The phrase "are difficult to separate" is correct and usable in written English.
It can be used when discussing concepts, ideas, or items that are not easily distinguished or divided from one another.
Example: "The themes of love and sacrifice in the novel are difficult to separate, as they intertwine throughout the story."
Alternatives: "are hard to distinguish" or "are not easily divided".
Adsorption is an effective technique for the purification and dehydration of gases and as a means of fluid fractionating, which are difficult to separate by other means of separation [3].
The results show that the PEG mixtures with molecular weights of 400 and 8000, and those of 1500 and 20,000, are separable, yet those of 1500 and 3500 are difficult to separate by the selected column.
Yet the issues are difficult to separate.
In the case of Judge Poe in Houston, some critics suggest his religious beliefs are difficult to separate from his legal ones.
The worse culprits are single-use plastic and plastic packaging: coffee cups (10,000 chucked every two minutes, just in the UK), straws (Americans use 500m every day), yoghurt cartons, cocktail stirrers, plastic razors, microbeads, and Tetra Pak cartons (because they're made of several ingredients that are difficult to separate: card, aluminium, plastic coating) and coffee pods.
The red shirt and the red wall are difficult to separate even with user interactions.
Data for short-tailed and sooty shearwaters were pooled, as these species are difficult to separate at sea.
However, the traditional carbon materials are difficult to separate from solution when they are used as adsorbent.
The resulting scatterplot suggests a number of groupings that may represent instars, although the smallest individuals fall very close together and are difficult to separate.
These inaccuracies are difficult to separate from the tendency of ITS2 as a non-coding region to accumulate indels.
Second, use of substituted phthalic anhydrides yields isomeric mixtures of rhodamines that are difficult to separate, requiring tedious chromatography.
