The quality of being like soap, having the characteristics of soap.
Soapiness is usually associated with cleanliness and a fresh smell.
Examples:
I like the soapiness of this shampoo.
The soapiness of the water makes it good for cleaning.
The soapiness of the air after it rains is nice.
The soapiness of the bubbles makes them fun to play with.
The soapiness of the ocean water was so strong you could smell it from far away.
The soapiness of the soap made me feel clean and fresh.