Q.Describe the shape of the $\text{CO}_2$ molecule and its hybridisation. Write the equation for the reaction of $\text{CO}_2$ with lime water, and explain why this is used as a standard laboratory test for the gas.

[!TLDR] CO2 is a linear, sp-hybridised O=C=O molecule; it turns lime water milky by precipitating CaCO3, the standard laboratory test for the gas. [!ANSWER] CO2 is linear (O=C=O), sp hybridised at carbon; $\text{Ca(OH)}_2+\text{CO}_2\rightarrow \text{CaCO}_3\downarrow+\text{H}_2\text{O}$, the white CaCO3 precipitate turning the lime water milky is the standard test for CO2.

Carbon dioxide is a simple, symmetric molecule with the structure $\text{O}=\text{C}=\text{O}$: two equivalent carbon-oxygen double bonds arranged at $180^\circ$ to each other, giving the molecule its characteristic linear shape, with the central carbon atom $sp$ hybridised (two $sp$ hybrid orbitals used for the two $\sigma$ components of the double bonds, with the remaining unhybridised $p$ orbitals on carbon forming the two $\pi$ bonds). Chemically, $\text{CO}_2$ is an acidic oxide and reacts readily with basic hydroxides; its reaction with lime water — a dilute solution/suspension of calcium hydroxide — is the classic laboratory test used to confirm the presence of the gas:\n$\text{Ca(OH)}_2 + \text{CO}_2 \longrightarrow \text{CaCO}_3\!\downarrow + \text{H}_2\text{O}$\nThe calcium carbonate produced is only sparingly soluble in water, so it precipitates out as a fine white solid, turning the originally clear lime water distinctly cloudy or 'milky' in appearance. This visible, unambiguous colour change (from clear to milky white) on simply bubbling the unknown gas through lime water makes the test quick, cheap and reliable, which is why it remains the standard introductory-level identification test for $\text{CO}_2$ (if excess gas continues to be passed, the milky precipitate will eventually redissolve as the more soluble bicarbonate, $\text{Ca(HCO}_3)_2$, forms, a useful further confirmatory observation). [!ANSWER] $\text{CO}_2$ is linear and $sp$-hybridised at carbon; it turns lime water milky via $\text{Ca(OH)}_2+\text{CO}_2\rightarrow \text{CaCO}_3\downarrow+\text{H}_2\text{O}$, the standard laboratory test for the gas.

State CO2's linear shape and sp hybridisation from its double-bond structure, then write and explain the lime-water test equation and why the milky precipitate forms.

Describing CO2 as bent/angular rather than linear; forgetting that excess CO2 redissolves the CaCO3 precipitate via bicarbonate formation, which is a commonly tested follow-up point.