(FCM) States of Matter Lesson

States of Matter

When we prepare our food there are many changes that occur within the food itself. We have previously looked at the chemical changes during the heating of food.  Remember, the heat caused the compounds to change from one form to another. Other changes may involve physical changes where the substance changes phase, but remains the same substance. Ice cream, cotton candy, popsicles, gelatin and slushies are a few examples of foods requiring phase changes to be made. Before we look at foods with phase changes, let's look at the different phases of matter so we can better understand the changes that occur when some foods are prepared.

Matter exists in four different phases which have properties that are very different from one another. The four phases of matter are solids, liquids, gases and plasma. On Earth we see three of these phases on a regular basis, but the fourth is rarely seen. Solids, liquids and gases are very common on Earth, but plasma is mainly seen in stars. Substances exist in various states of matter due to the amount of energy they possess; gaining or losing energy will cause the substance to change phase.

In the following diagram, phase changes are listed with the special term associated with that particular phase change. For example, a solid changing into a liquid is termed melting, while a liquid changing into the solid is termed freezing. To the right of the diagram is an arrow indicating an increase in the change of the enthalpy of the system. Enthalpy is a measure of the total energy in a system. The arrow refers to the four states of matter in this order: solid, liquid, gas, and plasma. For example, a solid has tightly packed molecules that generally vibrate in place. When you melt it, the atoms gain energy, begin to slide past each other and are vibrating faster. Gases and then plasmas have even more energy.

Phase Changes

See description of diagram Links to an external site.

Solids

Solids are characterized by having a regular shape, a regular volume, and are usually rigid. The atoms of a solid are tightly packed and usually form a crystalline structure that is arranged in an orderly geometric arrangement. Solids have a high density compared to the other phases of matter. Because solids have low energy, the molecules of a solid move relatively slowly, and are packed close together so that there are many molecules per unit of volume. Almost all foods that we consume are solids; some occur that way naturally, while others are transformed into solids from liquids or gases by removing energy. When we make ice cream, for example, we remove energy from a mixture of cream and sugar until a phase change occurs. We reduce the temperature by surrounding the liquid mixture with ice and salt that lowers the temperature below the freezing point.

The freezing point is the temperature at which the phase change from a liquid to a solid occurs. Other foods are also made in this way, slushies and popsicles for example, where a freezer is used to lower the energy of a liquid causing it to change to a solid. We can also use other methods to prepare solid food. Rock candy, for instance, is made when sugar is dissolved in hot water and is then allowed to cool, causing it to return to its solid form. This process forms a precipitate, a solid that forms when a substance comes out of solution. A wooden splint is placed in the solution and the sugar starts to crystallize on the wood until eventually enough crystals form to create a candy treat.

Liquids

Liquids are characterized by having a definite volume, but no fixed shape. Liquids have the ability to flow and take the shape of their container due to the increased amount of energy contained in their molecules. Unlike solids, liquid molecules are spread relatively far apart which allows them to slide past one another in a fluid motion. Liquids have a slightly lower density than solids, due to more space between the molecules. One of the few exceptions is water where the solid form has a lower density than the liquid form. This important factor allows life to exist on Earth and is also the reason ice floats in your glass of ice water.

Many foods that we eat come in the liquid form. Most condiments, like catsup (or ketchup) and mustard, are liquids. Other liquid foods include soups and stews. We also use liquid water to cook our food when we boil chicken, make pasta and gelatin treats for dessert. When water reaches its boiling point it will start to change from a liquid to a gas as more and more energy is added to the boiler.

Gases

Gases are characterized by having no definite volume or definite shape. Gases will fill the container they are placed in. Gases also have the ability to be compressed. Gas molecules are spread far apart and move very rapidly due to increased energy and weak molecular bonds between the individual molecules. Gases behave differently than liquids and solids due to these factors. There are several laws that explain their behavior. Gases respond to temperature, pressure, volume and number of molecules in a container and there are laws to express the relationship between each of these factors.

We also use gases to cook. Sometimes we boil water and convert it to steam. This steam is used to cook our food. Once water is heated past the 100°C mark, it turns into steam. One advantage to cooking with steam is the food is not directly placed in water where nutrients can be lost. Liquid water will change into steam when it reaches 100°C; however, unlike water, the steam can be compressed. In a pressure cooker, the hotter the steam becomes the faster it will cook.   Many types of foods are prepared using steam such as seafood, pasta, rice, and vegetables. Cooking with the gaseous form of water is a useful technique.  It makes a nutritious meal in a short amount of time.  

Cooking

Cooking with Steam	Cooking with a Pressure Cooker
Need a pan with an inserted steam basket, water below the basket is heated up to produce steam.	Need a pressure controlled pot where the top seals, when the water is heated up, the pressure increases.
Maximum temp is 100°C	Maximum temp is 121°C
Top is not sealed.	Top is sealed with a gasket, but there is a pressure release valve to prevent the cooker from exploding.
Preserves nutrients in food.	Preserves nutrients in food.

Use the presentation below to learn about the states of matter.

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