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# Use pH paper to test the pH of the three solu ons.

Acids & Bases

Ques ons

1. Why did the grape juice change color when an acid or base was added?
2. You added a base, sodium bicarbonate, to test tube 1 that contained citric acid and an acid to test tube 2 that contained base. Why did the grape juice return to its original color?
3. Name two acids and two bases you o en use. Lab 5: Chemical Processes

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Lab 5: Chemical Processes

Introduc on

Have you ever needed to place a cold pack on a sprained muscle? It’s the final seconds of the community league champion‐ ship basketball game, and your team is behind by one point. One of your team’s players takes a shot and scores. The game is over, and your team won! But something is wrong: the player is si ng on the floor, and appears to be in a lot of pain. The coach quickly brings a cold pack to the player, squeezes it, and places it on the swelling ankle. The bag immediately becomes cold—but how?

Though we o en use them interchangeably, heat and tem‐ perature have different defini ons—though they are close‐ ly related in the study of thermodynamics. Heat is the transfer of energy from one object to another due to a difference in temperature. Temperature, on the other hand, describes how much energy the atoms and molecules in a sub‐ stance have. This energy, o en called internal energy, describes how quickly the atoms or molecules in a substance move or vibrate around. When an object gains heat its molecules vibrate with more energy, which we can sense or measure as an increase in temperature. When you touch a hot object, it feels hot because a heat moves from the hot object (higher energy) to your skin (lower energy). Similarly, an object feels cold when heat is lost by your hand and gained by the cold object. Heat always transfers in the direc on of high temperature to low temperature—high energy to low energy.

Both physical processes and chemical reac ons can release or absorb energy in the form of heat. When a reac on or phys‐ ical change gives off energy it is called an exothermic process. To remember exothermic, think of ‘exi ng’ as in leaving or going out. An endothermic process does just the opposite—it takes in energy from its surroundings. The generalized chemical equa ons for exothermic and endothermic reac ons are:

The direc on energy moves determines whether the process is considered endothermic or exothermic, and tells you how the temperature of a system changes. In an endothermic reac on or physical change, energy is absorbed and the overall temperature of the system decreases. Some examples of endothermic processes include the mel ng of water in a so drink or the evapora on of a liquid. Similarly, an endothermic reac on takes in energy for chemical changes to occur. One example is what occurs in an instant cold pack like the ones used to decrease the swelling caused from a sports injury. The‐

exothermic:

endothermic:

reactants → products + energy

reactants + energy → products

Figure 1: The combus on of fuel, such as wood or coal, is a com‐ mon example of an exothermic reac on. Under the right condi‐ ons (usually the applica on of enough heat), a chemical reac‐ on occurs between wood and the oxygen in air. Fire is the re‐

Concepts to explore:  Understand the difference between endothermic and exothermic

processes  Understand the concept of enthalpy