【Foam】 ,  【froth】 ,  【spume】 ,  【scum】 ,  【lather】 ,  【suds】 ,  【yeast】  are comparable when they denote either a mass of bubbles gathering in or on the surface of a liquid or something as insubstantial as such a mass.

【Foam】 is the most comprehensive of these terms but is not interchangeable with all; it implies an aggregation of small bubbles such as rises to the top of a fermenting liquor or an effervescing or boiling liquid, or appears on the surface of the sea when agitated by high winds or covered with breaking waves; the term is also applicable to a bubbly slaver dribbling from the mouth of one in a rage or in great excitement or to the clotted sweat of an animal driven to exhaustion or suffering from intense heat.

Of all these words 【foam】 commonly has the most pleasant associations, usually connoting in poetry whiteness, delicacy, and grace.

【Froth】  is applicable to any 【foam】 , but it carries a stronger implication of insubstantiality, worthlessness, or, when there is direct or indirect reference to persons or animals, of mad excitement than  【foam】 carries.

【Spume】  is applicable where  【foam】  or  【froth】 might also be employed, but the term is chiefly used to denote the 【foam】 arising on an agitated body of water.

【Scum】 distinctively applies to the bubbly film that rises on boiling liquids, especially those containing organic matter or to a similar film which forms on molten metals or on the surface of a body of stagnant water.

Such scums ordinarily constitute impurities that are removed (as from broth or molten metal) or that constitute a contamination impairing the usability (as of stagnant water); this notion of worthlessness or obnoxiousness is carried over into extended use especially as applied to a class or body of persons.

【Lather】  and  【suds】  both apply to the 【foam】 produced by agitating water impregnated with soap or detergent.

【Lather】 , however, usually suggests a less frothy condition than  【suds】  and a heavier aggregation of small soapy bubbles <hard water does not produce a good  【lather】  for shaving>

【Suds】 , on the other hand, often denotes water so covered with a soapy 【foam】 that it is usable for laundering clothes.

【Lather】 , rather than  【suds】 , may be preferred when the 【foam】 induced by intense sweating or emotional excitement is denoted, but  【suds】 is more usual when the reference is to something that suggests the appearance of 【suds】 in a laundry tub or washing machine.

【Yeast】 basically applies to a 【froth】 or sediment composed of an aggregate of small fungal cells and found in saccharine liquids (as fruit juices and malt worts) in which it induces fermentation. The same substance is used as a leavening agent in bread; from this stems one line of its extended use in which it suggests a sign of activity, vitality, or agitation. But because 【yeast】 often appears as a 【froth】 on liquids and is accompanied by fermentation, the term has another line of extended use in which it is applied to a similar 【froth】 , 【foam】 , or 【spume】 , especially one appearing on the surface of an agitated sea.