Lighthouse Lab – Physical and Chemical Change

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Physical Changes to Matter
Take a sheet of scrap paper and fold it in half. By folding the paper, you have changed its shape. You have made a physical change to matter. A physical change is a change to matter in which no new matter is made.

When a physical change is made to matter, the amount of matter does not change. If you take a block of chocolate and break it into pieces, the chocolate will have a different shape and appearance, but the amount of chocolate does not change.

Physical change can also take place when matter is heated or cooled. If you heat the pieces of chocolate in a saucepan, they will melt into a liquid. If you turn off the heat and allow the chocolate to cool, it will form a solid again. When the chocolate changed from a solid to a liquid and back to a solid again, the amount of matter remained unchanged.

Many physical changes, including those that involve a change in state, are reversible. This means the matter can be changed back to its state or condition before the physical change occurred.

Chemical Changes
A chemical change occurs when two or more substances combine and a new substance is formed. Consider an iron nail that is left outside. Over time, the oxygen from air combines and reacts with the iron to form a new substance – rust. Rust forming on an iron nail is an example of a chemical change. Chemical changes are usually not reversible unless further chemical changes are made.

Chemical changes occur at different rates. Some changes can occur very rapidly, like the explosion of fireworks. Others occur slowly, like the slow burning of a wooden log. Chemical changes like the weathering of rocks can occur over millions of years.

We can identify when a chemical change is taking place or has taken place in the past by looking for evidence of a chemical change. Such evidence can include a change in color like the presence of red rust on iron or tarnish on silverware.

Many chemical changes give off heat and light like the burning of a flammable gas or wood.

Chemical changes can also be accompanied by a change in scent. A pizza will give off a pleasant smell as it cooks. If the pizza is overcooked, it burns and releases an unpleasant smell.

Chemical changes can also cause bubbling or fizzing. Placing aluminum foil in a strong acid will cause it to fizz, bubble and become warmer as hydrogen gas is created.

Unlike physical changes, chemical changes are irreversible. Once the chemical properties of matter are changed, it cannot return to its original state.

A chemical change is also called a chemical reaction. A chemical reaction has two parts. The matter before the chemical change occurred is called the reactant. The new matter produced as a result of the chemical reaction is called the product.

The Changing States of Water
Most matter changes state when it is heated or cooled. Some matter requires large increases or decreases in temperature before it changes state. Gold is a solid at room temperature. It needs to be heated to over 1,000oC (1,832oF) before it begins to melt. Other matter can change state more easily.

On Earth, water exists in three states – as solid ice, liquid water and as the gas water vapor. Let’s take a look at the temperature changes that must take place for water to change from one state to another.

At room temperature, water is in a liquid state. When water is cooled to 0oC (32oF), it begins to change into solid ice. This process is called freezing and the temperature at which this occurs is called its freezing point. So, the freezing point of water is 0oC.

At temperatures below 0oC, ice will remain in its solid state. If ice is heated, it can change from a solid to a liquid. The process in which matter changes from a solid to a liquid is called melting. The temperature at which this begins to occur is called the melting point. The melting point of matter is the same as its freezing point. So ice has a melting point, 0oC (32oF).

When water is heated it changes from a liquid into the gas water vapor. This process, called evaporation, occurs more rapidly the more water is heated. At 100oC (212oF), water begins to boil. Boiling is the process by which a liquid rapidly changes into a gas. When water boils, it changes into an invisible gas called steam. The temperature of a boiling liquid does not increase, even when more heat is added.

Have you ever noticed water droplets form on the side of a cold glass of water? The droplets of water form when water vapor in the air cools as it comes in contact with the cold glass.