HS-PS1-2: Explaining the Outcome of Simple Chemical Reactions
In plain English: HS-PS1-2 is the NGSS high school performance expectation that asks students to construct and revise explanations for the outcome of simple chemical reactions using the outermost electrons of atoms, periodic table trends and patterns of chemical properties. Examples include sodium with chlorine and carbon with oxygen or hydrogen. It is usually taught in Chemistry or Physical Science.
Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.
Clarification Statement: Examples of chemical reactions could include the reaction of sodium and chlorine, of carbon and oxygen, or of carbon and hydrogen.
Assessment Boundary: Assessment is limited to chemical reactions involving main group elements and combustion reactions.
Next Generation Science Standards · Disciplinary Core Idea: Matter and Its Interactions · Official standard
Students explain why a simple reaction gives the products it does: why sodium and chlorine make NaCl and not NaCl2, why carbon burns to CO2, and why the same pattern predicts the products of burning any hydrocarbon. Each explanation rests on three kinds of knowledge the standard names: the outermost electrons of the atoms involved, trends in the periodic table, and known patterns of chemical properties (for example, every Group 1 metal forms a chloride MCl). Students then revise their explanations when new evidence does not fit, which is the second half of the practice.
The NGSS Clarification Statement says: "Examples of chemical reactions could include the reaction of sodium and chlorine, of carbon and oxygen, or of carbon and hydrogen." The Assessment Boundary says: "Assessment is limited to chemical reactions involving main group elements and combustion reactions." Every reaction on this page uses only main group elements, and the combustion examples are hydrocarbons burning in oxygen. Reaction energy, rates and amounts in grams belong to other standards and are not assessed here. Observation data in the activities are invented for teaching and match typical classroom results.
Learning Objectives
By the end of this lesson, students will be able to:
Construct an explanation for the products of a simple reaction between main group elements based on how their atoms lose, gain or share outer electrons
Use periodic trends to explain why one reaction is more vigorous than another, or why one element displaces another
Use patterns of chemical properties, such as the products of combustion, to predict and explain the outcome of a new reaction
Write balanced equations that match the explanation
Revise an explanation when new evidence contradicts it, and state what changed and why
Prior Knowledge Required
Students should already be comfortable with:
Using the periodic table to find the number of outer electrons and predict ion charges and numbers of bonds HS-PS1-1
Signs that a chemical reaction has happened, such as a gas, a color change or a new solid MS-PS1-2
Atoms are rearranged, not created or destroyed, in a reaction MS-PS1-5
Reading chemical formulas and counting atoms in them
Show a short video of sodium burning in chlorine gas (video only; this reaction is never done in a classroom). Then show a dish of table salt.
Warm-Up Prompt
"A soft, shiny metal and a poisonous green gas react to make the white crystals you put on food. Write a first explanation: why do these two elements react, and why is the product NaCl rather than NaCl2?"
Students keep this as "Version 1" of their explanation. Collect a few ideas without correcting them; students revise their own Version 1 at the end of Direct Instruction.
Direct Instruction15-20 minutes
Building an explanation. A complete explanation of a reaction outcome has a claim (the products, their formulas and how vigorous the reaction is), evidence (what is observed) and reasoning from three sources: (1) outer electrons: metals with 1-3 outer electrons lose them, nonmetals gain or share electrons until they have 8; (2) periodic trends: metals lose electrons more easily down a group and toward the left, nonmetals attract electrons more strongly up a group and toward the right; (3) patterns of chemical properties: elements in a group react in the same way, and hydrocarbons burn in plenty of oxygen to give carbon dioxide and water. Revising means finding which part of the reasoning a new observation contradicts, changing that part, and saying what evidence caused the change. Diagram 1 models the electron transfer in the first example.
Sodium and chlorine
Explain why sodium and chlorine react to form NaCl.
Result: Na (Group 1) has 1 outer electron, held weakly; Cl (Group 17) has 7 and attracts one more strongly. Each Na atom transfers 1 electron to a Cl atom, giving Na+ and Cl-, which attract. One Cl2 molecule supplies two Cl atoms: 2Na + Cl2 → 2NaCl.
Carbon and hydrogen
Explain the formula of the simplest compound of carbon and hydrogen.
Result: Carbon has 4 outer electrons and shares one with each of 4 hydrogen atoms; each H shares its single electron. Carbon ends with 8 outer electrons and each H with 2: CH4, made by C + 2H2 → CH4.
Carbon and oxygen (combustion)
Charcoal (carbon) burns in plenty of oxygen. Explain the product.
Result: Carbon forms 4 bonds and oxygen forms 2, so each O shares two pairs with C (two double bonds): O=C=O. C + O2 → CO2.
Pattern of chemical properties
Methane, the main part of natural gas, burns in air. Predict and explain the products.
Result: Carbon atoms end up bonded to oxygen as CO2 and hydrogen atoms as H2O, the pattern for every hydrocarbon burned in plenty of oxygen: CH4 + 2O2 → CO2 + 2H2O.
Revising an explanation
Version 1: "Magnesium reacts with chlorine like sodium does, so the product is MgCl." New evidence: the reference formula of magnesium chloride is MgCl2.
Result: The pattern "same as sodium" fails because Mg is in Group 2, not Group 1. Revised: each Mg atom loses 2 outer electrons, one to each of two Cl atoms, forming Mg2+ and 2 Cl-. Mg + Cl2 → MgCl2.
Students now revise their warm-up Version 1 into Version 2 and underline the sentence that changed. Ask: "Which of the three sources of reasoning did you add?"
Guided Practice15 minutes
Show the results of a standard teacher demonstration (invented data, typical of this test): solutions of chlorine, bromine and iodine in water were added to solutions of potassium halides, then shaken with a little hexane, which takes up any free halogen and shows its color. Students do not handle the halogen solutions.
Halogen and halide results (invented data)
Halogen added
to KCl solution
to KBr solution
to KI solution
Chlorine (pale green)
not tested
hexane layer turns orange
hexane layer turns violet
Bromine (orange)
stays orange: no change
not tested
hexane layer turns violet
Iodine (brown in water, violet in hexane)
stays violet: no change
stays violet: no change
not tested
Identify the three mixtures that reacted. Orange means bromine was released and violet means iodine was released.
Write a balanced equation for each reaction, such as Cl2 + 2KBr → 2KCl + Br2.
Construct an explanation that uses outer electrons (every halogen atom gains 1 electron) and the trend in Group 17 (the outer level is closer to the nucleus higher in the group).
State the rule the data support: a halogen releases the halogens below it from their salts, but not the ones above it.
Independent Practice10-15 minutes
Students watch a video of lithium and potassium in water with phenolphthalein. Evidence (invented observation notes): both fizz, both solutions turn pink, the gas collected pops with a lit splint, and potassium reacts far more violently, with a lilac flame. They write Version 1 of an explanation for the products and the difference in vigor, then trade with a partner, who marks one piece of evidence the explanation does not account for. Students revise into Version 2.
Key points for Version 2: pink shows a basic hydroxide formed and the pop shows hydrogen, so 2K + 2H2O → 2KOH + H2 (and the same pattern for lithium). Potassium's outer electron is in a higher energy level than lithium's, farther from the nucleus, so it is lost more easily.
Closure10 minutes
Exit ticket: "Potassium reacts with bromine. Predict the product, write a balanced equation, and explain the formula using outer electrons." (2K + Br2 → 2KBr: each K gives 1 electron to a Br atom.) Ask two students to read their Version 1 and Version 2 from the warm-up and name the evidence that changed their thinking.
Differentiation Strategies
For Struggling Students
Give an explanation frame with three labeled boxes: "Outer electrons", "Trend", "Pattern", and one box for the balanced equation
Use electron-dot cards so students can physically move electrons from a metal to a nonmetal before writing a formula
For revisions, give a two-column organizer: "What my first explanation said" and "What the new evidence shows"
For Advanced Students
Ask students to explain why burning a hydrocarbon in limited air can give carbon monoxide or soot, and how the explanation must change
Have students predict whether bromine would release chlorine from a chloride solution, then design the test they would use to check it
Ask students to write one explanation that covers all of Group 1 reacting with water, then list the evidence that would make them revise it
Assessment Guidance
What to Look For
Strong explanations name the outer electrons of both atoms, use a trend when comparing reactions, and match a balanced equation. Look for revisions that point to the specific evidence and change the reasoning, not only the formula. Watch for students who write NaCl2 because chlorine is written Cl2, who say an atom "wants" electrons without naming the attraction of the nucleus, and who treat a trend for metals as a trend for nonmetals.
02
Classroom Activities
3 Activities
1
What Does a Candle Make When It Burns?
25 minGroups of 3
Paraffin wax is a mixture of hydrocarbons such as C25H52. Groups test the products of a burning tea light for water and carbon dioxide, then explain the products from the bonding patterns of carbon, hydrogen and oxygen.
Procedure
Safety: goggles on, long hair tied back, tea light standing in a tray of sand, and a teacher lights every candle. Limewater irritates eyes and skin; wipe spills at once.
Weigh the tea light. Light it and hold a cold, dry beaker upside down about 10 cm above the flame with tongs for 5 seconds. Record what forms inside.
Lower a clear jar over the candle until the flame goes out. Lift it, pour in 20 mL of limewater, cap and shake. Shake 20 mL of limewater in a second capped jar of room air as a control.
After 10 minutes of burning, blow out the candle and weigh it again.
Sample Data (invented)
Cold beaker: fogged with tiny droplets of a colorless liquid within 5 s. Limewater from the candle jar: milky within 10 s of shaking. Control jar of room air: still clear after 30 s. Tea light: 14.62 g before and 14.21 g after 10 minutes of burning.
Discussion Questions
Which observation is evidence of water, and which is evidence of carbon dioxide? Why was the room-air jar needed?
Use outer electrons to explain why carbon ends up in CO2 and hydrogen in H2O. Balance C25H52 + O2 → CO2 + H2O.
The candle lost 0.41 g. A student says the wax was destroyed. Revise that explanation using the evidence.
Modification for Distance Learning
Students watch a recorded version of the three tests, record the observations in a table and submit their explanation and balanced equation on a shared document.
2
Magnesium and Acid: Construct, Then Revise
20 minPairs
Pairs add magnesium ribbon to dilute hydrochloric acid, identify the gas, and write an explanation of the products. The teacher then hands out one new evidence card, and pairs revise.
Procedure
Safety: goggles on. Use 5 mL of 1 M hydrochloric acid in a test tube in a rack, and a 3 cm strip of magnesium ribbon. The splint test is done with the teacher at the station, away from any other flammable material.
Add the ribbon and loosely cover the tube with a second, upside-down test tube to collect gas.
Hold a burning splint at the mouth of the collecting tube and record the result.
The teacher evaporates a few drops of the final solution on a watch glass so students can see what is left.
Sample Data (invented)
Rapid bubbling, ribbon gone in about 90 s, tube feels warm. Squeaky pop with the burning splint. A white solid remains on the watch glass. Evidence card handed out after Version 1: "The white solid is magnesium chloride; its reference formula is MgCl2."
Discussion Questions
What does the pop tell you the gas is, and where did its atoms come from?
Many pairs first write MgCl for the white solid. What does the evidence card show, and which source of reasoning (outer electrons, trend or pattern) fixes the formula?
Write the balanced equation that matches your revised explanation.
Modification for Distance Learning
Use a recorded video of the reaction and the splint test. Post the evidence card after students submit Version 1 in the class discussion board.
3
Explanation Revision Stations
20 minGroups of 3-4
Four stations each show a first-draft explanation written by a fictional student and one new piece of evidence. Groups decide what the evidence contradicts and write a revised explanation. Rotate every 5 minutes.
Station Cards
Station 1: Draft: "Aluminum and oxygen form AlO, because aluminum is a metal and oxygen gains 2 electrons." Evidence: the reference formula of aluminum oxide is Al2O3.
Station 2: Draft: "Lithium reacts with water more violently than potassium, because lithium's outer electron is closer and easier to reach." Evidence: in video, a 3 mm piece of lithium takes 60 s to disappear and a piece of potassium 4 s.
Station 3: Draft: "Burning methane makes only carbon dioxide, because methane is a carbon compound." Evidence: a cold glass held over a methane flame fogs with a colorless liquid that boils at 100 °C.
Station 4: Draft: "Sodium and chlorine form NaCl2, because chlorine gas is made of Cl2 molecules." Evidence: sodium chloride crystals contain equal numbers of sodium and chloride ions.
Discussion Questions
At each station, which part of the draft does the evidence contradict: the outer-electron reasoning, the trend, or the pattern?
At Station 2, the potassium time is 15 times shorter. What does the trend in Group 1 predict, and how must the draft change?
Write the balanced equation for each revised explanation.
Challenge Variation
Each group writes a fifth station card: a flawed explanation of a main group reaction plus the evidence that exposes the flaw. Groups trade cards and revise.
03
Diagrams & Visual Aids
2 diagrams
Diagram 1: Electron Transfer from Sodium to Chlorine
Shell model of the reaction in Example 1. Sodium's single outer electron (green) moves to chlorine's outer level. Both ions end with filled outer levels: Na+ has 2, 8 and Cl- has 2, 8, 8. The green electron is shown in the Cl- ion to track it; once transferred, it is no different from the others.
Diagram 2: Bond Models for Carbon with Hydrogen and with Oxygen
Each line is a shared pair of electrons. Carbon has 4 outer electrons and forms 4 bonds in both molecules: four single bonds to hydrogen in CH4, and two double bonds to oxygen in CO2. Each oxygen forms 2 bonds (one double bond) and keeps 2 unshared pairs (dots), so every C and O atom has 8 outer electrons and every H has 2.
04
Homework Assignment
~30 min
HS-PS1-2 Homework: Constructing and Revising Explanations of Reactions
Directions: For every explanation, name the outer electrons of each atom, use a periodic trend or a pattern of chemical properties where it helps, and write a balanced equation. For revisions, say what the first explanation got wrong and which evidence shows it. All observations are invented.
Part 1: Construct an Explanation (Problems 1-2)
Lithium burns in oxygen to form a white solid, lithium oxide. (a) Predict the formula of the product and explain it with outer electrons. (b) Write the balanced equation.
Propane, C3H8, burns in a gas grill with plenty of air. (a) Predict the products using a pattern of chemical properties, and explain why carbon and hydrogen end up in those products. (b) Write the balanced equation.
Part 2: Revise an Explanation (Problems 3-4)
A student wrote: "Aluminum and bromine react to form AlBr, because bromine atoms gain one electron." A reference book gives the formula of aluminum bromide as AlBr3. (a) What part of the explanation is incomplete? (b) Write a revised explanation. (c) Write the balanced equation.
A student wrote: "Cesium reacts with chlorine less vigorously than lithium does, because cesium's outer electron is farther away, so the chlorine atom cannot reach it as easily." Evidence: cesium reacts with chlorine instantly at room temperature, but lithium must be warmed first. Revise the explanation using a periodic trend.
Part 3: Explain Two Products and a Pattern (Problems 5-6)
When magnesium ribbon burns in air, most of it forms magnesium oxide, but a small amount forms magnesium nitride. (a) Predict both formulas using outer electrons. (b) Write a balanced equation for each reaction.
Magnesium reacts very slowly with hot water to form magnesium hydroxide, Mg(OH)2, and hydrogen. (a) Use this pattern to predict the products when calcium reacts with water, and write the balanced equation. (b) Predict whether calcium reacts faster or slower than magnesium, and explain with a trend. (c) The solution turns cloudy. Revise your explanation if needed to account for this evidence.
Rubric
Criterion
Full Credit (2 pts)
Partial Credit (1 pt)
Outer-Electron Reasoning
Names the outer electrons of each atom and links them to the charges or bonds in every product
Correct formulas, but the electron reasoning is missing or incomplete
Trends and Patterns
Uses the correct trend or pattern whenever reactions are compared or predicted
Uses a trend or pattern but applies it in the wrong direction once
Revision and Equations
Revisions name the contradicting evidence and change the reasoning; every equation is balanced
Revision changes the answer without explaining why, or one equation is unbalanced
05
Quiz: 20 Questions
Interactive, with answers
Instructions
Every reaction here involves main group elements or the combustion of a hydrocarbon. Use a periodic table. Pick an option to check it, open the model answer for written questions, and use Reset quiz to start over.
Multiple choice: pick an option to check it. Short answer: write your answer, then reveal the model answer.
0 of 20 answered · 0 correct
Question 1 of 20 · Multiple Choice
Lithium reacts with chlorine. What is the formula of the product?
Answer: B
Lithium (Group 1) loses its 1 outer electron to form Li+, and chlorine gains 1 electron to form Cl-, so they combine 1:1 as LiCl. Choice A copies the 2 from Cl2 into the product. Choice D uses chlorine's 7 outer electrons as a subscript.
Question 2 of 20 · Multiple Choice
Which balanced equation matches the explanation that each calcium atom gives 2 electrons to one oxygen atom?
Answer: C
Ca2+ and O2- combine 1:1 as CaO, and one O2 molecule gives two O atoms, so 2 Ca atoms are needed: 2Ca + O2 → 2CaO. Choice A makes the wrong compound. Choice B ignores that oxygen gas is O2, and choice D writes calcium as if it were a two-atom molecule.
Question 3 of 20 · Multiple Choice
Why does strontium form Sr2+ ions when it reacts?
Answer: A
Strontium is in Group 2 with 2 outer electrons; losing them leaves the filled level below, the same arrangement as krypton. Choice B describes a Group 16 nonmetal. Choice C mixes up the period and the group, and choice D is false: the nuclear charge is 38+.
Question 4 of 20 · Multiple Choice
Silicon reacts with hydrogen to form a covalent compound. What formula does outer-electron reasoning predict?
Answer: D
Silicon is in Group 14 like carbon, with 4 outer electrons, so it shares one with each of 4 hydrogen atoms: SiH4. This follows the same pattern as carbon in its group. Choice C reverses the ratio.
The right side has 4 × 2 + 6 × 1 = 14 oxygen atoms, which takes 7 O2 molecules. Choice D is the number of oxygen atoms, not molecules. Choice C matches only the water, and choice B does not balance any count.
Question 6 of 20 · Multiple Choice
Hexane, C6H14, burns completely in air. What does the balanced equation give as the ratio of product molecules?
Answer: B
By the combustion pattern, the 6 carbon atoms end up in 6 CO2 and the 14 hydrogen atoms in 7 H2O (2 H per molecule): 2C6H14 + 19O2 → 12CO2 + 14H2O, a 6 : 7 ratio. Choice A forgets that each water molecule holds 2 hydrogen atoms. Choice D describes products that do not match complete combustion.
Question 7 of 20 · Multiple Choice
Which Group 1 metal would you predict to react most vigorously with chlorine gas?
Answer: D
Rubidium is the lowest of the four in Group 1, so its single outer electron is farthest from the nucleus and most easily transferred to chlorine. Choice A is the least reactive of the four, since lithium holds its outer electron most strongly.
Question 8 of 20 · Multiple Choice
A student explained: "Calcium and oxygen form Ca2O, because calcium is a metal like sodium." The reference formula is CaO. Which revision is best?
Answer: C
The draft fails because it used the Group 1 pattern for a Group 2 metal. With 2 outer electrons, calcium forms Ca2+, and one O2- balances it. Choice A reverses which atom loses electrons, and choice B gives oxygen the wrong charge.
Question 9 of 20 · Multiple Choice
When calcium reacts with sulfur to form CaS, what happens to the electrons?
Answer: A
Calcium has 2 outer electrons, held loosely, and sulfur (Group 16) needs 2 more to reach 8, so one Ca gives both electrons to one S, forming Ca2+ and S2-. Choice B reverses the direction. Choice D would give the formula CaS2.
Question 10 of 20 · Multiple Choice
Which element would you predict to react most vigorously with cold water?
Answer: D
All four are in Period 3. Sodium, on the left, has one outer electron that is held least strongly, so it reacts most vigorously. Magnesium (choice A) must lose 2 electrons and holds them more strongly; aluminum and silicon react even less with cold water.
Question 11 of 20 · Multiple Choice
Lithium reacts with water to form lithium hydroxide, LiOH, and hydrogen. Using this pattern, what are the products when rubidium reacts with water?
Answer: C
Rubidium is in Group 1 with lithium, so it follows the same pattern: 2Rb + 2H2O → 2RbOH + H2. Choice D uses the Group 2 formula pattern. Choices A and B do not match the evidence pattern at all.
Question 12 of 20 · Multiple Choice
A student explains: "Magnesium reacts more vigorously with water than sodium, because magnesium has two electrons to give away." Which observation contradicts this explanation?
Answer: C
If magnesium reacted more vigorously, it would fizz more; the observation shows the opposite, so the draft must be revised: sodium loses one weakly held electron more easily than magnesium loses two. Choices A and D are true of both metals, so they cannot tell the two apart, and choice B is not about the reaction.
Question 13 of 20 · Multiple Choice
Carbon reacts with sulfur, which is in the same group as oxygen. Using the pattern of carbon dioxide, which formula do you predict?
Answer: B
Sulfur has 6 outer electrons like oxygen and forms 2 bonds, so it follows the CO2 pattern: S=C=S, carbon disulfide. Choice D gives carbon 8 bonds. Choice A leaves carbon with only 2 bonds.
Question 14 of 20 · Multiple Choice
Hydrogen and oxygen react to form water. Which explanation of the formula H2O is best?
Answer: B
Oxygen needs 2 more electrons to reach 8, so it shares one pair with each of two hydrogen atoms, and each hydrogen gets 2. Choice A explains nothing about bonding. Choice C is false: hydrogen is a nonmetal and the bonds are covalent. Choice D reverses the direction electrons are pulled.
Question 15 of 20 · Short Answer
Barium reacts with fluorine. Construct an explanation for the formula of the product and write the balanced equation.
Barium (Group 2) loses its 2 outer electrons to form Ba2+. Each fluorine atom (Group 17) gains 1 electron to form F-, so two fluoride ions balance one barium ion: BaF2. One F2 molecule supplies both: Ba + F2 → BaF2.
Question 16 of 20 · Short Answer
A student wrote: "Fluorine is less reactive than iodine, because fluorine atoms are smaller and have fewer electrons to react with." Evidence: fluorine reacts explosively with hydrogen even in the cold and dark, while iodine reacts with hydrogen only when heated, and only partly. Revise the explanation.
The evidence shows fluorine is far more reactive, so the draft is wrong. Revised: both atoms have 7 outer electrons and react by gaining 1. Fluorine's outer level is closest to the nucleus, with few inner electrons shielding it, so it attracts an extra electron most strongly. Down Group 17 the outer level is farther away, so iodine gains an electron less readily and reacts less vigorously.
Question 17 of 20 · Short Answer
Butane, C4H10, is the fuel in disposable lighters. Predict the products of its complete combustion and write the balanced equation.
By the combustion pattern, carbon ends up in CO2 and hydrogen in H2O: 2C4H10 + 13O2 → 8CO2 + 10H2O. Check: 8 C, 20 H and 26 O atoms on each side.
Question 18 of 20 · Short Answer
Nitrogen and hydrogen react to form ammonia. Explain the formula of ammonia using outer electrons and write the balanced equation.
Nitrogen (Group 15) has 5 outer electrons and needs 3 more, so it shares one pair with each of three hydrogen atoms: NH3. Nitrogen ends with 8 outer electrons and each hydrogen with 2. N2 + 3H2 → 2NH3.
Question 19 of 20 · Short Answer
A student predicted that sodium and sulfur form NaS. The reference formula of sodium sulfide is Na2S. Revise the explanation and write the balanced equation.
The draft assumed the charges were equal. Sulfur (Group 16) has 6 outer electrons and gains 2 to form S2-, but each sodium atom gives only its 1 outer electron. So two sodium atoms are needed for each sulfur atom: 2Na + S → Na2S.
Question 20 of 20 · Short Answer
Sodium and potassium both form chlorides with the formula MCl, but potassium reacts more vigorously with chlorine. Explain both facts.
The same formula comes from a pattern: both are Group 1 metals with 1 outer electron, so each forms M+, which pairs with one Cl-. The difference in vigor comes from a trend: potassium's outer electron is in energy level 4, farther from the nucleus and more shielded than sodium's in level 3, so it transfers more easily.
0 of 20 answered · 0 correct
06
Frequently Asked Questions
10 Questions
What does HS-PS1-2 mean?
HS-PS1-2 asks students to build, and then revise, explanations for why simple chemical reactions give the products they do. The explanation has to use the outermost electrons of the atoms, trends in the periodic table and known patterns of chemical properties. The code stands for High School, Physical Science, core idea 1 (Matter and Its Interactions), performance expectation 2.
Is HS-PS1-2 a chemistry standard?
Yes. It is usually taught in a high school Chemistry course right after the periodic table, and it also appears in Physical Science and integrated science courses. It builds directly on HS-PS1-1, which uses the same outer-electron patterns to predict properties.
What reactions are in the HS-PS1-2 assessment boundary?
Only reactions involving main group elements, plus combustion reactions. The official examples are sodium with chlorine, carbon with oxygen and carbon with hydrogen. Reactions of transition metals, and topics such as reaction energy, rates and stoichiometry calculations, are beyond this standard.
What does "construct and revise an explanation" look like in class?
Students write a first explanation, meet new evidence, and rewrite the explanation so it fits. For example, a student who predicts MgCl sees that the known formula is MgCl2 and changes the reasoning to "magnesium has 2 outer electrons". The revision should say what changed and which evidence caused it.
Why do sodium and chlorine form NaCl and not NaCl2?
Each sodium atom has only one outer electron to give, and each chlorine atom needs only one, so they combine one to one. The 2 in Cl2 describes chlorine gas molecules; it means one Cl2 molecule reacts with two sodium atoms: 2Na + Cl2 → 2NaCl.
Why does burning a hydrocarbon always give carbon dioxide and water?
With plenty of oxygen, every carbon atom ends up sharing its 4 outer electrons with oxygen atoms (CO2) and every hydrogen atom bonds to oxygen (H2O). This is a pattern of chemical properties that students can use to predict the products for any hydrocarbon fuel. With too little oxygen, carbon monoxide or soot can also form.
How do periodic trends explain which reaction is more vigorous?
Metals react by losing outer electrons, which is easier the farther those electrons are from the nucleus, so metal reactivity increases down a group. Nonmetals react by gaining electrons, which is easier the closer the outer level is to the nucleus, so nonmetal reactivity increases up a group. That is why potassium reacts faster than lithium, and why chlorine releases bromine from bromide salts but bromine does not release chlorine.
What are common mistakes in explanations of reactions?
Writing NaCl2 because chlorine gas is Cl2, using the same charge for every metal, and applying the metal trend to nonmetals are frequent ones. Many students also give a correct formula with no reasoning about electrons; the standard asks for the reasoning.
What labs are safe for HS-PS1-2?
Burning a tea light and testing the products with a cold beaker and limewater, and reacting magnesium ribbon with dilute hydrochloric acid, are safe with goggles and teacher supervision of any flame. Reactions of sodium with chlorine or of Group 1 metals with water should be shown only by video.
What comes after HS-PS1-2?
HS-PS1-4 looks at the energy released or absorbed as bonds break and form, and HS-PS1-7 uses mathematics to show that atoms and mass are conserved in reactions such as those on this page. Students also use the bond types from HS-PS1-2 to explain bulk properties in HS-PS1-3.
07
Related Standards
5 standards
These standards connect to HS-PS1-2: prerequisites to review first, parallel standards at the same level, and next steps that build on it.
Before this lesson
MS-PS1-2Prerequisite
Analyze data on properties of substances before and after they interact to detect a reaction
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