There are many ways to descale a kettle

...Some more successful than others

The limescale in your kettle is mostly chalk, calcium carbonate (CaCO₃), which is a base. To remove a base you ideally need an acid. Agreed, you could use a stronger base to remove the limescale.

• Bicarbonate of soda, aka sodium bicarbonate, sodium hydrogen carbonate (NaHCO₃) is a base that is commonly claimed as being useful for descaling a kettle. It may eventually do the job, but then again you could scrub your garden path with a toothbrush. Its not ideal, there are better products.

You could also use a base/acid combination

• Baking powder is a usually a mix of sodium bicarbonate (a base) and cream of tartar (tartaric acid). The two when added to water fizz away producing carbon dioxide. When added to limescale the baking powder will fizz away removing the limescale, but as its also reacting with the bicarbonate of soda it will be a slow old job.

So, its best to use an acid, and there are several available.

• Hydrochloric acid, this stuff is just dangerous, its not suitable for a domestic enviroment. Avoid!
• Phosphoric acid, is a powerful descaler, but its not a nice chemical to work with:
  
  • It is a highly corrosive chemical which can irritate and burn the eyes as well as the nose, throat and lungs.
  • Phosphoric acid also has some rather complicated handling and storage requirements.
  All in all phosphoric acid is best avoided.
• Vinegar, which contains acetic acid. Its safe and is a common item in most kitchens, but vinegar has a few drawbacks:
  • Its a faily weak acid. A teaspoon of vinigar is mostly water, so it is going to take a while to work.
  • Vinegar is known for its strong flavour which is going to be difficult to get rid of. Vinegar flavour tea? No thanks! 🤣
• Lemons, which contain citric acid. They're safe and also another common kitchen item, but lemons also have a few drawbacks:
  • Like vinegar, lemon juice is mostly water, so it too will be slow.
  • And again, lemon juice has a strong flavour which will take some getting rid of. Lemon coffee? It might catch on. 🙄

However, when purified and concentrated Citric Acid has some useful properties...