Question

The "Chemistry in Focus" segment Green Chemistry discusses the use of gaseous carbon dioxide in place of CFCs and of liquid carbon dioxide in place of the dry-cleaning chemical PERC. Would you expect carbon dioxide to be very soluble in water? Explain your answer.

Short answer

Carbon dioxide (CO₂) is a non-polar molecule, while water is a polar solvent. Based on the principle "like dissolves like," non-polar solutes do not dissolve well in polar solvents. Thus, CO₂ would not be very soluble in water. However, it is important to note that CO₂ does dissolve in water to a certain extent, forming carbonic acid (H₂CO₃), which can further dissociate into bicarbonate (HCO₃⁻) and hydrogen ions (H⁺).

Step-by-step solution

Recognize the molecule of interest

In this case, the molecule is carbon dioxide, or CO₂.

Understand chemical properties of Carbon Dioxide

Carbon dioxide is a non-polar molecule. It has a linear molecular shape with a central carbon atom bonded with two oxygen atoms. CO₂ exhibits minimal polarity as the electronegativity difference between carbon and oxygen atoms is not significant enough to create significant polarity.

Understand properties of water

Water is a polar molecule. Its molecular shape is bent with an oxygen atom connected to two hydrogen atoms. The oxygen atom has a partial negative charge due to its high electronegativity and the hydrogen atoms have partial positive charges.

Analyze the solubility of CO₂ in water

Solubility of a substance in a solvent primarily depends on the principle "like dissolves like." Polar solutes dissolve well in polar solvents while non-polar solutes dissolve well in non-polar solvents. Carbon dioxide being a non-polar molecule interacts poorly with water, which is a polar solvent. However, it's worth noting that carbon dioxide does dissolve in water to some extent. In water, it forms carbonic acid (H₂CO₃), which can further dissociate into bicarbonate (HCO₃⁻) and hydrogen ions (H⁺).

Conclusion

Based on the analysis above, carbon dioxide being a non-polar molecule does not dissolve very well in water, a polar solvent. But it does dissolve to some extent, forming carbonic acid and other species.

Key concepts

Polar and Non-Polar Molecules

In chemistry, molecules can be categorized as either polar or non-polar based on their electric charge distributions. Polar molecules have regions with slight positive and negative charges due to differences in electronegativity between bonded atoms. A classic example of a polar molecule is water (H₂O). Non-polar molecules, such as carbon dioxide (CO₂), have either equal sharing of electrons or a symmetrical arrangement that offsets any polarity. For example, CO₂ is linear, with carbon in the center bonded equally to two oxygen atoms. There's no significant charge difference across the molecule, making it non-polar.
Understanding these distinctions is crucial because they dictate how molecules interact with each other. Polar molecules tend to interact well with other polar molecules (like water), while non-polar molecules prefer non-polar partners.

Solubility Principles

Solubility is a measure of how well a solute can dissolve in a solvent, which is largely dictated by the principle "like dissolves like." This means that polar solutes are more soluble in polar solvents, and non-polar solutes are better dissolved in non-polar solvents.
Solubility is not an absolute concept; it's a spectrum. Factors such as temperature and pressure can also affect solubility. Nonetheless, the basic rule stems from molecular polarity. For instance, sugar (a polar substance) dissolves well in water (a polar solvent), whereas oil (a non-polar substance) does not.
This principle helps predict and understand the interactions between different chemical substances in various contexts.

Chemical Properties of Carbon Dioxide

Carbon dioxide (CO₂) is a fascinating non-polar molecule. It has a linear shape, with two oxygen atoms symmetrically double-bonded to a central carbon atom. The electronegativity difference between carbon and oxygen is not enough to create significant polarity, maintaining CO₂ as non-polar overall.
Despite its non-polar nature, CO₂ interacts with water to a certain extent. When dissolved in water, CO₂ forms carbonic acid (H₂CO₃). This reaction isn't particularly strong under normal conditions, but it is an essential process in both atmospheric chemistry and the carbon cycle.
Even though CO₂ doesn't dissolve readily in water, its slight solubility plays a vital role in aquatic ecosystems and the environment.

Water as a Solvent

Water is one of the most versatile solvents known, owing to its polar nature and ability to dissolve a wide range of substances. This is due to its bent molecular shape, where an oxygen atom with a partial negative charge is connected to two hydrogens with partial positive charges.
Because water is polar, it is proficient at dissolving other polar molecules and ionic compounds. Its unique properties make it an excellent medium for chemical reactions, both in nature and various industries.
Although water predominantly dissolves polar substances, it's noteworthy that it can still interact with non-polar molecules like carbon dioxide to some degree. This interaction forms the basis of many natural processes such as carbonic acid formation in rainwater.