Sparkling water, also known as soda water or seltzer water, is made by dissolving CO2 in water. There are many in-home carbonation systems that will allow you to turn your tap water into sparkling water within seconds, and many of these systems are reasonably priced. If you choose to buy a carbonation system, you'll only have to worry about refilling the CO2 tank once it runs out. If you choose to create the carbonation system yourself, however, you'll need to buy a couple speciality products.
Fill your two-liter bottle with tap water and let it sit in the refrigerator overnight. Don't fill the bottle completely; leave about 1 1/2-inches of space at the top. You can place the bottle in the freezer an hour before starting your carbonation process to make the water even colder. While colder water makes for better carbonation, don't add ice cubes to your mix.
Remove the bottle of water from the refrigerator and screw the carbonation cap onto the top of it. The carbonation cap, also known as a carbonator cap, is a small cap designed to screw on to the top of a two-liter soda bottle. It is made for carbonating beverages and can be found at your local beer supply store.
Press down on the carbonation cap's stop valve to let out excess air from the bottle. The stop valve is a silver ball on the top of the cap. It works just like the air valve for a car tire; once you press it down, air will release. The 1 1/2-inches of space you left at the top of the bottle will be completely gone once you've squeezed out all the air. At this point, the bottle will be deflated.
Connect your ball lock to the tube on your CO2 tank. The device will easily slide right on. The ball lock is specifically made for carbonation and you can also find these at your local beer supply store. Make sure to buy a ball lock that fits the stop valve of your specific carbonation cap.
Press the ball lock firmly onto the carbonation cap until it snaps into place.
Slowly turn on your CO2 tank and release CO2 into the bottle until it resumes its original shape. The 1 1/2-inches of air at the top of the bottle has now been replaced with CO2.
Shake the bottle in an upright position for 30 seconds. You will hear the CO2 hissing as it gets absorbed into the water.
Shut off your CO2 tank and remove the ball lock and tube.
Let the bottle sit for a couple minutes.
Press the stop valve on the carbonation cap to release a little bit of the pressure. Remove the carbonation cap and replace it with a normal two-liter soda cap.
If you don't have a CO2 tank you can use a CO2 canister instead. These are essentially mini CO2 tanks and can only be used once. You can also find small CO2 tanks for a reasonable price.