Diffusion of food colouring
Put a drop of food colouring into still water at different temperatures and measure how far the colour spreads over time.
Core concept
Particles spread from a high concentration to a low concentration by diffusion, faster at higher temperatures
Topic: Cells
The particles of a liquid are always moving about at random. When a drop of food colouring is placed in still water, the colour particles bump into water particles and gradually spread out, from where there are many of them (high concentration) to where there are few (low concentration). This is diffusion. It carries on until the colour is spread evenly. Diffusion is how oxygen: A gas in the air that living things need to breathe and that fire needs to burn. All key words and carbon dioxide: A gas we breathe out. Plants take it in to make their food. All key words move into and out of cell: The tiny building block that every living thing is made of. All key words, and how food molecules pass from the gut into the blood. In warmer water the particles move faster, so diffusion is faster.
What you will learn
- Observe diffusion in a liquid.
- Measure how far a colour spreads over time.
- Explain the effect of temperature on the rate of diffusion.
Before you start
- Know that matter is made of moving particles.
Materials
- Food colouring, water at different temperatures
Equipment
- Beakers, dropper or straw, thermometer, ruler, stopwatch
Procedure
- Fill a beaker with still water at room temperature (25 °C). Gently place a drop of food colouring at the bottom.
- Record how far the colour has spread after 1, 5 and 10 minutes.
- Repeat with cold water (10 °C) and hot water (60 °C), recording after 5 minutes.
- Find how long it takes for the colour to spread through the whole beaker.
How this simulation is simplified
- At 25 °C the colour spreads 0.9 cm in the first minute, and the distance grows with the square root of the time.
- The rate increases by a factor e for every 35 °C rise; the beaker's radius is 5 cm.