is the number of oxygen molecules used, from the 2 to 7 ratio, not the carbon dioxide formed.
• D (correct): Ethane and carbon dioxide are in a 2 to 4 ratio, so 6 ethane molecules give 12 carbon dioxide molecules.

Answer: C

Correct answer: 7 g. In a sealed container total mass is conserved. Reactants total

so products also total 37 g. The gas is

• A: This subtracts one reactant from the other, which has no meaning for conservation of mass.
• B: This subtracts only the second reactant from the solid instead of comparing total reactants with total products.
• C (correct): The total product mass must equal 25 + 12 = 37 g, and 37 minus 30 leaves 7 g for the gas.
• D: This is the total mass of all products, not the mass of the gas alone.

Answer: A

Correct answer: 100 times. The pH scale is logarithmic: each one-unit drop means ten times more hydrogen-ion activity. Count the units between the values and multiply by ten once for each unit. From pH 6 to pH 4 is two units, so the factor is 10 × 10. The tempting error is to treat pH as a linear scale and conclude that a difference of 2 means twice as acidic. The lower pH is always the more acidic side.

• A (correct): Two pH units give $10^2$ = 100.
• B: The scale is logarithmic, not a simple twofold increase.
• C: One thousandfold applies to three pH units.
• D: Tenfold applies to one pH unit.

Answer: D

Correct answer: Oxygen atoms from the air combined with the iron. Burning steel wool is iron reacting with oxygen. Oxygen atoms from the air become part of the solid iron oxide, so the product weighs more. Counting the oxygen used, total mass is conserved.

• A: Reactions rearrange atoms; they never create new ones.
• B: Escaping gas would make the sample lighter, not heavier.
• C: An atom's mass does not change in a chemical reaction; the extra mass comes from added atoms.
• D (correct): The iron oxide includes oxygen taken from the air, so the solid gains that oxygen's mass.

Answer: A

Correct answer: About 100 times greater. The difference is

pH units.

The solution with the lower pH, baking soda at 9, has about 100 times the hydrogen-ion level of the ammonia cleaner.

• A (correct): The pH values differ by 2, and the lower pH has more hydrogen ions, so the factor is 10 squared, or 100.
• B: The direction is reversed; the lower pH always has the higher hydrogen-ion level.
• C: This multiplies 10 by the difference of 2 instead of raising 10 to the power 2.
• D: This treats pH as a linear scale; each pH unit is really a tenfold change.

Answer: A

Correct answer: 8. Divide each amount by its coefficient: nitrogen

and hydrogen

Hydrogen gives the smaller number, so it is limiting and only 4 sets can react. Each set forms 2 ammonia molecules, so

• A (correct): Hydrogen is limiting: 12 divided by 3 is 4 sets of the reaction, and each set makes 2 ammonia molecules.
• B: This adds the two reactant counts together, ignoring the ratio in the equation.
• C: This assumes all 6 nitrogen molecules react, but there is not enough hydrogen for that.
• D: This is the number of reaction sets, not the number of ammonia molecules, which is twice that.

Answer: C

Correct answer: The liquid is not fairly basic; it could be acidic or neutral. Phenolphthalein turns pink only in a fairly basic solution and is colorless in acids and neutral solutions. A colorless result therefore rules out a fairly basic liquid but cannot tell acidic from neutral.

• A: A neutral liquid would also leave phenolphthalein colorless, so acidity is not proven.
• B: An indicator gives only a rough range, and colorless also fits acidic liquids, so an exact neutral pH cannot be concluded.
• C (correct): Phenolphthalein stays colorless in acids and neutral solutions, so the result rules out only a fairly basic liquid.
• D: A fairly basic liquid would turn phenolphthalein pink, which did not happen.

Answer: C

Correct answer: It releases only a small fraction of its hydrogen ions in water. Acid strength is how completely the acid ionizes in water. A strong acid releases nearly all of its hydrogen ions, while a weak acid, such as acetic acid, releases only a small fraction. How much water is present is a separate idea, concentration.

• A: Any acid solution has a pH below 7; a pH above 7 is basic.
• B: That describes a dilute solution, which is about concentration, not strength.
• C (correct): Strength describes how completely an acid ionizes, and a weak acid ionizes only partly.
• D: Weak acids such as vinegar still react with and neutralize bases, forming a salt and water.

Answer: B

Correct answer: Use zinc powder instead of one lump of zinc. Rate rises when there are more effective collisions. Grinding the zinc into powder increases the surface area exposed to the acid, so the reaction speeds up. Cooling and diluting would slow it down.

• A: A bigger container does not change temperature, concentration or surface area, so the rate stays about the same.
• B (correct): Powder exposes far more surface area, so more acid particles can collide with the zinc at once.
• C: Dilution lowers concentration, so collisions happen less often and the reaction slows.
• D: Lowering the temperature slows particles and reduces effective collisions, so the rate drops.

Answer: A

Correct answer: Friction supplies the activation energy needed to start the reaction. Every reaction has an activation energy, a minimum energy colliding particles need. Striking the match gives enough energy through friction to get over that barrier, and then the heat released keeps the reaction going.

• A (correct): The reaction needs an initial push over the energy barrier, and the heat from friction provides it.
• B: The energy is already stored in the chemical bonds; striking only starts the reaction.
• C: Burning releases heat, so it is exothermic; once started, it keeps itself going.
• D: Rubbing on the box does not add oxygen; it provides the heat needed to begin the reaction.

Answer: A

Correct answer: It is oxidized, because it loses electrons. A 2+ charge means each magnesium atom gave up two electrons. By OIL RIG, oxidation is loss of electrons, so magnesium is oxidized while the oxygen that gains the electrons is reduced.

• A (correct): Forming a positive ion means losing electrons, and loss of electrons is oxidation.
• B: Losing electrons is oxidation; reduction is the gain of electrons.
• C: A gain of electrons would make a negative ion and is called reduction.
• D: Magnesium becomes positive, which shows it lost electrons rather than gaining them.

Answer: D

Correct answer: Water, because it accepts a proton. Hydrogen chloride gives a proton (hydrogen ion) to water, so it is the acid. Water takes that proton and becomes hydronium, so water is the proton acceptor, which makes it the base.

• A: Water ends up with an extra hydrogen ion, so it received a proton rather than giving one away.
• B: Hydrogen chloride loses its hydrogen ion and becomes chloride, so it did not accept a proton.
• C: Donating a proton is what an acid does, so this describes the acid in the reaction, not the base.
• D (correct): Water gains a hydrogen ion to become hydronium, and a proton acceptor is a base.

Answer: B

Correct answer: Double displacement. Barium trades its chloride partner for sulfate, and sodium trades sulfate for chloride. Two compounds exchanging partners is a double displacement reaction, and the white solid barium sulfate is the precipitate.

• A: Decomposition breaks one compound apart, but this reaction starts with two compounds.
• B (correct): Two compounds swap partners, AB plus CD giving AD plus CB, and one new product is a solid precipitate.
• C: Single displacement needs a free element replacing another, but both reactants here are compounds.
• D: Synthesis forms one product from several reactants, but this reaction makes two products.

Answer: D

Correct answer: The rock contains carbonate minerals that release carbon dioxide. Fizzing on a rock is the classic sign of carbonate minerals, such as those in limestone or marble. The acid reacts with the carbonate to produce carbon dioxide gas.

• A: A rock at ordinary temperature cannot boil the acid; the bubbles come from a new gas.
• B: Gold does not react with dilute acid, and acid reactions with rock do not release oxygen.
• C: Silver is a less reactive metal that does not release hydrogen from dilute acid.
• D (correct): Acids react with carbonates to form carbon dioxide gas, water and a salt, which causes the fizzing.

Answer: B

Correct answer: Magnesium. Fairly reactive metals such as zinc, magnesium and iron react with dilute acids:

Copper, silver and gold are less reactive and show no bubbles.

• A: Silver, like copper and gold, does not release hydrogen from dilute hydrochloric acid.
• B (correct): Magnesium is a fairly reactive metal, and it reacts with the acid to form a salt and hydrogen gas.
• C: Gold is very unreactive and does not corrode or bubble in dilute acid.
• D: Copper is too unreactive to release hydrogen from dilute hydrochloric acid, so the wire shows no bubbles.

Answer: C

Correct answer: It rises toward 7 but stays below 7. Adding water lowers the concentration of hydrogen ions, so the pH goes up. The solution becomes closer to neutral, but since only neutral water is added, the pH approaches 7 without passing it.

• A: The amount of acid stays the same, but it is spread through a larger volume, so the concentration and pH change.
• B: Pure water is neutral, so diluting an acid with it cannot push the pH past 7.
• C (correct): Dilution spreads the same acid through more volume, lowering hydrogen-ion level, but water alone cannot make it basic.
• D: A falling pH would mean more hydrogen ions per liter, but adding water lowers that level.

Answer: A

Correct answer: Calcium nitrate and water. Acid plus base gives salt plus water. Combine the metal ion of the base (calcium) with the negative ion of the acid (nitrate) to get calcium nitrate, and the hydrogen and hydroxide ions form water:

• A (correct): The metal ion from the base, calcium, pairs with the nitrate from the acid, and the hydrogen and hydroxide ions form water.
• B: Hydrogen gas forms when an acid reacts with a metal, not with a hydroxide base.
• C: Calcium hydroxide is a starting substance, and neutralization does not release nitrogen gas.
• D: Nitric acid is a reactant that gets used up, and no oxide forms in neutralization.

Answer: B

Correct answer: Tests show a solid formed with a formula different from either liquid. Heat, bubbles and color are clues that a simple physical change could also explain. Showing that a new substance with a different chemical formula has formed is the strongest evidence of a reaction.

• A: Bubbles can come from trapped air or warming, which are physical changes, so they are not proof.
• B (correct): Identifying a new substance with a different composition is the most direct proof of a chemical change.
• C: Dye can tint a liquid without any reaction, so the color change says nothing about new substances.
• D: A temperature change is only a clue, since mixing or dissolving can also change temperature.

Answer: D

Correct answer: A buffer resists moderate pH change but can be overwhelmed. A buffer contains a weak acid together with its conjugate base, so one component neutralizes added base and the other neutralizes added acid. That keeps pH nearly steady at whatever value the particular pair sets, not necessarily 7. The protection is limited: once added acid or base uses up one component, the pH shifts sharply. Blood is a real example, held near pH 7.4 by carbonic acid and bicarbonate.

• A: Hydrogen atoms are conserved, not destroyed; a buffer only binds or releases hydrogen ions.
• B: Buffers may operate at many pH values.
• C: Many buffers contain ions, such as the conjugate base of a weak acid.
• D (correct): Buffers resist limited additions of acid or base.

Answer: C

Correct answer: Carbon monoxide runs out; 8 carbon dioxide molecules form. Divide each amount by its coefficient: carbon monoxide

and oxygen

The smaller result, 4, means carbon monoxide is limiting. Four sets of the reaction form

carbon dioxide molecules, and 2 oxygen molecules are left over.

• A: The limiting reactant is right, but 4 is the number of reaction sets; each set makes 2 carbon dioxide.
• B: This assumes all 6 oxygen molecules react, which would need 12 carbon monoxide molecules, but only 8 are present.
• C (correct): 8 ÷ 2 = 4 is smaller than 6 ÷ 1 = 6, so carbon monoxide limits, and 4 sets each make 2 carbon dioxide.
• D: Comparing raw counts (6 is less than 8) ignores the coefficients; oxygen is actually in excess.

Answer: C

Correct answer: 1.6 g. Magnesium plus oxygen gives magnesium oxide, and mass is conserved when every substance is counted. The oxygen that joined is the product mass minus the magnesium mass:

• A: Conservation applies to all substances, including oxygen taken from the air, which is why the solid gained mass.
• B: This is the mass of the whole oxide product, not just the oxygen part of it.
• C (correct): The oxide holds all the magnesium plus the oxygen that joined it, so 4.0 − 2.4 = 1.6 g of oxygen.
• D: This adds the two masses, but the 4.0 g oxide already contains the 2.4 g of magnesium.

Answer: A

Correct answer: 1,000 times higher. Because pH is a base-10 logarithmic scale, each unit represents a tenfold change in hydrogen-ion activity. Going from pH 5 to pH 2 is three steps, so multiply by ten three times: 10 × 10 × 10. Since pH 2 is the lower value, it is the more acidic sample. Two common errors are using the plain difference, 3, as the ratio and multiplying the difference by ten to get 30.

• A (correct): A three-unit difference gives $10^3$ = 1,000; lower pH is more acidic.
• B: Subtracting pH values is not the activity ratio.
• C: Multiplying the difference by ten is not how logarithms work.
• D: The direction is reversed.

Answer: C

Correct answer: It rose about 1,000 times; the lake is now too acidic for many fish. The pH dropped by

units.

So the lake has about 1,000 times more hydrogen ions. At pH 4 it is well below the roughly 6.5 to 8.5 range that most fish need.

• A: This treats pH as a linear scale; each unit is a tenfold change, and pH 4 is well outside the healthy range.
• B: A lower pH means more hydrogen ions and a more acidic lake, not a more basic one.
• C (correct): A 3-unit drop means 10 cubed, or 1,000 times more hydrogen ions, and pH 4 is far below the 6.5 to 8.5 range most fish need.
• D: The factor is right, but pH 4 is far below the roughly 6.5 to 8.5 range where most aquatic life thrives.

Answer: B

Correct answer: Y has the higher hydrogen-ion level, even though hydrochloric acid is the stronger acid. The lower pH always has the higher hydrogen-ion level, so Y, at about pH 2, has roughly 10 times more than X, at pH 3. Hydrochloric acid is still the stronger acid because it ionizes almost completely; Y's acidity comes from a high concentration of a weak acid.

• A: Strength describes how completely an acid ionizes, but X is dilute, and its higher pH shows fewer hydrogen ions.
• B (correct): Y's lower pH shows more hydrogen ions, which a large amount of weak acid can supply; strength and concentration are separate ideas.
• C: Acetic acid is a weak acid that ionizes only partly; Y's low pH comes from its high concentration.
• D: Their pH values differ by 1 unit, which is a tenfold difference in hydrogen-ion level.

Answer: D

Correct answer: 13. One butane gives 4 carbon dioxide and 5 water, holding

oxygen atoms, which would need 6.5 oxygen molecules. To use whole numbers, double everything:

Each side then has 8 carbon, 20 hydrogen and 26 oxygen atoms.

• A: This is the number of oxygen atoms needed, not the number of two-atom oxygen molecules.
• B: This adds the product molecules 4 + 5 instead of counting the oxygen atoms they contain.
• C: This balances one butane, but 6.5 is not a whole number, so every coefficient must be doubled.
• D (correct): With 2 butane, the right side needs 8 carbon dioxide and 10 water, holding 26 oxygen atoms, which is 13 oxygen molecules.

Answer: A

Correct answer: P is exothermic and slow, while Q is endothermic and fast. Releasing heat makes P exothermic, and absorbing heat makes Q endothermic. How fast each goes depends on activation energy, temperature, concentration, surface area and catalysts, not on whether energy is released, so P is slow and Q is fast.

• A (correct): Energy change and rate are separate properties, so a reaction can release heat slowly or absorb heat quickly.
• B: Q absorbs heat, which makes it endothermic; speed alone does not tell whether energy is released.
• C: P gives off heat, which makes it exothermic; slowness comes from conditions such as activation energy, not the energy balance.
• D: A catalyst speeds a reaction but does not change the overall energy released or the final yield.

Answer: B

Correct answer: Acidic, because 1 sulfuric acid unit is left unreacted. The ratio is 1 sulfuric acid to 2 sodium hydroxide. The 8 sodium hydroxide units can neutralize

acid units. That leaves

sulfuric acid unit, so the mixture is acidic with a pH below 7.

• A: A mixture is neutral only when the amounts match the ratio; here acid is left over.
• B (correct): The 8 hydroxide units neutralize only 4 acid units at the 1 to 2 ratio, so 1 acid unit remains and the pH stays below 7.
• C: This subtracts 5 from 8 as if the ratio were 1 to 1, ignoring that each acid unit uses 2 base units.
• D: Each acid unit needs 2 base units, so 8 base units are not enough for all 5 acid units.

Answer: D

Correct answer: Exothermic, releasing 350 kJ overall. Breaking bonds absorbs energy and forming bonds releases it. The net energy released is

Because more energy is released than absorbed, the reaction is exothermic.

• A: This adds the two amounts, but bond breaking absorbs energy, so it must be subtracted.
• B: The direction is reversed; the reaction would be endothermic only if breaking bonds took more energy than forming them released.
• C: The 1,850 kJ is released when bonds form, not absorbed, and the net change is the difference of the two.
• D (correct): Forming bonds releases 350 kJ more than breaking bonds required, so the extra energy goes to the surroundings.

Answer: C

Correct answer: The buffered sample can undergo a smaller pH decrease than plain water. Pure water has nothing to absorb added hydrogen ions, so even a few drops of acid can drop its pH by several units. A buffer holds a base component, such as the conjugate base of a weak acid, that reacts with incoming hydrogen ions and ties them up in a weak acid. Equal starting pH says nothing about this capacity, which comes from the buffer's dissolved components.

• A: Adding acid does not usually create a strong base.
• B: Salt presence alone does not determine an initial greater acidity at equal pH.
• C (correct): Buffer components can react with some added acid and moderate pH shift.
• D: Initial pH does not establish equal buffer capacity.

Answer: D

Correct answer: Single displacement. Look at the shape of the equation: an element plus a compound gives a different element plus a different compound. Chlorine replaces bromine beside sodium, so this is single displacement.

• A: Synthesis has several reactants joining into one product, but here two products form.
• B: Decomposition starts from a single compound, but this reaction has two reactants.
• C: Double displacement needs two compounds swapping partners; chlorine gas here is an element, not a compound.
• D (correct): An element (chlorine) takes the place of another element (bromine) in a compound, which is the A + BC gives AC + B pattern.

Answer: D

Correct answer: 6 iron oxide units form; 3 oxygen molecules are left over. Divide each amount by its coefficient: iron

oxygen

The smaller result, iron, is limiting, so the reaction runs 3 times: 3 x 2 = 6 iron oxide units form and 3 x 3 = 9 oxygen molecules are used, leaving

• A: This treats oxygen as limiting (12 divided by 3, times 2), but 12 oxygen molecules are more than the 9 the iron can use.
• B: This assumes a 1 to 1 ratio and ignores the coefficients 4, 3 and 2.
• C: The amount of iron oxide is right, but iron uses only 9 of the 12 oxygen molecules, so 3 remain.
• D (correct): Iron is limiting: 12 iron atoms make 6 iron oxide units and use 9 oxygen molecules, leaving 3.

Answer: B

Correct answer: Using a few large lumps of marble with the same total mass. Rate depends on how often particles collide effectively. Temperature, concentration and stirring all raise the collision rate, while replacing small chips with large lumps reduces the exposed surface area, so the reaction slows.

• A: Higher temperature makes particles collide more often and with more energy, so the reaction speeds up.
• B (correct): Large lumps expose less surface area than many small chips, so fewer acid particles can collide with the solid at once.
• C: More acid particles per unit volume means more frequent collisions, which speeds the reaction up.
• D: Stirring brings fresh acid to the surface of the marble, which speeds most reactions in solution.

Answer: D

Correct answer: It is dissolved in water. State labels give the physical state: (s) solid, (l) liquid, (g) gas and (aq) aqueous. Aqueous means dissolved in water, so the silver nitrate is in solution.

• A: The label (aq) describes physical state, not acidity; silver nitrate is a salt, not an acid.
• B: A gas is labeled (g), not (aq).
• C: A pure liquid is labeled (l); (aq) means the substance is dissolved in water.
• D (correct): The label (aq) stands for aqueous, meaning the substance is in a water solution.

Answer: A

Correct answer: Ammonia, because it accepts a proton. Follow the hydrogen ion: $\mathrm{HN}{\mathrm{O}}_3$ loses one and becomes $\mathrm{N}{\mathrm{O}}_3^{-}$, while $\mathrm{N}{\mathrm{H}}_3$ gains one and becomes $\mathrm{N}{\mathrm{H}}_4^{+}$. The proton donor is the acid and the proton acceptor is the base, so ammonia is the base.

• A (correct): Ammonia gains a hydrogen ion to become ammonium, and a proton acceptor is a base.
• B: Ammonia gains a hydrogen ion here; donating a proton would make it an acid.
• C: Nitric acid loses its hydrogen ion to become nitrate, so it gives a proton rather than accepting one.
• D: Nitric acid does donate a proton, but that makes it the acid in this reaction, not the base.

Answer: B

Correct answer: Magnesium chloride and water. Acid plus base gives a salt plus water. Combine the metal ion from the base (magnesium) with the negative ion from the acid (chloride) to get magnesium chloride, and

supplies the water.

• A: Hydrogen gas forms when an acid reacts with a reactive metal, not with a metal hydroxide.
• B (correct): The acid's hydrogen ions join the base's hydroxide ions to make water, and magnesium ions pair with chloride ions as the salt.
• C: Hydrogen chloride is the acid that is used up, so it cannot be the product of the reaction.
• D: Neutralization makes a salt and water; it does not release chlorine gas or form an oxide.

Answer: A

Correct answer: Logs burning in a campfire. An exothermic process releases energy and warms its surroundings. Burning wood gives off heat and light, while the cold pack, photosynthesis and heated limestone all take energy in.

• A (correct): Combustion releases energy as heat and light, so the surroundings get warmer.
• B: The cold pack feels cold because the process absorbs heat, which makes it endothermic.
• C: Photosynthesis takes in light energy to build sugars, so it is endothermic.
• D: This decomposition needs a steady input of heat, so it absorbs energy rather than releasing it.

Answer: C

Correct answer: Zinc. Rust needs iron, oxygen and water. Galvanizing puts a layer of zinc over the steel, keeping oxygen and water away from the iron and slowing corrosion.

• A: Copper is used for wiring and pipes, but it is not the coating used in galvanizing.
• B: Lead is not used to galvanize steel; the process uses a different, lighter-colored metal.
• C (correct): Galvanizing means coating iron or steel with zinc, which shields it from oxygen and water.
• D: Silver is far too costly for guardrails and is not the metal used in galvanizing.

Answer: C

Correct answer: The same amount of product forms sooner, because the energy barrier is lower. A catalyst lowers the activation energy, so more collisions succeed and the reaction goes faster. It is not used up, and it changes neither the final amount of product nor the overall energy change.

• A: A catalyst is not a reactant and does not raise the final yield; it only changes the speed.
• B: A catalyst does not change the overall energy released or absorbed by the reaction.
• C (correct): A catalyst provides a pathway with lower activation energy, so the reaction finishes faster with the same final yield.
• D: A catalyst speeds a reaction and is regenerated by the end, so it is not consumed.

Answer: A

Correct answer: 4. Each pH unit is a tenfold change, and fewer hydrogen ions means a higher pH. One-hundredth is a factor of 100 = 10 x 10, so sample B is 2 units higher than sample A, at pH

Ten times more hydrogen ions is 1 unit lower, so sample C is at pH

• A (correct): B is 2 pH units above A, at pH 5; ten times more hydrogen ions is 1 unit lower, so C is at pH 4.
• B: This raises the pH for C, but more hydrogen ions means a lower pH, not a higher one.
• C: This moves the pH the wrong way at each step; fewer hydrogen ions means a higher pH, not a lower one.
• D: This is the pH of sample B; the final step to sample C was skipped.

Answer: B

Correct answer: C_2H_5OH + 3O_2 → 2CO_2 + 3H_2O. Ethanol has 2 C, 6 H and 1 O. Carbon: write 2 in front of carbon dioxide. Hydrogen: 6 H needs 3 water molecules. Oxygen on the right is

ethanol supplies 1, so 6 more must come from 3 oxygen molecules.

• A: Six water molecules hold 12 hydrogen atoms, but one ethanol molecule has only 6.
• B (correct): Each side has 2 carbon, 6 hydrogen and 7 oxygen atoms, so every atom is accounted for.
• C: This forgets the oxygen atom inside ethanol: the left side has 16 oxygen atoms but the right has 14.
• D: Carbon and hydrogen match, but the left side has only 5 oxygen atoms while the right has 7.