In Experiment 3, white smoke is produced at the end nearer $\ce{HCl}$ because:

• a$\ce{HCl}$ diffuses faster than $\ce{NH3}$
• b$\ce{NH3}$ diffuses faster than $\ce{HCl}$, meeting closer to the $$ end
• cThe tube is uneven
• dAir pushes the gases