Connect the boxes:
1. 2.
3. 4.
5. 6.
1→ 2→ 4→ 6
5→ 3→ 4→ 2
1→ 5→ 4→ 2
5→ 6→ 4→ 2
NaOH + NaCl → NaCl (neutrtalization reaction)
Box 3 and 4 are not used.