Evidence of Chemical Change

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 two molecules exchange atoms, forming new substances. Animation from: http://www.ias.ac.in/initiat/sci_ed/resources/chemistry/sn2.gif

Chemical Change Example: Baking Soda and Vinegar Baking soda and vinegar are two chemicals with different physical properties Baking soda (sodium bicarbonate) is a powdery white odorless solid Vinegar (acetic acid) is an acidic liquid with a strong odor NaHCO3 Sodium Bicarbonate (solid) CH3COOHVinegar (liquid) Think of baking soda and vinegar as two different couples dancing. The atoms stay with their own molecule when they are dancing on opposite sides of the room.

Chemical Change Example: Baking Soda and Vinegar A chemical change occurs when you combine baking soda and vinegar Atoms from one molecule are more attracted to the other molecule The two molecules break apart and their atoms rearrange to form three new substances NaHCO3 Sodium Bicarbonate (solid) CH3COOHVinegar (liquid) The two dancing couples split apart and switch partners when they get too close

Chemical Change Example: Baking Soda and Vinegar A chemical change occurs when you combine baking soda and vinegar The baking soda and vinegar molecules break apart and their atoms rearrange to form new substances No atoms were created or destroyed – only rearranged The reaction produces three new substances and an exothermic* temperature change C2H3NaO2 Sodium Acetate (solid) CO2 Carbon Dioxide (gas) H2O Water (liquid) The dancing couples split apart and dance with new people instead. That’s cold!*

Why is the reaction of baking soda and vinegar such a great example of a chemical reaction? We know that a chemical change occurs when you combine baking soda and vinegar because there is evidence Two reactants are combined: Vinegar (acetic acid) is a transparent colorless liquid with a strong odor Baking soda (sodium bicarbonate) is a white powdery solid with a salty taste Three new substances with different properties are produced: Carbon dioxide gas is colorless and odorless (gas produced) Water is colorless and odorless liquid (liquid produced) Sodium acetate is a white solid precipitate (solid precipitate forms) There is an endothermic temperature change, which means the reaction gets cold

Evidence of Chemical Change We will discuss four signs of chemical reactions in 6th grade science 1. Formation of a gas 2. Color change 3. Temperature change 4. Formation of a solid precipitate

Evidence of Chemical Change 1. Production of a Gas You might see small bubbles (fizz) or large bubbles when a gas is produced. You might be able to hear the gas bubbles popping Be careful! Bubbles don’t always mean that a chemical change occurred. You can see and hear bubbles when you open a soda can, but that is not a chemical reaction.

Evidence of Chemical Change 2. Color Change Color is a property of chemical substances. You might see a new color when atoms and molecules rearrange after a chemical reaction Be careful! Different colors don’t always mean that a chemical change occurred. You can see a new color when you add food coloring to frosting, but that is not a chemical change!

Evidence of Chemical Change 3. Change in Temperature Chemical reactions can result in an increase or decrease in temperature Exothermic reactions get hotter (EXO = HOT) Endothermic reactions get colder (ENDO = COLD) Be careful! Changing temperatures don’t always mean that a chemical change occurred. You can melt ice cream in the sun or freeze it in the freezer, but those are not chemical changes!

Evidence of Chemical Change 4. Production of a Precipitate A new solid forms The solid comes out of a liquid solution because it does not dissolve The solid usually falls to the bottom of the beaker Be careful! The formation of a solids does not always mean a chemical reaction occurred. Putting water in the freezer causes the water to become a solid, but that is not a chemical reaction!

Is it a Chemical Change? Test your knowledge! Write CHEMICAL if the example is a chemical change. Write PHYSICAL if the example is a physical change. List evidence to support your answer For example: Adding sprinkles to ice cream is a PHYSICAL CHANGE because the mixture can be separated and the substances retain their original properties For example: Adding baking soda to vinegar is a CHEMICAL CHANGE because a gas is produced, a solid precipitate forms, and the reaction is endothermic

Is It a Chemical Change? A forest fire destroys acres of land. There is a lot of heat and smoke, and the trees turn to charcoal. ANSWER: CHEMICAL CHANGE EVIDENCE: Gas produced – Smoke Color change – wood to charcoal Temperature change - exothermic

Is It a Chemical Change? When making hot cocoa, transparent colorless water turns brown when you mix in the cocoa powder. ANSWER: PHYSICAL CHANGE EVIDENCE: The water is turning brown because the cocoa is brown. Both substances retain their original properties and the mixture can be separated

Is It a Chemical Change? An iron wheelbarrow is left out in the rain and the meal rusts, producing iron oxide. ANSWER: CHEMICAL CHANGE EVIDENCE: Color change – new reddish brown color Precipitate – rust is a new solid

Is It a Chemical Change? On a hot day, water droplets form on the outside of a cold glass of ice water. ANSWER: PHYSICAL CHANGE EVIDENCE: Condensation is a phase change. Water molecules from the air condense on the cold glass. No new substance is formed.

Is It a Chemical Change? Ice cream melts after it falls on the ground. ANSWER: PHYSICAL CHANGE EVIDENCE: Melting is a phase change. Solid ice cream turns into a liquid when heat is added. No new substance is formed.

Is It a Chemical Change? A raw egg cooks in a frying pan. The transparent colorless egg white solidifies and turns white. The yellow yolk solidifies. ANSWER: CHEMICAL CHANGE EVIDENCE: Color change – colorless to white Precipitate forms - the egg white and yolk solidify when heated

Is It a Chemical Change? Chalky white solid Alka- Seltzer tablets contain baking soda and citric acid. Alka-Seltzer tablets fizz when dropped into water. ANSWER: CHEMICAL CHANGE EVIDENCE: Gas formation – the fizz you see is the result of gas bubbles

Is It a Chemical Change? Crayons change white paper to red. ANSWER: PHYSICAL CHANGE EVIDENCE: The paper is turning red because the crayon is red. No new substance is formed.

Is It a Chemical Change? Soda fizzes, and bubbles rise as you pour it in a glass. ANSWER: PHYSICAL CHANGE EVIDENCE: The bubbles were already there, dissolved under pressure in the bottle. No new substance was formed.

Is It a Chemical Change? When you crack a glow stick, the chemicals inside combine to produce a small amount of heat and carbon dioxide gas along with bright glowing light. ANSWER: CHEMICAL CHANGE EVIDENCE: Temperature change - exothermic Gas produced – carbon dioxide gas is a new substance