Seltzer is carbonated water, which is plain water that has been carbonated with added carbon dioxide gas. It is also known as soda water or sparkling water. Seltzer is the simplest form of carbonated water, and it is often used as a base for flavored carbonated drinks like La Croix and Spindrift. Seltzer is occasionally flavored using natural fruit essences and oils. Seltzer is different from club soda, which has added minerals like sodium bicarbonate and potassium sulfate, giving it a slightly saltier taste than seltzer. Tonic water is another type of carbonated water that is sweetened with high-fructose corn syrup and flavored with quinine, which gives it a little bitterness. Seltzer is often used as a mixer in cocktails, and it is a favorite of bartenders for mixed drinks. Seltzer is also used to make hard seltzer, which is a type of highball drink containing seltzer, alcohol, and often fruit flavorings.