Kitchen Chemistry

Cooking with hot water. Why?

When cooking we sometimes use hot water. But why?
Well when we studied particle theory we saw that particles of any substance, especially a liquid like water, move faster as more heat is applied.
This means that we can cook faster as the particles travel faster but also we can dissolve more solutes in water. As a volume of liquid water is heated the amount of salt or sugar that can be dissolved in it increases. This makes for the formation of some amazing rock candy.

We will have a go at making some of this rock candy at a later time but first let's see how the amount of solute that can be dissolved in a given volume of water changes with temperature.

We will need:
- one teaspoon
- one cup of sugar
- ice cubes
- one kettle
- warm and cold tap water
- thermometer. This guarantees accuracy of recording results, however, since this is done at home if a thermometer is not available use the temperature given below at each step.

.

step 1 - Start off by filling the glass full of cold water from the fridge. Cool it down by using some ice cubes. This water should be around 2-4 ^oC. Using a spoon slowly add the sugar whiles stirring constantly. Keep adding sugar until no more dissolves and sugar crystals remain at the bottom of the glass of water undissolved. Wait two minutes to see if any sugar crystal remain on the bottom of the glass .

Step 2 - Repeat step 1 using normal tap water. This should be at around 14 ^oC

Step 3 - Repeat step 1 using hot water from the tap. This should be around 45 ^oC

Step 4 - Repeat step 1 using hot water from the kettle. Be careful as it is very hot. This should be around 80 ^oC.
As you can see from the video on the right the sugar dissolves very quickly at this temperature.

Fill the table below

Use graph paper to represent you results in the form of a graph.