Home / Study Guides / Chemistry
Chemistry Study Guide
Chemistry explores the substances that make up matter and the changes they undergo. This guide introduces atoms, elements, compounds, chemical formulas, balancing equations, acids and bases, mixtures, and basic laboratory safety. Worked examples and practice questions help you check your understanding.
1. Matter, atoms, and elements
Matter is anything that has mass and occupies space. It is made of particles. An atom is the smallest unit of an element that retains that element's chemical identity.
- Protons: Positively charged particles found in the nucleus.
- Neutrons: Particles with no electric charge, found in the nucleus.
- Electrons: Negatively charged particles found around the nucleus.
An element contains one type of atom, defined by its number of protons. A compound contains two or more different elements chemically bonded in a fixed proportion.
Example: Water, H₂O, contains hydrogen and oxygen. Each water molecule has two hydrogen atoms and one oxygen atom.
2. Chemical symbols and formulas
Chemical symbols are shorthand for elements. For example, H represents hydrogen, O represents oxygen, Na represents sodium, and Cl represents chlorine.
A chemical formula shows which elements are present and how many atoms of each are represented in a molecule or formula unit.
- H₂O: Two hydrogen atoms for every oxygen atom in a water molecule.
- CO₂: One carbon atom and two oxygen atoms in each carbon dioxide molecule.
- NaCl: Sodium chloride, composed of sodium and chloride ions in a 1:1 ratio.
The small number written after an element symbol is called a subscript. If there is no subscript, the number is understood to be one.
3. Balancing chemical equations
A chemical equation represents a chemical reaction. It must be balanced because atoms are conserved during an ordinary chemical reaction: the number of atoms of each element must be the same on both sides.
Worked example: formation of water
Start with the unbalanced equation:
H₂ + O₂ → H₂O
- The left side has two oxygen atoms, but the right side has one. Place a coefficient of 2 before H₂O.
- The equation becomes
H₂ + O₂ → 2H₂O. The right side now has four hydrogen atoms. - Place a coefficient of 2 before H₂.
The balanced equation is:
2H₂ + O₂ → 2H₂O
There are now four hydrogen atoms and two oxygen atoms on each side. Change coefficients to balance equations; do not change subscripts in chemical formulas.
4. Acids, bases, and pH
Acids and bases have different chemical properties. In a typical aqueous solution at about 25°C:
- A pH below 7 is acidic.
- A pH of 7 is neutral.
- A pH above 7 is basic or alkaline.
Examples of acidic substances include lemon juice and vinegar. Many household cleaning products are alkaline, but their exact properties vary by product.
The pH scale is logarithmic: a difference of one pH unit corresponds to a tenfold change in hydrogen-ion activity under the relevant conditions.
5. Pure substances and mixtures
A pure substance has a defined chemical composition. It may be an element or a compound. A mixture contains two or more substances physically combined, rather than chemically bonded into one substance.
- Homogeneous mixture: Has a uniform composition throughout, such as salt dissolved in water.
- Heterogeneous mixture: Has a non-uniform composition, such as sand mixed with water.
Mixtures can often be separated using physical methods. Filtration can separate an insoluble solid from a liquid. Evaporation can recover a dissolved solid from a solution, while distillation can separate substances based on differences in boiling behaviour.
6. Basic laboratory safety
- Follow your teacher's or laboratory supervisor's instructions.
- Wear appropriate eye protection and other required protective equipment.
- Never taste chemicals or directly inhale their vapours.
- Read labels carefully and use only the quantities specified.
- Report spills, breakages, or injuries immediately.
- Do not mix chemicals unless the procedure specifically instructs you to do so.
Practical experiments should be carried out with appropriate supervision and equipment.
7. Practice questions with answers
Try each question before opening its answer.
Question 1: How many hydrogen atoms are in one H₂O molecule?
There are two hydrogen atoms, as indicated by the subscript 2 after H.
Question 2: What does the formula CO₂ represent?
One carbon atom and two oxygen atoms in each carbon dioxide molecule.
Question 3: Balance H₂ + Cl₂ → HCl
The balanced equation is H₂ + Cl₂ → 2HCl. Both sides contain two hydrogen atoms and two chlorine atoms.
Question 4: Is a solution with pH 3 acidic, neutral, or basic?
It is acidic because its pH is below 7 under the usual aqueous-solution classification at about 25°C.
Question 5: Which method can separate sand from water?
Filtration can separate insoluble sand from water.
Continue learning with EduSolveX
Use the EduSolveX question solver to request an explanation of a chemistry topic. Check AI-generated explanations against your textbook and class notes.
Explore our other guides: Mathematics, Physics, Biology, Hindi, and English.