Acids, Bases, and Water

A group of students observe an insect sitting on the surface of water. They discuss how surface tension, cohesion, and adhesion can explain this observation. They disagree on whether surface tension is caused by cohesion or adhesion.

Which student has the best explanation?

• Destiny: “Cohesion makes water molecules push each other away, which makes the water very stable for insects to walk on. This is why cohesion causes surface tension.”

• Rosa: “The attraction between water molecules is what causes cohesion.  Cohesion is the force that pushes up on the surface of the water to counteract the force of the insect.”

• Franklin: “Surface tension happens when water molecules stick to the insect. Water at the surface sticks to the insect, so surface tension is caused by adhesion."

• Paolo: “Molecules at the surface have stronger cohesion with the molecules around them, so the surface acts like a thin film. This is why cohesion causes surface tension."

Which phenomenon does not occur due to adhesion?

• Water droplets forming on a waterproof jacket

• A puddle of water evaporating at a slower rate

• Dew forming on a leaf in the early morning

• A wet, plastic tablecloth sticking to a table

A small needle floats on the surface of water. Soap, which separates water molecules, is added to the water. When soap is added to the water, the needle sinks. 

Which statement best explains what causes the small needle to sink?

• Soap increases the cohesion by decreasing the attraction between water molecules.

• Soap increases the cohesion by increasing the attraction between water molecules.

• Soap decreases the surface tension by increasing the attraction between water molecules.

•  Soap decreases the surface tension by decreasing the attraction between water molecules.

Which of the following describes the following information?

• Cohesion

• Adhesion

• Surface tension

Which of the following describes the following information?

• Cohesion

• Adhesion

• Surface tension

Water droplets spread out and stick to the metal squares in the middle of a window screen when it rains.

• Cohesion

• Adhesion

• Surface tension

A meniscus forms as water molecules stick to the wall of the glass.

• Cohesion

• Adhesion

• Surface tension

Water droplets cling to the ends of pine needles.

• Cohesion

• Adhesion

• Surface tension

When it rains on a car windshield, water forms droplets.

• Cohesion

• Adhesion

• Surface tension

Water moves up a straw unassisted.

• Cohesion

• Adhesion

• Surface tension