Name: why some substances dissolve in water
Uploaded: 2025-05-22T09:05:33.000Z
Duration: 3 min
Channel: JoVE

K12 Standards

Minnesota Academic Standards in Science

9-12

Chemistry

3.2.1 9C.3.1.1.3

Solubility

溶液とは、溶質と溶媒という 2 つの主要な物質が均質に混ざり合ったものです。溶媒は溶質よりも多く存在します。
希薄溶液では、溶媒に対する溶質の割合は小さいですが、濃縮溶液ではその割合が大きくなります。
溶液を形成するとき、溶質と溶媒は化学的に反応しません。代わりに、分子は再分布し、個々の溶媒分子が溶質分子を取り囲み、引力を通じて相互作用します。このような溶解過程を溶媒和と呼びます。
溶媒和を有利に進めるには、溶質-溶媒相互作用は、溶媒-溶媒および溶質-溶質相互作用よりも強くなければなりません。
溶液中では、溶質と溶媒は任意の物質状態にすることができます。ソリューション自体は 1 つの "フェーズ" です。
溶媒和の反対は、強い溶質-溶質相互作用によって引き起こされる沈殿物です。溶媒和速度が降水速度と等しい場合、溶解度平衡が確立されます。
溶質の溶解度は、一定量の溶媒に溶解平衡したときの最大可能濃度です。溶解性は、温度や分子極性などの他の物理的条件の影響を受けます。
非極性溶質は分子間ロンドン分散力によって非極性溶媒に溶解しますが、極性溶質またはイオン溶質はイオン-双極子または双極子-双極子相互作用を介して極性溶媒に溶解します。
水は極性溶媒です。親水性、または水を好む化合物は、イオン電荷または極性基により、十分に強い水と溶質の相互作用を可能にするため、水溶性です。溶質-溶質相互作用は、非極性溶媒と極性溶質分子との間の可能な相互作用よりもはるかに強いため、非極性溶媒には溶けにくいです。
油のような水に不溶性で脂溶性の非極性化合物は、溶媒間相互作用と競合する水素結合を形成できないため、疎水性または水恐怖症です。
両親媒性または両親媒性化合物は、極性基と非極性基の両方を有する。例えば、石鹸は脂肪酸の金属塩です。水中では、石鹸分子はミセルと呼ばれる球体に集まります。彼らの非極性の疎水性脂肪酸の尾は内側を向き、極性の親水性の頭は外側を向いています。
石けんを使って油汚れを落とすと、尾部が無極性の油を取り囲んで水溶性のミセルを形成し、油を洗い流すことができます。

溶解度

溶液、溶解度、溶解度平衡

溶液は、主成分である溶媒と微量成分である溶質で構成される均一な混合物です。 溶液の物理的状態 (固体、液体、または気体) は通常、溶媒の物理的状態と同じです。 溶質濃度は、希薄 (比較的低濃度) や濃縮 (比較的高濃度) などの定性的な用語で表されることがよくあります。

溶液中では、溶質粒子（分子、原子、イオン）は溶媒分子に密に囲まれており、引力によって相互作用します。 この溶解プロセスは溶解と呼ばれます。 水が溶媒である場合、このプロセスは水和として知られています。 溶解の場合、溶質と溶媒の相互作用は、溶質と溶質および溶媒と溶媒の相互作用よりも強くなければなりません。 沈殿は溶解の逆であり、溶質間の強い相互作用によって起こります。

溶解度は、所定量の溶媒に溶解できる溶質の最大量の尺度です。 温度、圧力、分子極性は、溶解度に影響を与える重要な要素の一部です。 溶解度平衡は、溶質種の溶解と沈殿が同じ速度で起こるときに確立されます。

似たものに溶ける

溶質が特定の溶媒に溶けるかどうかを予測するには、「似たものは似たものに溶ける」という経験則があります。 極性またはイオン性溶質は、溶媒分子とのイオン-双極子相互作用または双極子-双極子相互作用により極性溶媒に溶解します。 このような相互作用は、非極性溶媒では不可能です。 非極性溶質は、分子間分散力によって非極性溶媒に溶解します。

親水性、疎水性、両親媒性化合物

水は極性溶媒です。 水に溶ける溶質は「親水性」と呼ばれます。 たとえば、固体の KCl を水に加えると、極性水分子のプラス (水素) 端はマイナスの塩化物イオンに引き寄せられ、水のマイナス (酸素) 端はプラスのカリウム イオンに引き寄せられます。 水の分子は個々の K+ イオンと Cl− イオンを取り囲み、イオンを結合する強い力を弱め、それらを溶解イオンとして溶液中に移動させます。

水に溶けない溶質は「疎水性」と呼ばれます。 このような溶質は、油と同様、溶質間の相互作用がより強いため、周囲の水分子と水素結合を形成できません。 その結果、溶質粒子は凝集し、溶けずに残ります。

極性基と非極性基の両方を持つ化合物は、「両親媒性」と呼ばれます。 たとえば、脂肪酸の塩である石鹸です。 それらは、非極性炭化水素の疎水性尾部と極性親水性頭部を持っています。 石鹸や洗剤の洗浄作用は、関与する分子の構造の観点から説明できます。 石鹸や洗剤の分子の炭化水素 (非極性) 末端は、油、グリース、汚れ粒子などの非極性物質に溶解するか、非極性物質に引き寄せられます。 イオン末端は水に引き寄せられます（極性）。 その結果、石鹸や洗剤の分子は汚れの粒子と水の間の界面に並び、非極性と極性の 2 つの異なる種類の物質の間の一種の役割を果たします。 その結果、汚れ粒子はコロイド粒子として浮遊し、容易に洗い流されます。

このテキストは、OpenStax Chemistry 2e、セクション 11.1: 溶解プロセス、セクション 11.3: 溶解度、およびセクション 11.5: コロイドから改変されたものです。

溶液、溶解度、および溶解度平衡
溶液は、主成分である溶媒と溶質である溶質で構成される均質な混合物です...

Education

JoVE Core

Organic Chemistry

有機化学

Unit 1

ユニット1

Covalent Bonding and Structure

共有結合と構造

KEY TERMS AND CONCEPTS

主要用語と概念

why some substances dissolve in water

Solution formation depends on how solutes and solvents interact. A solution is a homogeneous mixture made of a solvent, which is the larger part, and a solute, which is the smaller part. The physical state of a solution is usually the same as the solvent. Concentration is often described as dilute for a low amount of solute or concentrated for a high amount of solute.

When a solute dissolves, its particles are surrounded by solvent species. This process is called solvation. If water is the solvent, it is called hydration. For solvation to happen, solute-solvent attractions must be stronger than the attractions within the solute and within the solvent. The opposite process is precipitation, which happens when solute particles are pulled together strongly enough to leave solution.

Solubility is the maximum amount of solute that can dissolve in a given amount of solvent. Temperature, pressure, and molecular polarity can all affect solubility. Solubility equilibrium occurs when dissolution and precipitation happen at the same rate. At that point, the amount dissolved stays constant.

A useful rule for predicting solubility is “like dissolves like.” Polar or ionic solutes usually dissolve in polar solvents because of ion-dipole or dipole-dipole attractions. These interactions do not work well with nonpolar solvents. Nonpolar solutes usually dissolve in nonpolar solvents through dispersion forces.

Water is a polar solvent, so substances that dissolve well in water are called hydrophilic, or water-loving. For example, when KCl is added to water, the hydrogen end of water is attracted to chloride ions, and the oxygen end is attracted to potassium ions. Water molecules surround the ions, weaken the forces holding them together, and allow them to move into solution as solvated ions. Substances that do not dissolve in water are called hydrophobic, or water-fearing. Oil is one example. It cannot form hydrogen bonds with water, so the particles cluster together and stay undissolved.

Some compounds contain both polar and nonpolar parts. These are called amphipathic, or amphiphilic. Soaps are an example because they are salts of fatty acids. They have a nonpolar hydrocarbon tail and a polar hydrophilic head. This structure helps soaps and detergents clean because the nonpolar end interacts with oil, grease, or dirt, while the ionic end interacts with water. The molecules line up at the boundary between the dirt and the water. They act like a bridge between polar and nonpolar matter, which helps suspend dirt as colloidal particles so it can be washed away.

Why Some Substances Dissolve in Water

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Learn how solution formation, solvation, solubility, and the rule “like dissolves like” explain why some substances dissolve in water and others do not.

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Arizona Science Standards

High School

Physical Science

HS+C.P1

HS+C.P1U1.4

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9C.3.1.1.2

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9C.3.2.1.2

3 Developing possible explanations of phenomena or designing solutions to engineering problems
 3.2 Constructing explanations and designing solutions 
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Why Some Substances Dissolve in Water

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