Why are green algae but not brown algae
considered part of the plant kingdom?

Green algae are considered part of the plant kingdom because green algae contain the same cartenoids and chlorophyll a and b as plants that are on land.

Explanation:

Brown algae cannot be classified as part of plant kingdom because they have different accessory pigments and types of chlorophyll molecules in addition to chlorophyll a .

Green algae has chlorophyll a and b, xanthophylls while brown algae has chlorophyll a and c and fucoxanthin. It's also classified as kingdom protista because they are non plant photoautotrophs eukaryotes.

if you mean hydrogen and oxygen then water is the product

the temperature of the gas, in k, not celsius.