Chemical Reactions

What is Chemical Reactions?

A chemical reaction is one of the most fundamental events in nature: atoms that were bonded in one arrangement break apart and reconnect in a completely new arrangement, forming substances with entirely different properties. When wood burns in a campfire, the carbon and hydrogen in the wood combine with oxygen from the air to produce carbon dioxide, water vapor, and light — a combustion reaction. The wood and oxygen are reactants; the CO2 and water are products. One of the most powerful ideas in science is that no atoms are created or destroyed during this process — they are only rearranged. This is the Law of Conservation of Mass, and it is why balanced chemical equations have equal numbers of each type of atom on both sides. This simulation shows four important reaction types at the molecular level: combustion, synthesis, decomposition, and single displacement. You can control temperature, reactant concentration, and — for Pro users — add a catalyst to see how reaction speed changes without changing the final outcome.

Parameters explained

Temperature(°C)

Temperature sets how much kinetic energy the reacting particles have before they collide. When you raise the slider, particles move faster, collide more often, and hit with enough energy to break old bonds and form new ones more frequently. Lower temperatures slow those collisions, so the same reaction pattern can take longer to unfold. Keep the Concentration slider fixed while changing Temperature to isolate this variable. Then compare the Combustion, Synthesis, and Decomposition presets to see that temperature changes the rate of successful collisions, not the rule that atoms must be conserved in the balanced reaction.

Reactant Concentration(mol/L)

Concentration controls how many reactant particles are available in the reaction space. A higher value packs more particles into the same volume, which increases the number of collisions per second and usually speeds up product formation. A lower value spreads particles farther apart, so successful collisions are less frequent. Use this slider after choosing a preset: Combustion, Synthesis, and Decomposition each keep their own balanced equation, but concentration changes how quickly the visible reaction proceeds. Students can compare two trials with the same Temperature to explain reaction rate using particle crowding instead of changing the identities of the reactants.

Common misconceptions

• MisconceptionMatter is destroyed during chemical reactions — things just disappear.

  CorrectMatter is never created or destroyed in a chemical reaction. When something burns, it seems to disappear, but the atoms from the original substance recombine with oxygen from the air to form gases like CO2 and water vapor that disperse into the atmosphere. If you could collect all the products — including the gases — and weigh them, the total mass would equal the mass of the original substances plus the oxygen that reacted. This is the Law of Conservation of Mass, one of the most tested and reliable principles in all of science.

• MisconceptionExothermic reactions always make fire or explosions.

  CorrectExothermic reactions release energy as heat, but only some of them are dramatic enough to produce visible flames or explosions. Hand warmers use an exothermic reaction (iron oxidation) that just produces warmth. Even dissolving some salts in water releases heat. Note: mixing baking soda and vinegar is actually endothermic — the mixture gets slightly cooler, not warmer. The amount and rate of energy release or absorption — not just the sign (exo vs. endo) — determines how dramatic the reaction appears.

• MisconceptionThe preset button changes the rules of conservation of mass.

  CorrectA preset changes which reaction example you are viewing, not the rule that atoms are conserved. Combustion, synthesis, and decomposition have different reactants and products, but each balanced equation still keeps the same number of each atom type before and after the reaction. The preset is a shortcut for comparing reaction patterns. It does not allow atoms to appear, disappear, or change into different elements.

• MisconceptionBalancing chemical equations means changing the formulas of the molecules.

  CorrectWhen balancing equations, you can only change the coefficients (numbers in front of each formula) — never the subscripts inside the molecular formula. Changing subscripts would change the identity of the substance: H2O is water, but H2O2 is hydrogen peroxide, a completely different compound. Coefficients tell you how many molecules participate, while subscripts tell you how many atoms are in each molecule. Balancing adjusts the 'how many' without changing 'what kind.'

• MisconceptionHigher temperature always means a faster reaction with more products.

  CorrectHigher temperature does typically increase reaction rate, but it does not change the amount of product formed if the reactants are limited by concentration, not by rate. Once all reactants are used up, no more product forms regardless of temperature. Additionally, some reactions are reversed at high temperatures — heating can cause products to decompose back into reactants. Temperature affects speed and sometimes equilibrium position, but the stoichiometry (ratio of reactants to products) is fixed by the balanced equation.

How teachers use this lab

1. Conservation of mass verification: choose the Combustion preset and set concentration to 1.0 mol/L. Students count the atoms on each side of the displayed equation, confirm they balance, and then calculate the expected mass of products. Connect to the historical experiment where Lavoisier showed that mass is conserved in combustion by conducting reactions in sealed containers (MS-PS1-2).
2. Reaction type classification: run the Combustion, Synthesis, and Decomposition presets at temperature 25°C and concentration 1.0 mol/L. For each, students record the reactants, products, and molecular animation, then write a general pattern such as A+B → AB or AB → A+B. Students sort real-world examples into the matching categories (MS-PS1-2).
3. Temperature as a variable: choose the Synthesis preset and hold concentration at 1.0 mol/L. Run the reaction at temperature 20°C, 200°C, and 400°C, counting how many product molecules form per unit time shown on screen. Students graph temperature vs. reaction rate and explain the relationship using the idea that higher temperature means more energetic collisions (MS-PS1-5).
4. Concentration as a variable: choose the Combustion preset and hold temperature at 25°C. Run low, medium, and high concentration trials, recording the time needed for visible product formation. Students compare particle crowding across trials and use collision frequency to explain why reaction rate changes while the balanced equation remains the same (MS-PS1-5).
5. Exothermic vs. endothermic sorting: run Combustion and Decomposition at the same temperature and concentration settings. Students observe the temperature or energy change shown in the simulation for each, classify as exo or endothermic, and then sort a list of real reactions such as ice packs, hand warmers, photosynthesis, and burning toast into the same categories using evidence from the simulation.