Physical and Chemical Changes

What is a physical change? A change that alters the form of an object without changing what type of matter it is.

What are some examples of physical changes? Molding clay Cutting a piece of paper Sharpening a pencil Melting ice

What happens when you tear a sheet of paper into tiny pieces? You only change how the paper is shaped. You only change the size of each piece, not the type of elements inside.

Change of state If you place a piece of ice on the sidewalk outside, it will melt. After some time, it will disappear as the water turns to gas. Are these physical changes? Yes, a substance can change from solid to liquid to gas. The substance does not change its chemical identity in this process. Only its state changes. Ice is water in solid form. As it melts, it is water in liquid form. Finally, when it evaporates, it is water in gas form. In each state, the particles are still water.

Combining two substances to form a mixture is a physical change. The substances in a mixture can be separated. Examples: Sugar dissolved in water

What do all physical changes have in common? All physical changes alter the form of an object without changing what type of matter it is.

What are some examples of physical changes you see around school? Crushing aluminum cans Shaping clay Blowing up a basketball Balling up paper

Chemical Changes A chemical change occurs when atoms link together in new ways to create substances different from the original substances. It is also known as a chemical reaction.

We often notice a change in odor when we cook food. We often detect a chemical change by noticing a change of odor or aroma. We often notice a change in odor when we cook food.

Food that is being cooked often gives off an aroma, or smell Food that is being cooked often gives off an aroma, or smell. Are all the smells of cooking food the same? Different foods give off different odors. Think of chocolate chip cookies baking… Think of a steak being grilled… Do these smell the same?

Why do foods have different smells? Foods have different compounds in their structures.

Look at the baking soda and vinegar reaction diagram. P. 415 What substances are on the left side of the diagram? Sodium bicarbonate (baking soda) and acetic acid (vinegar) These substances are called the REACTANTS

In the baking soda and vinegar reaction, what substances are on the right side of the arrow? Water, carbon dioxide, and sodium acetate These substances are called the PRODUCTS. How does a chemical equation show that a chemical change has taken place? The substances to the left of the arrow are different from those to the right of the arrow.

Water-Forming Reaction Look at the diagram. What elements combine to form water? Hydrogen and Oxygen How many water molecules are shown as a product of the reaction? TWO!

Reactants are vinegar and baking soda. What are the reactants and the products of the vinegar and baking soda reaction? Reactants are vinegar and baking soda. Products are water, carbon dioxide, and sodium acetate.

If 32 atoms of hydrogen react completely with 16 atoms of oxygen, how many molecules of water will be made? Why? 16 molecules of water will be made because each water molecule has one oxygen atom and two hydrogen atoms.

How can you spot a chemical change? A chemical change produces new substances!

New substances produced by chemical change The new substances have different properties than the previous ones. You can often see, hear, or smell the formation of new substances as a chemical change occurs.

How can you tell that a new substance has been formed in a chemical change? The new substance may look different from the old substance.

When iron rusts, the new substance formed is iron oxide When iron rusts, the new substance formed is iron oxide. In what way does iron oxide look different from iron? Iron is gray and shiny. Rust is dull and reddish brown.

A change in color is often an indicator that a chemical change has occurred!

Bubbles! When an antacid tablet is placed in water, a chemical reaction begins and bubbles form. The appearance of bubbles indicates that a chemical change occurred.

Precipitate More than just gases can form in a chemical change. A precipitate is a solid formed from the chemical reactions of some solutions. Have you ever seen soap scum in you shower or sink? If so, it means a precipitate has formed (and you need to clean!).

Heat and Light Release of heat and light can indicate a chemical change. A burning candle produces a flame. The heat and the light come from the combination of atoms in the candle and the wick with oxygen from the air.

What are the five indicators of a chemical change What are the five indicators of a chemical change? Discuss in your group. 1. Color change 2. Forms tarnish 3. Release of a gas 4. Formation of a precipitate 5. Release of energy

Is frying an egg a chemical change? Why or why not? Yes, frying an egg involves a chemical change. The egg yolk and albumen change color.

What signs tell you that a log burning in a fire is a chemical change? Gas and sap are released from the log. The log changes color and heat and light energy are released.

Look at the picture of the space shuttle Look at the picture of the space shuttle. How can you tell that energy is being released? The fire beneath the shuttle is the result of the oxygen and hydrogen combining and releasing heat and light. What work is being done as a result of this chemical change? The space shuttle is being moved.

How are compounds and chemical reactions related? Compounds are formed by chemical reactions.