Recognizing a Chemical Reaction
Introduced| English | Meaning |
|---|---|
| chemical reaction/ˈkemɪkl rɪˈækʃn/ | A process in which substances are transformed into new substances by rearranging atoms and electrons. |
| law of conservation of mass/lɔː ɒv ˌkɒnsəˈveɪʃn ɒv mæs/ | law of conservation of mass |
Signs that something new was made
- Bubbles fizz up when vinegar meets baking soda.
- A nail turns orange-brown after weeks in the rain.
- A glowstick suddenly shines without any heat.
- Certain clues reveal that new substances have formed.
Clues of a reaction
- A chemical reaction 化学反应 makes new substances from old ones.
- Watch for a colour change, a gas, a precipitate, or heat and light.
- These signs hint that bonds broke and reformed.
Magnesium burning: a chemical reaction rearranges atoms and releases energy as light and heat
Select all signs that suggest a chemical reaction.
Difficulty 2/5 · 140 coins + a small bonus on first completion today
Colour change, gas, and a precipitate all hint at new substances.
Mass is conserved
- Atoms are only rearranged, never created or destroyed.
- So the total mass stays the same before and after.
- This is the law of conservation of mass 质量守恒定律.
In a chemical reaction, the total mass of reactants equals the total mass of products.
Difficulty 1/5 · 100 coins + a small bonus on first completion today
Atoms are conserved, so mass is conserved.
Mass is conserved even when a ____ escapes during the reaction.
Difficulty 1/5 · 100 coins + a small bonus on first completion today
The escaped gas still has mass, so total mass is unchanged.
Balancing the equation
- A balanced equation has equal atoms of each element on both sides.
- Adjust the coefficients (never the subscripts) to balance it.
- $2\text{H}_2 + \text{O}_2 \to 2\text{H}_2\text{O}$ is balanced.
Chemical reaction or not?
Decide whether each observation is a sign of a chemical reaction or just a physical change.
To balance an equation, you adjust the...
Difficulty 1/5 · 100 coins + a small bonus on first completion today
Change coefficients only; changing subscripts changes the substance.
Changing a subscript to balance an equation is allowed.
Difficulty 1/5 · 100 coins + a small bonus on first completion today
Changing a subscript changes the compound itself, so it is not allowed.
Is $\text{H}_2 + \text{O}_2 \to \text{H}_2\text{O}$ balanced?
- The left has 2 oxygen atoms; the right has 1, so it is not balanced.
- Fix it with coefficients: $2\text{H}_2 + \text{O}_2 \to 2\text{H}_2\text{O}$.
In $2\text{H}_2 + \text{O}_2 \to 2\text{H}_2\text{O}$, how many oxygen atoms are on each side?
Difficulty 2/5 · 140 coins + a small bonus on first completion today
$\text{O}_2$ has 2 O; $2\text{H}_2\text{O}$ has 2 O -- balanced.
Balance by changing coefficients (the big numbers in front), never subscripts — changing a subscript changes the substance itself. A single sign like bubbles suggests but does not prove a chemical change (dissolving can fizz too). And mass is always conserved, even when a gas escapes.
A chemical reaction turns old substances into new ones, hinted at by colour change, gas, a precipitate, or heat and light. Atoms are only rearranged, so the law of conservation of mass holds. Balance an equation by adjusting coefficients, never subscripts.