Does najis become pure when it dries?

It depends on what the najis is on. On clothes, skin and carpets, drying alone does not make najis pure, and you need to wash it with water. On bare ground, the Hanafi school and some other scholars say that najis becomes pure when it dries and the trace is gone, while the other schools say water is needed.

Clothes, skin and carpets

For anything you wear or touch, the rule is the same: remove the impurity and wash the spot with clean water until the trace is gone. Letting it dry does not change the fact that it is still there, even if you can no longer see it. This is why a dried urine stain on a shirt still needs washing before you pray. See How do I clean a najis carpet?

Floors and bare ground

The question of dried najis on the ground comes from two hadiths:

A Bedouin urinated in the mosque, and the Prophet ﷺ ordered the Companions to pour a bucket or a tumbler of water over the place (Sahih al-Bukhari 220).
Hamza ibn Abdullah reported that in the lifetime of the Prophet ﷺ, dogs used to urinate and pass through the mosque, and people did not sprinkle water on it (Sahih al-Bukhari 174).

What the schools say

Hanafi: najis on the ground becomes pure when the sun or wind dries it and no colour, smell or trace is left. You can pray on that ground, but not do tayammum with it. They connect this to the second hadith.
Shafi'i and Hanbali: the ground is purified only with water, as in the Bedouin hadith. Drying alone is not enough.
Maliki: in the main view, najis also needs water to remove it, even from the ground.

The Hanafi view is a real concession, especially for outdoor prayer areas. It does not apply to clothes, skin or carpets, where the dried impurity can still cling.

What this means at home

Outdoors, such as a garden or a park: if you are praying on dry soil or grass where a dog or a child has relieved itself in the past, you can follow the Hanafi view and pray there once all trace is gone.
Indoors, such as a bathroom floor or kitchen tiles: wipe up the impurity and pour water over it, then dry.
Clothes: wash the stained area, or the whole garment in the machine.
If you are unsure whether something was ever impure: your clothes and floors are considered clean. Do not act on suspicion alone.

Floors and laundry have their own guides. See Is a wet bathroom floor najis? and Does a washing machine purify najis clothes?

The principle behind it

Purity needs a purifier, and the Prophet ﷺ pointed to water for this. The Hanafi view is based on the idea that drying removes the substance of the impurity from the soil. In any case, if you follow your own school and clean carefully, your prayer is valid.

Key points

Drying does not purify najis on clothes, skin or carpets; wash with water.
The Hanafis say dried najis on bare ground becomes pure once the trace is gone.
Shafi'i, Hanbali and Maliki scholars require water for the ground too (Sahih al-Bukhari 220).
Where you are unsure, assume purity.

Tip: If something impure falls on a hard floor at home, do not wait for it to dry. Wipe it up and pour a little water over the spot straight away, and you will satisfy every view.