The textbook says molecules exert both attraction and repulsion on one another. That sounds a little mystical: the two forces point in opposite directions — how can they exist at the same time? And what decides whether attraction or repulsion gets the final say? This guide uses three experiments you can do in a kitchen (two merging water drops, a glass slide resting on a water surface, and a syringe with its tip plugged) to let students see and feel intermolecular forces for themselves.

Safety first (read this section before you start)

  • The syringe must have its needle removed — use the barrel only. The smallest pharmacy size (2.5 mL or 5 mL) is plenty; if yours came with a needle, have an adult remove it and put it safely away first.
  • Choose a glass slide with smooth edges (a microscope slide or an offcut of picture-frame glass works). Hold it by the faces, not the edges; if cuts are a worry, swap in a smooth rigid plastic card (a membership card the size of a bank card is perfect) — the effect is the same.
  • Keep a firm grip when pressing the syringe plunger, and don't ram it to the bottom. Water refusing to compress is the normal result — don't fight it.
  • The table will catch splashes, so lay down a towel; wipe up any water afterward to prevent slips.
  • No flames and no electricity anywhere in this experiment — it is about as low-risk as they come — but invite a family member anyway: an extra pair of hands, and an extra witness.

Materials and equipment

  • A smooth plastic board, plastic wrap or wax paper (for the "merging drops" — the less absorbent the surface, the better);
  • A dropper (or a straw: pinching the top to release water drop by drop is all you need);
  • A glass slide or smooth rigid plastic card, a length of cotton thread, and a rubber band;
  • A basin of fresh water (wider than the slide);
  • A syringe with the needle removed (2.5–10 mL, any size);
  • A ruler, paper and pen for recording — and ideally a phone that can shoot slow motion.

Step-by-step procedure

Experiment 1: two water drops merge (seeing attraction). Wipe the board dry and lay it flat, then use the dropper to place two roughly equal drops about 1 cm apart. With a toothpick, nudge one drop toward the other — push at the board surface right beside the drop's edge, easing it along. The instant the two edges touch, stop and watch closely: the drops snap together into one larger drop in a single moment, without the slightest hesitation. If your phone has a slow-motion mode, film it — the replay is even more striking.

Experiment 2: a glass slide stuck to the water surface (measuring attraction). Loop cotton thread around each end of the slide and tie the loops to the bottom of a rubber band, making a little cradle. First hold the rubber band up so the slide hangs still in the air, and measure the band's length with the ruler — write it down. Now lower the slide flat onto the water surface and slowly raise the rubber band: you'll feel yourself pulling noticeably harder, and the band stretches clearly longer than before, until the slide pops off the water with an audible snap. Measure the band's greatest length in the instant before release, and compare it with the hanging length.

Experiment 3: water that won't compress (feeling repulsion and spacing). Pull the plunger all the way out to fill the barrel with air, plug the tip tightly with a finger, and press the plunger with your other hand: it slides in easily past the halfway mark. Release it, then fill the syringe with water (working out the air bubbles), plug the tip the same way, press just as hard — the plunger barely moves at all, and your finger gets sore from the pressure. The difference between those two tries is this experiment's data.

Bonus round: a homemade "lead cylinder" demo. In class, a teacher presses two freshly shaved lead cylinders together until they stick into one piece that can hold a hanging weight. At home, use two candles instead: shave both end faces flat and smooth with a knife, press them together hard with a few twists, and they stick — lift the top candle and the bottom one comes with it. Freshly cut faces of modeling clay do the same thing.

Interactive simulation: this experiment has a drag-and-play simulator in our Interactive Lab. The lab interface is currently Chinese-only — the controls are simple sliders and switches, so it is still easy to explore.

What you should observe

Compare your students' notes against these:

  • The moment the two drops' edges touch, they merge instantly into one, the contact line vanishing as if zipped shut; the big new drop even wobbles a couple of times before settling.
  • The rubber band holds its length while the slide hangs in air; lifting the slide off the water stretches the band visibly longer first — the extra pull is what it takes to overcome the attraction between water and glass.
  • After the slide comes free, its underside is wet: the "tear" happened inside the water itself — water molecules were pulled apart from water molecules.
  • The air in the syringe compresses easily to less than half, and the plunger springs partway back when released; with water, no matter how hard you push, the plunger barely moves.
  • The two candles' shaved faces stick when pressed hard — lifting one lifts the other — yet a casual twist snaps them apart: too few molecules on the joint faces ever got close enough to grip.

The science

The third statement of the kinetic theory of matter is: molecules exert both attraction and repulsion on each other at the same time. The key words are "at the same time" — the two forces don't take turns; they are two opponents locked in a permanent tug-of-war, and what we ever observe is only their net force.

Both forces weaken as the distance between molecules grows, but repulsion falls off far faster. So there is one special separation, the equilibrium distance r₀ (on the order of 10⁻¹⁰ m): at exactly r₀, attraction equals repulsion and the net force is zero; squeeze closer than r₀ and repulsion surges, pushing outward; stretch beyond r₀ and attraction takes the lead, pulling back inward; and past roughly 10 times r₀, both forces become too feeble to matter.

Hold that ruler up to the three experiments and everything clicks. Experiment 1: however close the two drops sit, as long as they aren't touching, almost all their molecules are far beyond 10 r₀ apart — total strangers. The instant the edges meet, the molecules at the contact point come within attraction's range and haul the two drops together tighter and tighter: the merge happens in a blink. Experiment 2: once the slide rests on the water, glass molecules and water molecules are close enough for attraction to engage; lifting means overcoming gravity plus that attraction, so the rubber band must stretch farther. Experiment 3: water molecules already sit near r₀, so pushing them closer means beating the steeply rising repulsion — no human finger can; air molecules, by contrast, sit about ten molecular diameters apart, where the forces are negligible, and compressing air merely squeezes out the empty space between molecules.

One muddle is worth clearing up while we're here: the hard-to-lift wet slide is often explained away as "all due to air pressure." Strictly speaking both effects contribute — if air pressure were the whole story, a dry slide laid flat on water should be just as hard to lift. Add one more control: touch the slide to the water edge-on, held vertically, then lift. The resistance is far smaller, because the contact area shrank and so did the number of molecules doing the pulling. That is exactly the signature of a force that depends on how close, and how many, the molecules are.

The bonus-round candle trick is the home version of the classroom lead cylinder demo: shaving flat, pressing hard and twisting all serve one purpose — bringing as many molecules on the two faces as possible into attraction's range. Which answers the reverse question too: why can't a broken mirror be made whole? The broken faces look flat, but magnified they are all canyons and ridges; press them together and only a scattered handful of molecules come within a few r₀ of each other — attraction never gets a chance.

Notes for teachers and parents

  • Time and grouping: 20 minutes, easily done at home; run it as a demonstration with students taking turns to feel each step.
  • Say this before hands touch anything: don't force the syringe too hard or it may crack.

The step most often skipped. Do the three experiments in the order "merging drops → slide off water → syringe of water" — that's attraction, attraction, repulsion — then spring the question "how can the same molecules both push and pull?" The lesson flows most naturally that way.

How to tell they truly understand. A student who can point at the immovable syringe plunger and say "that's repulsion holding it," and at the merging drops and say "that's attraction pulling," has met the goal.

Take it further, and common misconceptions

  • Misconception 1: "Attraction and repulsion between molecules take turns." Wrong. Both forces exist at all times; distance only decides which one gets the upper hand, and what shows is the net force.
  • Misconception 2: "Solids resist compression, so there are no gaps between their molecules." Wrong. The gaps are always there; what resists is the steeply rising repulsion.
  • Misconception 3: "Gases compress easily, so gas molecules only attract." Wrong. Gas molecules are so far apart that attraction and repulsion are both negligible — what gets squeezed out is empty space.
  • Misconception 4: "The wet slide is hard to lift purely because of air pressure." Not quite. Run the "vertical slide, edge only" control and you'll see molecular attraction doing real work.
  • Misconception 5: "Water won't compress because water molecules are hard and prop each other up." The more accurate statement is that the repulsion between water molecules props it up; "hardness" as an everyday word is nothing but the summed repulsion of billions upon billions of molecules.
  • Think it over: why does glue bond two sheets of paper so firmly? (The liquid glue floods the canyons of both surfaces, and once it sets, it has pulled the molecular distances into attraction's range.)
  • And one more: how does a water strider stand on water? (Surface-layer water molecules are pulled inward by the molecules below, stretching the surface into a "skin" — surface tension, another handiwork of molecular attraction.)
  • Read on with why the temperature holds steady while ice melts to see how molecular spacing and forces define solid, liquid and gas; the companion interactive simulation splits attraction and repulsion into two arrows you can watch at leisure.

When a student can stand before that immovable syringe and say "repulsion is holding it," and before the merging drops and say "attraction is pulling," intermolecular forces stop being an empty line in a textbook.