Both have a solution mixed in  water for example most foam are AFFF which makes it suitable for class A and B fires 

What is the main difference with water additive? 

Why water additive cannot be used on flammable liquid? (water may spread?) 

Thanks

 

There is a difference in terminology between the UK and some mainland European countries with regards to extinguishers.

In most countries they don't have Foam extinguishers as such (unless referring to obsolete chemical foam or specialist foam branchpipe extinguishers) but what they call Water with additive, which traditionally was a water based extinguisher with a spray nozzle & AFFF charge that was of a suitable concentration and quantity to be able to seal Class B fires as well as tackle Class A fires.

In the UK, where low expansion branchpipe extinguishers were common in the past the spray version was classed as foam spray. At this time in the UK water extinguishers were plain water jet for Class A fires only.

In the 90's water extinguishers with a spray nozzle were introduced that had a synthetic Class A additive that breaks the surface tension of the water increasing the penetration capability allowing a 3 litre extinguisher to achieve the same Class A rating as a 9 litre water jet. The additive mix when discharged looks like an AFFF discharge and is chemically similar, but usually not in the right formulation or concentration to effectively tackle a Class B fire and wouldn't usually 'seal' like foam.