Final answer:
The root word of 'abbreviations' is 'abbreviate', and it refers to the practice of shortening words or phrases, often used to write more efficiently, similar to the use of chemical symbols in chemistry.
Step-by-step explanation:
The root word of 'abbreviations' in the sentence 'Sienna used abbreviations in her notes so she could write faster' is 'abbreviate'. The process of abbreviating involves shortening words or phrases to save time or space. In the context provided, Sienna likely used common abbreviations or possibly a form of shorthand to quickly jot down essential information. Similar to how in the field of chemistry, chemical symbols are used as a form of shorthand to represent compounds succinctly.