Final answer:
Adding a black powder to a clear liquid resulting in fizzing, bubbling, and heat indicates an exothermic chemical reaction, producing a new substance with different properties, such as iron sulfide from iron and sulfur.
Step-by-step explanation:
When a black powder is added to a clear liquid, with fizzing, bubbling, and heat being produced, this suggests a chemical reaction is taking place. In the case described, where heat is applied to a mixture in a test tube and it becomes hot to the touch, we are likely seeing an exothermic reaction, which releases heat. One example of such a reaction is when iron filings and sulfur powder react to form iron sulfide. This particular reaction changes the appearance and properties of the reactants into a new product, in this case, a compound with different properties such as color and magnetic behavior.
Observations that indicate a chemical change include temperature changes, light emission, unexpected color changes, and bubonic formation when no boiling occurs. These changes suggest the formation of new substances. This process is well-documented in engagement with Chemical Equations which help to understand the substances and quantities involved in chemical reactions.