Why Does Food Colouring Spread Faster in Hot Water? Particles in Motion

Two cups, two drops of food colouring and a stopwatch. Within seconds, the hot cup is a swirl of colour while the cold one barely changes. It’s a vivid demonstration of something we can’t see: particles in motion.

The race

You need: two identical clear cups, ice-cold water, warm tap water (not hot), food colouring and a timer.

Fill one cup with ice water (without the ice) and the other with warm water. Let them sit still for a minute so the water stops moving.
Drop one drop of food colouring into each, at the same moment, from the same height.
Don’t stir. Watch and time how long each takes to spread through the cup.
Take a photo every 30 seconds, if you can.

Predict first: which spreads faster, and why?

What happens

In warm water, colour spreads quickly. In cold water, it spreads slowly. A third cup at room temperature falls in between.

The science

All matter is made of tiny particles in constant motion. In a liquid, water particles move around and bump the dye particles, spreading them out in all directions, a process called diffusion. Warm water has more thermal energy, so its particles move faster and collide harder, so diffusion is quicker.

You can’t see particles, but the colour is evidence of what they’re doing. That’s the idea behind the particle model: a model that explains many different observations.

Fair-test checks

Same amount of water in each cup
Same amount of colouring, same drop height
No stirring, no bumping the table
Start both at the same time

Common mix-ups

“The colour spreads because it’s heavier and sinks.” Look: it spreads sideways and upward too.
“Particles stop moving in cold water.” They slow down but never stop.
“Heat is a substance that flows in.” Temperature is a measure of average particle motion.

This is Why does food colouring spread faster in hot water? from Hot and Cold: Particles on the Move. Also try Why do bike tires go soft in winter? Sign up for three lessons free.