SKILL BUILDER PAG 80

VARIABLES AND CONTROLS

How did dissolving salt in one beaker affect the water temperature?

Dissolving common salt in water is a process that is not energetically neutral. It ‘takes the electrical energy to break the red salt crystal and the hydrogen bonds between the water molecules. At room temperature, the result is such that the entire process absorbs the energy; the enthalpy of the dissolution of the common salt in water is ΔH ≈ 3.9 kJ mol-1.

The agitation of the water causes an increase in temperature due to the increase of kinetic energy, however, the dissolution of the salt consumes part of the energy generated, so that the temperature of the salt solution is lower than the water alone.

What were the controls? Variables?

The independent variable is the factor that a scientist manipulates to obtain results.

The dependent variable will respond to changes in the independent variable.

Control variables are the things that are kept constant in each test of the experiment.

In this case the independent variable is the dissolution of the salt in water, the dependent variable is the temperature of the water that will change due to the presence of the salt. The control variable is the amount of water added, which is the same from the beginning to the end of the experiment in both beakers.

PRACTICING THE SKILL

1.    Ten grams of sugar dissolves more quickly in a liter of pure,hot,tap wáter tan it does in a liter of pure,cold,tap wáter. List the controls, the independent variable, and the dependent variable.

Controls: Same amount of water, same amount of sugar, both containers are covered.

Independent variables: water temperatures: cold and hot.

Dependent variables: time in which the sugar dissolves.

2.    Identify the dependent variable from the following hypothesis: Of Premium gasoline is more efficient, so it should increase the fuel mileage of our family car.

Dependent variable: family car efficiency