An experiment is performed in which a crystalline substance is added to a beaker filled with room-temperature water. The following observations were made: 1. The crystals dissolve. 2. A precipitate forms. 3. The bottom of the breaker feels warm to the touch. 4. Bubbles begin to form in the water. Which observations are evidence that a chemical reaction took place?

Which statement accurately describes the relationship between air pressure, air density, or altitude? as altitude increases, pressure increases.as altitude increases, air density increases.air pressure and density are lowest at sea level.denser air exerts more pressure than less dense air.

Achange in the number of neutrons in an atom will change an blank . when the number of protons changes in an atom, a new element will form.

The scheme below is from a series of reactions that are part of a synthesis of vitamin a. answer the following questions with reference to this scheme. (i) what is "reagent a"? (ii) draw a step-by-step mechanism which explains the formation of compound c from compound b (iii) which reagents would you use to form compound e from compounds c and d (reagents b and c)? for each reagent suggested above in (ii) explain the role of the reagent in the reaction to (iv) form compound e. you may wish to do this by drawing a mechanism. 1. addition of reagent a но reagent a 2. н,о" thо oh нон-с compound a. compound b. compound c .ch-оh 1. reagent b "сно 2. reagent c сh oh compound e. compound d.