Please help! Thanks in advance.

This is a characteristic of the elements in the periodic table that shows a pattern. it may increase or decrease across or down the table.

How many moles of oxygen gas are produced from the decomposition of six moles of potassium chlorate

When the chemicals iron sulfide (fes) and hydrochloric acid (hcl) are combined, bubbles appear from the mixture. 1. does the appearance of bubbles indicate a physical or chemical change? 2. why do the bubbles indicate this change? 3. what property is this?