Chloe Zhang
Water molecules are highly polar, which makes them stick together (cohesion) and also stick to other surfaces (adhesion). When water is poured into a plastic graduated cylinder, an inverted U-shaped meniscus is observed due to the properties of water's hydrogen bonding and
| Characteristics | Values |
|---|---|
| Shape of meniscus | Inverted U-shaped |
| Cause of shape | Hydrophobic nature of plastic, water's surface tension and hydrogen bonding |
| Volume measurement | At the bottom of the meniscus |
| Water behaviour | Sticks to itself and other surfaces |
| Water surface | Higher at the edges |
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What You'll Learn
Water's hydrogen bonding
Water molecules are highly polar, which makes them stick together (cohesion) and also stick to other surfaces (adhesion). This property of water is due to its hydrogen bonding and surface tension. When water is poured into a plastic graduated cylinder, an inverted "U-shaped" meniscus is observed due to the hydrophobic nature of the plastic. The water molecules stick to each other and pull away from the hydrophobic surface, forming a downward-facing meniscus.
Water molecules (H2O) consist of two hydrogen atoms covalently bonded to one oxygen atom (H-O-H). The hydrogen and oxygen atoms share electrons unequally due to differences in electronegativity. As a result, the oxygen atom has a partial negative charge, while the hydrogen atom has a partial positive charge. This polarity allows water molecules to form hydrogen bonds with each other and with other molecules.
A hydrogen bond is an intermolecular force that occurs between molecules. It is the attraction between a partially positively charged hydrogen atom and an electronegative atom, typically oxygen (O), nitrogen (N), or fluorine (F). In water, hydrogen bonds form between neighbouring hydrogen and oxygen atoms of adjacent water molecules. Each water molecule can form hydrogen bonds with up to three other water molecules.
The hydrogen bonds in water are relatively weak compared to covalent or ionic bonds, and they continuously form, break, and reform. They are responsible for water's high cohesiveness, as they hold the water molecules together. This cohesiveness creates surface tension, where air and water meet, and allows water to form spheres in space due to its attraction to itself.
The strength of hydrogen bonds in water varies due to thermal fluctuations and the disparity between donor or acceptor strengths of individual water molecules. While the bonds are weak, they are strong enough to be maintained at and below ambient temperatures. The lifetime of a single hydrogen bond is very short, ranging from 0.1 picoseconds at 100°C to 20 picoseconds at -20°C.
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Surface tension
When water is poured into a plastic graduated cylinder, an inverted "U-shaped" meniscus is observed. This occurs due to the properties of water's hydrogen bonding and surface tension. Water molecules are highly polar, which makes them stick together (cohesion) and also stick to other surfaces (adhesion).
The forces of attraction acting between molecules of the same type are called cohesive forces, while those acting between molecules of different types are called adhesive forces. The balance between the cohesion of the liquid and its adhesion to the material of the container determines the degree of wetting, the contact angle, and the shape of the meniscus. When cohesion dominates, the wetting is low and the meniscus is convex at a vertical wall. Conversely, when adhesion dominates, the wetting is high and the meniscus is concave.
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Hydrophobic materials
In the context of a plastic graduated cylinder, the hydrophobic nature of the plastic causes water to stick to itself rather than the surface, forming an inverted or downward meniscus. This occurs due to the surface tension of water and the polar nature of water molecules, which creates a stronger adhesion to the plastic walls of the cylinder than the cohesion among the water molecules.
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Water adhesion
The adhesive property of water is observed when water appears to climb" up the sides of a glass or plastic container, forming a curved or 'U-shaped' meniscus. This occurs because the water molecules are attracted to the glass or plastic walls, adhering to them more strongly than to each other. The water molecules are pulled up along the walls of the container, creating a concave meniscus.
Additionally, water adhesion plays a role in creating surface tension. Water molecules' attraction to each other and to other substances allows them to form a "skin" that holds the water droplet together in a flattened sphere. This surface tension enables certain insects, such as the water strider, to stay afloat on the surface of the water.
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Capillary action
When water is poured into a plastic graduated cylinder, an inverted "U-shaped" meniscus is observed. This occurs due to the properties of water's hydrogen bonding and surface tension. Water molecules are highly polar, which makes them stick together (cohesion) and also stick to other surfaces (adhesion), such as the plastic walls of the cylinder.
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Frequently asked questions
Yes, water does stick to a plastic graduated cylinder. This is due to the water's surface tension and hydrogen bonding.
A meniscus is a curve in the surface of a molecular substance, such as water, when it touches another material.
An inverted U-shaped meniscus occurs due to the hydrophobic properties of the plastic graduated cylinder. The water molecules are attracted to each other and the plastic walls of the cylinder, forming a downward-facing curve.
To accurately measure the volume of water, you should read the bottom of the meniscus at eye level. This ensures a consistent and precise reading.
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