Today we're going to be looking at states of matter
in the kitchen.
Now everything you can see,
taste, touch, and smell in
this kitchen is made of something called matter.
Now, all matter is made up of very small particles
and usually comes in three main forms or states.
These are known as solids,
liquids and gases.
The three common states of matter,
solids, liquids and gases have different observable properties and behave
in different ways.
A solid has a fixed volume and a fixed shape.
Solids are usually tough and rigid.
Their particles are close together and vibrate while in a
fixed position.
Solids cannot be easily compressed.
A liquid has a fixed volume,
but does not have a fixed shape,
as liquids will take on the shape of the container
they are in.
The energy levels of particles in liquids is greater than
solids.
The particles move freely enough to collide and move past
each other,
and liquids cannot be easily compressed.
A gas has no fixed volume or shape.
A gas takes on both the shape and the volume
of the container,
spreading out to fill the entire space.
The particles have lots of energy and move randomly in
all directions,
and gases can be compressed.
Did you know we can change the state of matter
by either heating or cooling it?
The kitchen is a great place to demonstrate these transformations.
Also known as changes of state.
There are four main changes of state, matter can go
through.
Freezing is when a liquid changes state into a solid,
while melting is when a solid changes state back into
a liquid.
Vaporization is when a liquid changes state
into a gas while, condensation is when a gas changes
state back into a liquid.
A great way to show matter going through each change
of state is to use water.
You see, water is a unique substance because the properties
of water allow it to actually exist in all three
states and I can show you all four changes of
state right here in the kitchen.
So the first change of state I'm going to
show you is from a liquid to a solid.
Ok, this water is definitely still in its liquid state,
but let's put it in the freezer and see what
happens.
By placing the water, the liquid, into the freezer and
dropping it to a temperature below 0 degrees Celsius,
we were able to change the state of water from
a liquid to a solid and this solid is known
as ice and the whole process is known as freezing.
So the next change of state I'm going to demonstrate
is turning a solid piece of ice into liquid,
but it requires some heat.
By applying the heat to the solid state of water
known as ice,
we were able to change its state into a liquid
and this whole process is known as melting.
The next change of state
we are going to look at is the change from a
liquid into a gas.
There are two ways water can change state from a
liquid into a gas,
and these are known as evaporation and boiling,
and both are examples of the phase change, vaporization. Boiling
is when water is heated and bubbles form within the
liquid and rise to the surface.
The temperature at which water boils is around 100 degrees
Celsius.
This is known as its boiling point.
You can actually see this change of state happening right
before your eyes.
The liquid or water is turning into steam.
Steam is very hot and actually invisible.
The misty cloud above the boiling pot we can see
is in fact known as wet steam, which is a
mixture of steam,
the gas and tiny water droplets.
Evaporation occurs at temperatures below 100 degrees Celsius and typically
takes place on the surface of the liquid where the
liquid will slowly change state into an invisible gas or
water vapor. Evaporation occurs at a much slower rate than
boiling.
The final change of state we are going to look at
today is condensation,
and that's changing from a gas back into a liquid.
When the cold surface of the lid comes in contact
with this hot gas or steam
it will actually change state back into a liquid.
Now we understand how heating and cooling affects the state
of matter.
We can use this knowledge to our advantage
when we perform everyday tasks like cooking food and I'm
going to do just that today.
I'm going to cook up
these veggies and chicken by changing the state of water
from a liquid to a gas. So this cooking method,
we're using today is known as steaming and you can
see why, it primarily uses steam to cook
the food and I'd say,
this chicken is almost done.
Now, who would have thought there was so much science
right here in the kitchen and today we were able
to steam this small piece of chicken and veggies in
a short amount of time.
But what if we were trying to cook for say,
a large group of people?
Well, people have been utilizing steam as a means of
cooking food for thousands of years and not just in
kitchens.
Today I'm going to show you a cooking technique that
is thousands of years old and it's still used today
and it also uses heat to change the state of
water, a liquid, into a gas.
But this time it's not done on the stovetop
it's done underground. Cooking underground using steam is what is
called an underground or earth oven.
Ground ovens are used by Aboriginal and Torres Islander People
Australia wide. Ground ovens
were used to cook a variety of food items -
everything from fish, stingray, turtle,
dugong and even vegetables, and they're also a great way
to cook for large groups of people.
But today, I'll be cooking and steaming these sweet potatoes
and this fish I caught earlier today. A ground oven,
is prepared by digging out a hole around 100 centimeters
long and around 60 centimeters deep. A fire is built
beside the hole with dry rocks placed in the fire
to
heat them up. Next the fish and sweet potato are
prepared. The fish is wrapped in fresh, native ginger leaves
and placed inside wet paper bark and secured with twine.
The hole is then filled with hot coals and heated
rocks from the fire. The prepared food
is placed on a layer of green leaves or branches
and sprinkled with water.
The hole is then covered with a thick layer of
paperbark that acts as a seal or lid to
the oven.
The hole is quickly covered with sand to prevent loss
of steam and heat.
The food is then left to cook for several hours.
The ground oven is a brilliant way to cook large
quantities of food
and it's a cooking method that has stood the test
of time.
This cooking method looks like a lot of effort,
but it actually has its advantages.
There's not a great deal of attention required when cooking
the food,
but also, all the heating and steaming and cooking is
done underground.
Let's take a closer look.
The secret to a ground ovens success is in its
ability to conserve the heat generated by the rocks,
while the earth and paperbark lid prevent heat loss to
the outside environment.
Let's go in for a closer look.
The hot rocks and embers heat up the air at
the base of the oven,
making it less dense or lighter.
This now, hot air rises away from the base to
the top of the oven. As the hot air rises
it pulls the cooler air from the top of the
oven down to the bottom to replace it.
This will continue until the heat is evenly distributed throughout
the oven.
As almost all foods contain water,
as the temperature rises within the oven,
the heated liquid water present in the food reaches boiling
point at around 100 degrees Celsius and changes state from
a liquid into a gas to produce
steam. The wrapping of the green ginger leaves and wet
paperback
provide additional moisture and insulation around the food.
The thick, waterproof paperbark traps the heat and steam within
the food parcel,
preventing it from escaping. After several hours,
the food will be cooked and the hot rocks will
have lost their heat,
dropping the overall temperature within the oven.
The cooling process forces the remaining steam inside the paperbark
parcels to change state again from a gas, the steam,
back into liquid water by means of condensation.
This is why much of the original moisture in the
food
is retained.
The intricate knowledge around food preparation and design has allowed
Aboriginal and Torres Strait Islander People to use the properties
of water to improve essential daily activities like cooking and
eating.
Today we have learned how heating and cooling of matter
can create a change in its state.
These phase changes are happening all around us every day
from freezing to steaming.
The knowledge and processes of changing states between matter plays
an important role in all cooking processes we see today
and it has influenced both modern and traditional cooking techniques
around the world.