It's a salt of carbonic acid. it's called sodium bicarbonate. It contains the anion HCO3. It usually refers to bicarbonate of soda (baking soda), a white powder that is a common ingredient in antacids. It is also a byproduct of your body's metabolism.
A bicarbonate is a salt containing the anion HCO3-, which is the most important buffer in the blood, it is regulated by the kidney, which excretes it in excess and retains it when needed; it increases with ingestion of excess anti-acids, diuretics and steroids; it is decreased with diarrhoea, liver disease, renal disease and chemical poisoning.
This is an example of foreshadowing. This is when the plot gives you details, so in this case the nightmare, that symbolizes a dreadful event will take place sometime later in the story. Foreshadowing is a technique used to hint at something that will transpire in the story and the foreshadowing nearly always appears at the beginning of the story.
