Physical and Chemical Changes

Let’s Review: In A Physical Change.... A substance is changed physically, but not chemically. It is still the same substance. Ex: Ice melts into water. It is still H 2 O, just in a different physical form. It changes shape or size It dissolves. It changes state of matter (freezes, melts, evaporates, condenses)

A substance’s chemical formula is changed and the substance is changed into something completely new. It no longer has the physical or chemical properties it had before. In A Chemical Change....

Examples of what I mean….

SIGNS OF A CHEMICAL CHANGE

It’s a chemical change if... A new substance is formed If the combination of two substances bubbles, (makes a gas) it’s a chemical change The gas is a new substance.

For Example.... Example: Alka-Seltzer Place some water in a test tube. Put an Alka-Seltzer tablet into the water and IMMEDIATELY place a balloon over the test tube

What Happened? What did you observe happening? Was a new substance created? How do you know?

Oxygen was created The chemical reaction that just occurred was: C 6 H 8 O 7 + 3NaHCO 3 → 3H 2 O + 3CO 2 + Na 3 C 6 H 5 O 7 citric acid + baking soda → water + carbon dioxide + sodium citrate carbon dioxide is released as a gas. Carbon Dioxide was one of the new substances being created.

It’s a chemical Change if.... An explosion occurs. In a chemical reaction, gas is formed as a new substance. Sometimes, the gas that forms is highly combustible and an explosion occurs! An explosion is evidence a chemical reaction did occur. Calcium Carbide explosions

It’s a chemical change if.... A precipitate is formed. A precipitate is a solid that forms and separates out from a liquid mixture. This goes along with the “a new substance is formed” idea.

For example... Limewater reaction: Using a straw, blow a couple of breaths into the limewater solution. What happens?

Was there a chemical change? Which caused a chemical change? Air or your breath? Breath reacted with limewater, but room air didn’t. What do you think is different about exhaled air that caused a chemical change? Here is the reaction: Ca(OH) 2 + CO 2 → CaCO 3 + H 2 O

It’s a chemical change if... There is a color change. A color change often indicates a new substance was formed. ( remember that it is possible to have a color change in a physical change too!)

For Example.... Add 3ml of the sodium iodate solution (solution A) to a test tube. Add 3ml of the sodium sulfate, citric acid and starch solution (solution B) to the test tube. Swirl the test tube around. DON’T TAKE YOUR EYES OFF YOUR TEST TUBE!!!!!!

AND THERE YOU HAVE A COLOR CHANGE! Now add 1-2 crystals of sodium thiosulfate to your test tube and swirl it around. Did another chemical reaction just occur? How do you know?

It’s a chemical change if... Temperature changes without YOU heating or cooling the substances. (Without you putting it in ice or on a hot plate) The substances reacting with each other cause the temperature to change.

Whenever chemical reactions occur, energy is transferred to or from the surroundings. An exothermic reaction is one which transfers heat energy to the surroundings An endothermic reaction is one which takes heat energy from the surroundings

Exothermic Exothermic reactions feel warm or hot because they are releasing heat or light. Example: Hand warmers are an exothermic reaction. Example: Lighting a match is an exothermic reaction. Exothermic reaction

Endothermic reaction Endothermic reactions feel cold because they are taking heat away from your body. Example: Vinegar and Sodium bicarbonate

It’s a chemical change if…. It burns Example: A match burning Water burningburning

Physical or Chemical change? Gas is created? Chemical change Mixing salt and water? Physical change Rust Chemical change

What kind of change is it if you.. someone... Burn paper? Chemical change Evaporate salt water? Physical change