Activator – Write down the entire sentence! A chemical reaction is when two or more substances interact and form a new _____________ with new ______________. In a chemical reaction, the mass of the reactants must be __________ to the mass of the products, according to the law of __________________ _______ ___________.

Chemical Changes September 28, 2017

Objective SWBAT: identify evidence of a chemical reaction taking place

Connection We know that when substances are combined, a couple different things can happen: They can form a mixture, where they keep their identity and properties Or they can go through a chemical reaction and create new substances with new properties Today we are going to talk about evidence that tells you if a chemical reaction is happening or not.

Formation of a Gas When two chemicals (liquid and liquid, solid and liquid) are mixed together, they sometimes create bubbles of gas. These bubbles are evidence that a chemical reaction is taking place.

Formation of a Gas Example: Mixing Baking Soda and Vinegar When sodium bicarbonate (baking soda) is mixed with acetic acid (vinegar), a chemical reaction happens that creates carbon dioxide gas. NaHCO3 + HC2H3O2 → NaC2H3O2 + H2O + CO2 Vinegar + Baking Soda  Sodium Acetate + water + carbon dioxide

Formation of a Precipitate When two chemicals (liquid and liquid, liquid and gas) are mixed together, they sometimes can form a solid substance called a precipitate. This solid (or precipitate) is evidence that a chemical reaction is taking place.

Formation of a Precipitate Example: Blowing into Limewater When you breathe out you are exhaling Carbon Dioxide. When this Carbon Dioxide mixes with Calcium Hydroxide, they go through a chemical reaction to create Calcium Carbonate, which is chalk. CO2 + Ca(OH)2  CaCO3 + H2O Carbon Dioxide + Limewater  Calcium Carbonate + water

Change in Temperature Some chemical reactions are exothermic, which means the temperature of the mixture increases (feels hot to touch) Some chemical reactions are endothermic, which means the temperature of the mixture decreases (feels cold to touch)

Change in Temperature Example: Elephant Toothpaste When hydrogen peroxide is mixed with yeast, it releases oxygen molecules which can be captured in tiny bubbles. This is a reaction that releases heat, otherwise known as an exothermic reaction. 2H2O2   2H2O + O2 hydrogen peroxide  water + oxygen

Exit Ticket Tell me one chemical reaction that you witnessed today and tell me how you know it was a chemical reaction. (Give me the evidence!)

Homework Complete the study guide for tomorrow’s quiz!