Chemistry, Matter, and Measurement

Chemistry as General Science

Chemistry appears inside General Science because GS covers physical and biological science knowledge. PiCAT chemistry is not a full laboratory course. It is a practical check of whether you understand matter, atoms, formulas, common reactions, acids and bases, and measurement units.

The best approach is to connect vocabulary to structure. If you know what an atom contains, what a formula records, and what a unit measures, many chemistry items become direct. If you rely only on word recognition, small details such as a subscript or prefix can cause a miss.

Matter and States

Matter has mass and takes up space. It is commonly described as solid, liquid, gas, or plasma. A solid keeps its shape and volume. A liquid keeps volume but takes the shape of its container. A gas expands to fill its container. Plasma is an ionized gas found in places such as stars, lightning, and some specialized devices.

Phase changes describe matter moving between states. Melting changes solid to liquid. Freezing changes liquid to solid. Vaporization changes liquid to gas. Evaporation is vaporization from a surface below the boiling point. Condensation changes gas to liquid. Sublimation changes solid directly to gas.

Temperature and heat are not identical. Temperature measures average particle motion. Heat is energy transferred from warmer matter to cooler matter. This distinction helps with questions about boiling, cooling, insulation, and thermal transfer.

Atoms, Elements, and Ions

An atom has a nucleus with protons and neutrons, with electrons outside the nucleus. Protons have positive charge, electrons have negative charge, and neutrons have no electrical charge. The number of protons is the atomic number, and it identifies the element.

If an atom gains or loses electrons, it becomes an ion. Losing electrons makes a positive ion because there are more protons than electrons. Gaining electrons makes a negative ion. Changing the number of neutrons creates an isotope, not a different element.

Mass number is protons plus neutrons. For a neutral atom, the number of electrons equals the number of protons. These relationships are common in basic science questions because they test structure rather than memorized element lists.

Compounds, Formulas, and Bonds

A molecule is made of atoms bonded together. A compound contains two or more different elements chemically combined. A formula uses element symbols and subscripts to show the atoms present. In CO2, there is one carbon atom and two oxygen atoms. In CaCl2, there is one calcium atom and two chlorine atoms.

Do not move subscripts casually. H2O and H2O2 are different substances. The formula is not just a decoration; it is the atom count.

Chemical bonds involve electrons. An ionic bond forms when electrons are transferred and oppositely charged ions attract. A covalent bond forms when atoms share electrons. Metals also have metallic bonding, where electrons can move through a lattice of metal atoms.

Reactions and Conservation

Chemical reactions rearrange atoms. They do not create or destroy atoms in ordinary chemical reactions. This is why balanced equations matter: the same number of each type of atom appears before and after the reaction.

Common reaction clues include burning, rusting, gas production, color change, temperature change, and formation of a solid precipitate from solutions. Physical changes, such as cutting, dissolving, melting, or evaporating, may change form without making a new substance.

Combustion usually involves reaction with oxygen and releases energy. Oxidation includes reactions where electrons are lost; rusting is a familiar slow oxidation process involving iron and oxygen with moisture.

Acids, Bases, and pH

The pH scale describes acidity or basicity. A pH below 7 is acidic, 7 is neutral, and above 7 is basic or alkaline. Each whole pH step represents a tenfold change in hydrogen ion concentration, so pH 3 is much more acidic than pH 5.

Acids often taste sour in everyday examples and can react with some metals. Bases can feel slippery and can neutralize acids. Do not use taste or touch as a lab procedure; on the exam, these are descriptive ideas, not safety instructions.

Measurement and Units

Chemistry and physical science use metric units often. Mass may be measured in grams or kilograms. Volume may be milliliters or liters. Length may be millimeters, centimeters, meters, or kilometers. Density is mass divided by volume.

Key prefixes:

Convert units before substituting into formulas. If density is requested in grams per milliliter, mass must be in grams and volume in milliliters. If volume is given in liters, convert when needed.

PiCAT Chemistry Routine

For any chemistry item, identify whether it is asking about structure, count, change, or measurement. Structure means atoms, ions, bonds, and formulas. Count means protons, electrons, neutrons, mass number, or subscripts. Change means phase change, reaction, acid-base behavior, or conservation. Measurement means units, density, pH, or metric prefixes.

This routine keeps you from overreading. Many PiCAT chemistry questions are short because the tested rule is basic. Read the symbols carefully, convert units first, and let the rule do the work.