Alkali Metals Reactivity
Observe how reactivity increases down Group 1
What this lesson covers
Why it matters
Sodium dances on water; potassium explodes. Why does the heavier metal react more violently?
The idea in plain words
Balance check: confirm the displacement of hydrogen from water by potassium.
Predict first
Why does potassium react more violently with water than sodium?
Down the group, atomic size increases. The outer electron is shielded and further away, making it easier to lose — increasing reactivity.
- Potassium has more protons, so it pulls electrons tighter.
- Potassium's outer electron is further from the nucleus and easier to lose. — correct
- Potassium is softer, so it breaks apart faster.
- Sodium forms a protective oxide layer instantly.
What you do
First, drop sodium. Note the fizzing. Then, drop potassium. Compare the violence of the reaction.
Check yourself
What gas is produced when sodium reacts with cold water?
The equation 2Na + 2H₂O → 2NaOH + H₂ shows hydrogen gas is released.
Which observation distinguishes potassium's reaction from sodium's?
Reactivity increases down the group. Potassium's reaction is described as explosive, while sodium's is vigorous and may catch fire.
Why is the reaction between sodium and water classified as a displacement reaction?
A more reactive element (Na) displaces a less reactive element (H) from its compound (H₂O).
- Oxygen
- Hydrogen — correct
- Nitrogen
- Potassium reacts explosively; sodium is vigorous but may just catch fire. — correct
- Sodium reacts explosively; potassium is slow.
- Both react at the same rate.
- Potassium produces a blue flame; sodium produces a yellow flame.
- Sodium displaces hydrogen from water to form NaOH and H₂. — correct
- Water displaces sodium from the air.
- It is a combination reaction.