Chemical Changes

What is a chemical change? chemical change - matter changes into a new substance through a chemical reaction. The animation to the right shows a chemical change. The blue and white molecule is more attracted to the red and white atoms than the green atom so they switch, forming a new substance. Animation from:

Why did baking soda and vinegar bubble? A chemical change occurred when you combined baking soda and vinegar. ◦ Vinegar is acidic. ◦ When the molecules in baking soda mix with the acid, some of their atoms are more attracted to the acid and they break apart to form new molecules. Baking soda and vinegar are like two couples dancing. When they are separate from each other they stay with their own molecule… Baking Soda Vinegar

Why did baking soda and vinegar bubble? A chemical change occurred when you combined baking soda and vinegar. ◦ Vinegar is acidic. ◦ When the molecules in baking soda mix with the acid, some of their atoms are more attracted to the acid and they break apart to form new molecules. Baking Soda Vinegar But when the two molecules mix, atoms from one molecule are more attracted to the other molecule…

Why did baking soda and vinegar bubble? A chemical change occurred when you combined baking soda and vinegar. ◦ Vinegar makes the water more acidic. ◦ When the molecules in baking soda mix with the acid, some of their atoms are more attracted to the acid and they break apart to form new molecules. NaC 2 H 3 O 2 Sodium Acetate CO 2 Carbon dioxide …So they switch dance partners and form new substances. Notice that nothing was created or destroyed… just changed. H 2 O water

Evidence for a Chemical Change 1. Formation of a gas  Seeing bubbles or gas after mixing substances together is evidence that a chemical change took place.  When you mixed baking soda or baking powder and vinegar, the bubbles you saw were carbon dioxide gas—a new substance that had formed.  Bubbles don’t always mean that a chemical change occurred. Can you think of any examples of bubbling that do not create a new substance?

Why does red cabbage juice turn the powders different colors? A chemical change occurres when red cabbage juice is added to cream of tartar and laundry detergent. ◦ Red cabbage juice is an indicator, because it contains molecules that change color when an acid or base is added to them. ◦ Cream of tartar is an acid, so it gave particles to the indicator, making it turn from blue to pink. ◦ Laundry detergent is a base, so it took particles from the indicator, making it turn from pink back to blue.

Evidence for a Chemical Change 2. Color Change ◦ Seeing a color change after mixing substances together is evidence that a chemical change took place. ◦ Color is a property of how the molecule is arranged. If there is a change to the molecule, the color can change. ◦ Color change doesn’t always mean that a chemical change occurred. Can you think of any examples of a substance changing color that does not create a new substance? Sometimes it takes a while for the molecules to rearrange and the color to change. From:

Evidence for a Chemical Change 3. Temperature Change  Noticing a temperature change after mixing substances together is evidence that a chemical change took place.  The substance can get warmer or cooler, depending on whether the molecules give energy while swapping places (hot - exothermic), or need energy to swap (cold - endothermic).  Temperature change doesn’t always mean that a chemical change occurred. Can you think of any examples of a substance getting warmer or colder that do not create a new substance?

Evidence for a Chemical Change 4. Formation of a Precipitate  A precipitate forms when a substance comes out of solution and forms a solid. Seeing a solid substance forming after mixing substances together is evidence that a chemical change took place.  Don’t confuse freezing a substance to make it a solid with forming a solid precipitate. Freezing is a physical change that does not change what the substance is made of! Can you think of any examples of solids forming in a liquid that do not create a new substance?

Is it a Chemical Change? Write down “YES” on your board if it is a chemical change, and “NO” if it isn’t. If YES, list what evidence supports this. (Example: gas formation).

Is it a Chemical Change? A forest fire destroys acres of land. There is a lot of smoke, and the trees turn to charcoal. ANSWER: YES EVIDENCE: - Smoke, - color change, - temperature change

Is it a Chemical Change? When making hot cocoa, clear water turns brown when you mix the cocoa in. ANSWER: No (the water is turning brown because the cocoa is brown)

Is it a Chemical Change? An old wheelbarrow is left out in the rain and rusts. ANSWER: Yes EVIDENCE: Color change, precipitate forming (rust)

Is it a Chemical Change? On a hot day, water forms on the outside of a cold glass of water. ANSWER: No (No new substance is forming)

Is it a Chemical Change? Ice cream melts after it falls on the ground. ANSWER: No (No new substance is forming)

Is it a Chemical Change? A raw egg gets cooked. ANSWER: Yes EVIDENCE: Color change

Is it a Chemical Change? When Alka Seltzer is dropped into water it fizzes. ANSWER: Yes EVIDENCE: gas formation

Is it a Chemical Change? Crayons change white paper to red. ANSWER: No (The paper is turning red because the crayon is red.)

Is it a Chemical Change? Soda fizzes, and bubbles rise as you pour it in a glass. ANSWER: No (The bubbles were already there, just under pressure in the bottle.)

Is it a Chemical Change? When you crack a glow stick, hydrogen peroxide mix with other chemicals, causing it to glow and warm up. ANSWER: Yes EVIDENCE: - color change - temperature change