What does Friar Laurence say will happen when Juliet drinks the potion in Romeo and Juliet?

Friar Laurence says that the potion Juliet drinks will mimic death. 

Juliet married Romeo in secret.  When he got himself banished for brawling in the streets and killing Tybalt, she would have been forced to marry another man.  Since she did not want to marry Paris, she went to Friar Laurence for help.  He knew her situation because he had secretly married her to Romeo. 

She told him she wanted to kill himself, but he had other ideas.  He told her that he would give her a special "vial" to save the day.

When presently through all thy veins shall run
A cold and drowsy humour, for no pulse
Shall keep his native progress, but surcease:
No warmth, no breath, shall testify thou livest;
The roses in thy lips and cheeks shall fade
To paly ashes, thy eyes' windows fall,
Like death, when he shuts up the day of life … (Act 4, Scene 1)

Friar Laurence was gifted with herbs and potions, and knew just the trick.  He had developed a special potion which had the qualities to mimic death. If Juliet drank it, everyone would think she was dead and she would be buried in her family’s tomb.  Later, she could be reunited with Romeo. 

The problem was that Friar Laurence was also supposed to get word to Romeo, but a plague delayed the letter.  Romeo killed himself after thinking she was dead, right there in her tomb.  She awoke to that.  Seeing he had taken poison, she stabbed herself.

After Juliet died for real, Friar Laurence explained to her parents and Romeo’s parents what happened.

Then gave I her, so tutor'd by my art,
A sleeping potion; which so took effect
As I intended, for it wrought on her
The form of death: meantime I writ to Romeo,
That he should hither come as this dire night,
To help to take her from her borrow'd grave,
Being the time the potion's force should cease. (Act 5, Scene 3)

Friar Laurence took full responsibility, but the Capulets and Montagues forgave him.  They were devastated by the loss of their children, but they realized their part in their deaths.  With their children's deaths, they buried their feud.