Pure substances are materials that have a constant composition and distinct chemical properties. There are two types of pure substances: elements and compounds. An element is the simplest form of matter and cannot be broken down into simpler substances by chemical means. Examples include individual atoms on the periodic table like iron (Fe), oxygen (O), and carbon (C). A compound is formed when two or more different elements are chemically bonded together in a fixed ratio. These can be broken down into simpler substances. For instance, water (H₂O) is a compound made of hydrogen and oxygen, and table salt (NaCl) is a compound of sodium and chlorine.
A mixture, on the other hand, consists of two or more substances that are physically combined but not chemically bonded. Mixtures are classified as either homogeneous or heterogeneous. A homogeneous mixture has a uniform composition throughout, meaning you can't see the different components. Saltwater is a great example; once the salt is dissolved in water, you can't distinguish between the two. Other examples include air (a mixture of gases) and brass (a mixture of copper and zinc). In contrast, a heterogeneous mixture does not have a uniform composition, and its individual components are often visible. A classic example is sand and water; the sand settles at the bottom and is clearly separate from the water. Other examples include a salad, where you can see all the different vegetables, or a mixture of oil and water.
Tuesday, May 13, 2025
Law of Mass Conservation
The law of mass conservation is a fundamental principle in chemistry stating that mass is neither created nor destroyed in a chemical reaction. Put simply, the total mass of the substances that react (the reactants) must equal the total mass of the new substances that are formed (the products). The atoms are simply rearranged to form new compounds, but the total amount of matter remains the same. This principle was first articulated by Antoine Lavoisier in the late 18th century and is a cornerstone of modern chemistry.
Example Problem
Here's a common type of problem that illustrates this law:
If 10 grams of sodium (Na) react completely with chlorine (Cl) to produce 25.4 grams of sodium chloride (NaCl), how many grams of chlorine were used in the reaction?
Reactants: Sodium (Na) and Chlorine (Cl)
Product: Sodium Chloride (NaCl)
According to the law of mass conservation:
Mass of Reactants = Mass of Products
So,
Mass of Na + Mass of Cl = Mass of NaCl
We can plug in the known values:
10 g + Mass of Cl = 25.4 g
To find the mass of the chlorine, we subtract the mass of the sodium from the mass of the sodium chloride:
Mass of Cl = 25.4 g - 10 g
Mass of Cl = 15.4 g
Therefore, 15.4 grams of chlorine reacted with the 10 grams of sodium.
Example Problem
Here's a common type of problem that illustrates this law:
If 10 grams of sodium (Na) react completely with chlorine (Cl) to produce 25.4 grams of sodium chloride (NaCl), how many grams of chlorine were used in the reaction?
Reactants: Sodium (Na) and Chlorine (Cl)
Product: Sodium Chloride (NaCl)
According to the law of mass conservation:
Mass of Reactants = Mass of Products
So,
Mass of Na + Mass of Cl = Mass of NaCl
We can plug in the known values:
10 g + Mass of Cl = 25.4 g
To find the mass of the chlorine, we subtract the mass of the sodium from the mass of the sodium chloride:
Mass of Cl = 25.4 g - 10 g
Mass of Cl = 15.4 g
Therefore, 15.4 grams of chlorine reacted with the 10 grams of sodium.
Understanding Scientific Notation
Scientific notation is a way of writing very large or very small numbers in a more compact and manageable form. It's expressed as a number between 1 and 10 (the coefficient) multiplied by a power of 10. For instance, the number 5,800,000 can be written as 5.8 x 10⁶. Here, 5.8 is the coefficient, and 6 is the exponent. Similarly, a very small number like 0.000043 is written as 4.3 x 10⁻⁵. The key is to move the decimal point until you have a number between 1 and 10, and the number of places you moved the decimal becomes the exponent of 10. If you move the decimal to the left, the exponent is positive; if you move it to the right, the exponent is negative.
Calculations with Scientific Notation
Performing calculations with numbers in scientific notation is straightforward. When multiplying, you multiply the coefficients and add the exponents. For example, (2 x 10³) * (3 x 10⁴) = (2 * 3) x 10³⁺⁴ = 6 x 10⁷. When dividing, you divide the coefficients and subtract the exponents. For instance, (8 x 10⁷) / (2 x 10⁴) = (8 / 2) x 10⁷⁻⁴ = 4 x 10³.
For addition and subtraction, the exponents on the 10s must be the same. If they aren't, you'll need to adjust one of the numbers. For example, to add (2 x 10³) and (3 x 10⁴), you could rewrite 3 x 10⁴ as 30 x 10³. Then, you can add the coefficients: (2 + 30) x 10³ = 32 x 10³. Finally, you would adjust the answer to be in proper scientific notation, which would be 3.2 x 10⁴.
Calculations with Scientific Notation
Performing calculations with numbers in scientific notation is straightforward. When multiplying, you multiply the coefficients and add the exponents. For example, (2 x 10³) * (3 x 10⁴) = (2 * 3) x 10³⁺⁴ = 6 x 10⁷. When dividing, you divide the coefficients and subtract the exponents. For instance, (8 x 10⁷) / (2 x 10⁴) = (8 / 2) x 10⁷⁻⁴ = 4 x 10³.
For addition and subtraction, the exponents on the 10s must be the same. If they aren't, you'll need to adjust one of the numbers. For example, to add (2 x 10³) and (3 x 10⁴), you could rewrite 3 x 10⁴ as 30 x 10³. Then, you can add the coefficients: (2 + 30) x 10³ = 32 x 10³. Finally, you would adjust the answer to be in proper scientific notation, which would be 3.2 x 10⁴.
Temperature Conversions: A Simple Guide
Converting between Fahrenheit, Celsius, and Kelvin is straightforward with the right formulas.
To convert from Fahrenheit to Celsius, you first subtract 32 from the Fahrenheit temperature and then multiply the result by 5/9. For example, to convert 68°F to Celsius, you would calculate (68 - 32) * 5/9, which equals 20°C.
To go from Celsius to Fahrenheit, you do the reverse. Multiply the Celsius temperature by 9/5 and then add 32. So, to convert 20°C back to Fahrenheit, the calculation is (20 * 9/5) + 32, which brings you back to 68°F.
Converting from Celsius to Kelvin is the simplest of the three. You just need to add 273.15 to the Celsius temperature. Therefore, 20°C is equal to 293.15 K. Because the Kelvin scale is an absolute temperature scale, there are no negative numbers, and the degree symbol is not used.
To convert from Fahrenheit to Celsius, you first subtract 32 from the Fahrenheit temperature and then multiply the result by 5/9. For example, to convert 68°F to Celsius, you would calculate (68 - 32) * 5/9, which equals 20°C.
To go from Celsius to Fahrenheit, you do the reverse. Multiply the Celsius temperature by 9/5 and then add 32. So, to convert 20°C back to Fahrenheit, the calculation is (20 * 9/5) + 32, which brings you back to 68°F.
Converting from Celsius to Kelvin is the simplest of the three. You just need to add 273.15 to the Celsius temperature. Therefore, 20°C is equal to 293.15 K. Because the Kelvin scale is an absolute temperature scale, there are no negative numbers, and the degree symbol is not used.
Monday, May 12, 2025
New ACT Preparation Courses - May 2025
Is your high school student taking the ACT test this year?
Here is an opportunity to help your student get ready for the test by---
- finding out what to expect on the test
- learning test-taking strategies
- reviewing the content that the test covers
I am offering the test preparation course at two different times (each track will cover the same material). See below.
Track 1 will meet in person in Eden Prairie. Track 2 will meet over zoom. If none of the dates listed below work for you and you are still interested in the class, please contact me through the contact form and we can try to work something out.
Track 1 will meet for two two-hour sessions. Each session will cover two sections of the ACT.
It will meet on Thursday mornings from 10 am to noon.
Dates: May 22 and May 29
Location: Eden Prairie
Track 2 will meet for two two-hour sessions. Each session will cover two sections of the ACT.
It will meet on Thursday afternoons from 2 pm to 4 pm.
Dates: May 22 and May 29
Location: online through Zoom
Cost: $100.00 per student for the entire course. Includes materials.
Registration deadline: Friday, May 16, 2025
Register here.
Here is an opportunity to help your student get ready for the test by---
- finding out what to expect on the test
- learning test-taking strategies
- reviewing the content that the test covers
I am offering the test preparation course at two different times (each track will cover the same material). See below.
Track 1 will meet in person in Eden Prairie. Track 2 will meet over zoom. If none of the dates listed below work for you and you are still interested in the class, please contact me through the contact form and we can try to work something out.
Track 1 will meet for two two-hour sessions. Each session will cover two sections of the ACT.
It will meet on Thursday mornings from 10 am to noon.
Dates: May 22 and May 29
Location: Eden Prairie
Track 2 will meet for two two-hour sessions. Each session will cover two sections of the ACT.
It will meet on Thursday afternoons from 2 pm to 4 pm.
Dates: May 22 and May 29
Location: online through Zoom
Cost: $100.00 per student for the entire course. Includes materials.
Registration deadline: Friday, May 16, 2025
Register here.
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