U4.1 · Introduction for ReactionsPractice · 反应入门

A temperature change alone doesn’t prove a new substance formed. When $\ce{KNO3}$ dissolves, its ions spread into the water and absorb heat, but the solution still holds $\ce{K+}$ and $\ce{NO3-}$, and evaporating it gives back $\ce{KNO3}$. A precipitate, a gas ($\ce{CO2}$), and a color change with a new solid (Cu) all point to new substances.

The reaction makes $\ce{CO2}$, but the setup is closed, so the gas stays in the balloon. A chemical reaction only rearranges atoms; the kinds and numbers of atoms don’t change, so the total mass is unchanged (conservation of mass). With an open flask, $\ce{CO2}$ would escape and the measured mass would drop, which still doesn’t violate conservation of mass.

In a chemical change atoms regroup into different molecules: the H–H and Cl–Cl bonds break and new H–Cl bonds form. In the other three the molecules themselves stay the same; only their spacing or arrangement changes, so those are physical changes.

The lost mass is the $\ce{CO2}$ that escaped; mass is still conserved. n($\ce{CaCO3}$) = 5.00 ÷ 100.09 = 0.04996 mol, giving 0.04996 mol of $\ce{CO2}$ (1 : 1), which is 0.04996 × 44.01 = 2.20 g.

A reaction breaks old bonds and forms new ones, regrouping the atoms, but atoms are neither created nor destroyed, so the count of each kind of atom and the total mass are conserved. Many reactions form no gas or precipitate; neutralizing hydrochloric acid with NaOH, for instance, shows only a temperature change.