Physical changes are when things get changed without altering chemical consistencies, which is melting solid butter into liquid one, or boiling water. Chemical changes are things such as caramelizing sugar when making sweets, or when carbon dioxide is created and released when baking bread.
Answer:
Krebs cycle is known as citric acid cycle because the citric acid is the first product formed in Krebs cycle. This cycle is also known as TCA cycle because the first product (citric acid) formed in Krebs cycle is a tricarboxylic acid (TCA).
Here's a chart that will help you color in the periodic table for #20:
Potassium Sulfate = K₂SO4
SO4 = negative 2, so you need 2 K+ ions to counteract it, hence K₂