Stages 3 · 4
Chemistry
Count the atoms: substances change, the atoms only regroup.
Seeds
CHEM-1
Atoms are rearranged, never lost
CHEM-2
Structure decides properties
CHEM-3
Reactions trade energy
CHEM-4
Reactions have a speed and a balance point
CHEM-5
The periodic table is a pattern
Stages
3
Structure and Evidence
How does one substance turn into another?
- Atoms are rearranged, never lost In a reaction atoms regroup into new molecules; the count of each kind of atom, and so the mass, stays the same. Try it
- Snap atoms into molecules Each atom makes a fixed number of bonds, so only certain molecules can be built. Try it
- Same atoms, different stuff Properties come from how atoms are arranged, not from the atoms themselves: graphite and diamond are both pure carbon. Try it
- pH: every step is ten times Each step down the pH scale means ten times more hydrogen ions. Try it
- New stuff, or the same stuff changed? A change is chemical only when a new substance forms; melting, boiling and dissolving keep the same molecules, burning makes new ones. Try it
- What does a nail need to rust? Iron rusts only when water and oxygen are both present; take away either one and the nail stays almost bright. Try it
- A mole is a number, like a dozen A mole is a fixed count of particles, 6.02 × 10²³; a mole of magnesium weighs twice a mole of carbon because each magnesium atom is twice as heavy. Try it
4
Systems and Argument
Why do reactions happen, and how fast?
- Where reaction heat comes from Breaking bonds costs energy and making bonds releases it; a reaction is exothermic when the new bonds are stronger. Try it
- Hotter means harder hits Warming speeds a reaction mainly because many more collisions carry enough energy to react, not just because there are more collisions. Try it
- Equilibrium is busy, not stopped At equilibrium the forward and backward reactions run at the same rate, so amounts stay constant while particles keep changing. Try it
- Why the table has rows and columns Elements in a column behave alike because they have the same number of outer electrons. Try it
- Why salt water carries a current and sugar water doesn't Salt is built of charged ions that come apart and move freely when it dissolves or melts; sugar dissolves as whole neutral molecules, so nothing carries the charge. Try it
- Adding base: the pH jump at the end H⁺ and OH⁻ pair into water one for one, so the pH barely moves until the counts are almost equal, then a single drop pushes it up several units. Try it
Ideas we expect to meet
- Burning destroys matter; mass is lost in a reaction.
- Atoms have the properties of the substance: copper atoms are orange, ice molecules are cold.
- Breaking a chemical bond releases energy.
- At equilibrium the reaction has stopped.
- The periodic table is ordered by atomic mass.