Pick a reaction. Use + and − to change how many of each molecule take part (the coefficients). When every kind of atom has the same count on both sides, press Run the reaction and watch the same atoms move into the products. Solve 4 of the 6 reactions.
Colors: hydrogen white, carbon dark gray, nitrogen blue, oxygen red, magnesium green, iron brown. Masses use whole atomic mass units (u) so they are easy to add. To practice balancing with symbols only, use the Balancing Chemical Equations game.
Everything starts on the balance: the flask, the liquid and the second reactant in a small tube or paper. Run each reaction in an open beaker and in a capped bottle (a closed system), record the readings and explain them. Mission: test 3 reactions both ways and explain 3 open results on the first try.
| # | Reaction | System | Start (g) | End (g) | Change (g) |
|---|
Masses are worked out from the equations: 4.20 g of baking soda with excess vinegar (100 g) makes 2.20 g of carbon dioxide; a fizzy tablet with 1.00 g of citric acid makes about 0.69 g. In a real lab you lose a little less, because some carbon dioxide stays dissolved. Steel wool gains mass because part of it reacts with oxygen from the air. Real-lab safety: use a plastic soda bottle or a large zip bag with no more than 1 g of baking soda, and never cap glass — trapped gas builds up pressure.
Add the solid to the liquid, stir and watch the thermometer for 2 minutes. Exothermic reactions release thermal energy, so the mixture gets warmer. Endothermic reactions take in thermal energy, so the mixture gets colder. Classify each trial in the table. Mission: classify 4 different reactions correctly on the first choice.
| # | Reaction | Amount | Start (°C) | Highest / lowest (°C) | ΔT (°C) | Exo or endo? |
|---|
Typical classroom results (starting at 20 °C). Safety in a real lab: goggles; hydrochloric acid and quicklime need a teacher; the magnesium reaction makes flammable hydrogen gas. Dissolving salts in instant cold packs also gets cold, but dissolving is a physical change, not a reaction.
Engineers use exothermic and endothermic reactions in real products. Pick a client brief, choose the amount of reactant and one design feature, test, and improve. Meet 2 of the 3 briefs. (Simplified heat model: the reactant is used up over time while heat leaks to the 20 °C surroundings.)
Scientists decide whether a reaction happened by comparing the properties before and after: melting point, density, color, magnetism, solubility, smell. A new substance means a chemical reaction. Get 7 of 8 right in one round.
Click an equation, then the group it belongs in (or its “Place card here” button). Use the pattern under each name. All 12 right with at most 2 mistakes per check; more than 2 sends every card back.
10 questions from a bank of 20. Score 7 or more to complete the mission. Each new round picks new questions.
When you heat iron and sulfur together you get a black solid that a magnet cannot pick up and that melts at 1194 °C. Neither iron nor sulfur behaves like that: it is a new substance (iron sulfide). That is the real test of a chemical reaction. Bubbles, a color change, light or a temperature change are clues, but the proof is a substance with different properties (melting point, density, color, magnetism, solubility). Melting, dissolving and bending are physical changes: the substance stays the same.
Hydrogen burns in oxygen to make water: 2H₂ + O₂ → 2H₂O. On the left there are 4 hydrogen atoms and 2 oxygen atoms; on the right, 4 hydrogen atoms and 2 oxygen atoms. The atoms have only changed partners. The big numbers in front (coefficients) count molecules; the small numbers (subscripts) are part of each formula and never change. A balanced equation has the same number of each kind of atom on both sides.
If the same atoms are there before and after, the mass must be the same. But a balance only weighs what is on it. In an open system, a gas can escape (baking soda + vinegar loses carbon dioxide, so the reading goes down) or a gas from the air can join in (steel wool takes oxygen from the air, so the reading goes up). In a closed system, such as a capped bottle, nothing gets in or out, and the reading stays the same. Antoine Lavoisier showed this in the 1770s by heating metals in sealed vessels.
Breaking chemical bonds takes energy; forming new bonds releases energy. If more energy is released than taken in, the reaction is exothermic and the surroundings warm up (burning fuel, hand warmers, quicklime + water). If more is taken in, it is endothermic and the surroundings cool down (citric acid + baking soda, baking soda + vinegar). Energy is conserved too: it moves between the chemicals and the surroundings.
Balance the rusting of iron in the Atom Rearranger and count the atoms. In the mass lab, run baking soda + vinegar in the capped bottle, then press Open the cap. In the temperature lab, add 1 and then 3 scoops of yeast to hydrogen peroxide: what changes and what stays the same?
| Idea | What it means | Example on this page |
|---|---|---|
| Reactants → products | Starting substances on the left of the arrow; new substances on the right | CH₄ + 2O₂ → CO₂ + 2H₂O |
| Coefficient | Big number in front: how many molecules or atoms | The 2 in 2H₂O |
| Subscript | Small number in a formula: atoms in one molecule (never change it to balance) | The 2 in H₂O |
| Conservation of mass | Total mass of reactants = total mass of products | 48 g Mg + 32 g O₂ → 80 g MgO |
| Open system | Matter can enter or leave | Open beaker: CO₂ escapes, reading drops |
| Closed system | No matter can enter or leave | Capped bottle: reading stays the same |
| Exothermic | Releases thermal energy; surroundings get warmer | Quicklime + water, magnesium + acid |
| Endothermic | Takes in thermal energy; surroundings get colder | Citric acid + baking soda |
| Catalyst | Speeds up a reaction without being used up | Yeast in hydrogen peroxide |
| Precipitate | Solid that forms when two solutions react | Calcium carbonate from CaCl₂ + Na₂CO₃ |
Related pages: physical and chemical changes (signs of a reaction, particle diagrams, label quiz), balancing chemical equations, stoichiometry, reaction rate: magnesium + acid and forms of energy.
2H₂ + O₂ → 2H₂O. With molar masses H = 1.008 and O = 16.00 g/mol: 2 mol H₂ = 4.032 g, 1 mol O₂ = 32.00 g, total 36.03 g; 2 mol H₂O = 2 × 18.02 = 36.03 g. The moles of molecules change (3 → 2), but the atoms and the mass do not. For the baking soda reaction, 4.20 g NaHCO₃ ÷ 84.0 g/mol = 0.0500 mol, which makes 0.0500 mol CO₂ × 44.0 g/mol = 2.20 g, exactly the loss in the open beaker. Practice more on the stoichiometry page.
ΔH ≈ (energy to break bonds) − (energy released forming bonds). For H₂ + Cl₂ → 2HCl: break H–H 436 + Cl–Cl 242 = 678 kJ; form 2 × H–Cl 431 = 862 kJ; ΔH = 678 − 862 = −184 kJ (exothermic). For CH₄ + 2O₂ → CO₂ + 2H₂O: break 4 C–H (4 × 413) + 2 O=O (2 × 498) = 2648 kJ; form 2 C=O (2 × 799) + 4 O–H (4 × 463) = 3450 kJ; ΔH ≈ −802 kJ. Average bond energies give estimates; measured values differ a little.
An energy diagram plots stored chemical energy along the reaction. Exothermic: products lower than reactants, ΔH negative. Endothermic: products higher, ΔH positive. The hump is the activation energy, the energy needed to start breaking bonds; that is why steel wool needs a spark and a match needs striking. A catalyst (like the enzyme catalase in yeast) gives a lower hump, so the reaction is faster, but ΔH stays the same.
Many classes put a balloon over the flask instead of a cap. When the balloon inflates, it pushes aside air, and the air pushes it up (buoyancy): about 1.2 g less on the balance for every liter the balloon swells, so with 4.2 g of baking soda (about 1.2 L of gas) the reading drops by more than a gram even though no mass escaped. A rigid capped bottle avoids this. It is a good example of checking a method, not just the result.
Reactive metals, high in the reactivity series (Mg, Zn, Fe), lose their outer electrons easily, so they react with oxygen and with acids; copper is less reactive and does not react with dilute hydrochloric acid. A more reactive metal replaces a less reactive one: Fe + CuSO₄ → FeSO₄ + Cu.
Standards: NGSS MS-PS1-2, MS-PS1-5, MS-PS1-6 (middle school); HS-PS1-2, HS-PS1-4, HS-PS1-7 (Going further). Objective: students show with particle models and data that atoms are rearranged and mass is conserved, explain open vs closed system results, classify reactions as exothermic or endothermic from temperature data, and design a device that uses a chemical reaction to heat or cool.
Misconceptions to target: gas has no mass; burning destroys matter; atoms change into other atoms; coefficients and subscripts are the same thing; cold reactions “add cold”. Missions are saved on the student’s device; the certificate prints as proof of completion. A real-lab follow-up: baking soda + vinegar in a large zip bag or plastic soda bottle on a balance, with no more than 1 g of baking soda (never a capped glass container).
A chemical reaction is a process in which one or more substances (the reactants) change into new substances (the products) with different properties. The atoms of the reactants are rearranged: old bonds break and new bonds form. Burning, rusting, cooking an egg and baking soda fizzing in vinegar are chemical reactions.
Key takeaway: reactants are rearranged into new substances.In a chemical reaction, matter is not created or destroyed, so the total mass of the products equals the total mass of the reactants. This is true because the same atoms are present before and after; they are only rearranged. Antoine Lavoisier showed it in the 1770s by carefully weighing reactions in sealed containers.
Key takeaway: mass before = mass after, because the atoms are the same.The reaction makes carbon dioxide gas. In an open cup, the gas bubbles out into the air, so its mass is no longer on the balance and the reading goes down. Mass has not been destroyed. If you do the same reaction in a capped bottle (a closed system), the reading stays the same.
Key takeaway: the gas escapes from the open system.Burning steel wool is iron reacting with oxygen from the air to make iron oxide. The oxygen atoms join the iron, so the product weighs more than the iron alone. The oxygen came from the air, which was not on the balance at the start. In a sealed jar, the total mass would not change.
Key takeaway: oxygen atoms from the air are added.In an open system, matter can enter or leave, such as gas escaping from an open beaker or oxygen entering from the air. In a closed system, such as a sealed bottle, no matter can get in or out. Mass is always conserved, but only a closed system shows it directly on a balance.
Key takeaway: open lets matter in or out; closed does not.Count the atoms of each element on both sides of the arrow, multiplying each subscript by the coefficient in front. In 2H₂ + O₂ → 2H₂O there are 4 hydrogen and 2 oxygen atoms on each side, so it is balanced. To balance an equation, change only the coefficients, never the subscripts.
Key takeaway: same number of each kind of atom on both sides.An exothermic reaction releases thermal energy, so the surroundings get warmer: burning fuel, hand warmers, quicklime with water. An endothermic reaction takes in thermal energy, so the surroundings get colder: citric acid with baking soda, baking soda with vinegar. The difference comes from bond energies: breaking bonds takes energy and making bonds releases it.
Key takeaway: exothermic releases energy (warmer); endothermic absorbs it (colder).Common signs are a color change, bubbles of gas, a solid forming from two solutions (a precipitate), a temperature change, light, or a new smell. Each sign is only a clue, because some physical changes show them too. The best evidence is a new substance with different properties, such as a different melting point or density.
Key takeaway: clues are color, gas, precipitate, temperature, light; proof is a new substance.A disposable hand warmer contains iron powder, salt, water held in a powder, and activated carbon. When the packet is opened, oxygen from the air reaches the iron and it rusts quickly. Rusting is exothermic, so the packet warms up. The size and number of air holes control how fast the iron reacts, and so how hot it gets and how long it lasts.
Key takeaway: fast rusting of iron releases heat.No. A catalyst gives the reaction an easier path with a lower activation energy, so it goes faster, but the starting and final substances are the same, so the energy released or absorbed is the same. Yeast contains the enzyme catalase, which speeds up the breakdown of hydrogen peroxide into water and oxygen.
Key takeaway: a catalyst changes the speed, not the energy change.