Understanding how to develop hypotheses and plan experiments is a foundational skill in chemistry. This entry will guide you through using scientific theories to formulate hypotheses, designing experiments, selecting appropriate techniques and materials, conducting experiments, and ensuring representative sampling.

In chemistry, a practical, investigation, and experiment, while closely related, have distinct differences:

1. Practical: This refers to hands-on activities conducted in the lab to apply and practice theoretical knowledge. It usually involves following set procedures to develop skills and understand techniques.
2. Experiment: This is a specific procedure carried out to test a hypothesis or observe a phenomenon. It follows a structured approach, including a control, variables, and repeated trials to ensure accuracy and reliability of results.
3. Investigation: This is a more open-ended activity where students explore a scientific question or problem. It involves planning, conducting, and analysing experiments, often requiring critical thinking and problem-solving to draw conclusions.

Before diving into hypotheses and methods, it’s essential to understand variables. Variables are parts an experiment that can change. There are three kinds of variables:

1. Independent variable: The variable that you change or manipulate in the experiment.
2. Dependent variable: The variable that you measure or observe in response to changes in the independent variable.
3. Control variables: Variables that are kept constant to ensure that the test is fair.

In an experiment investigating how temperature affects the rate of a reaction, the independent variable is temperature, the dependent variable is the reaction rate, and control variables might include the concentration of reactants and the volume of the reaction mixture.

In some experiments, particularly those involving mixtures or reactions, it is important to collect representative samples to ensure the accuracy of your results.

Applying sampling techniques:

1. Identify when sampling is needed: Determine if your experiment requires sampling, such as monitoring pH throughout a reaction.
2. Use representative sampling methods: Ensure samples are taken consistently and represent the whole mixture or reaction. For example, taking multiple samples at different time points.
3. Avoid contamination: Use clean apparatus and techniques to prevent contamination of samples.

Developing Hypotheses

A hypothesis is a testable statement based on scientific theories and explanations. In chemistry, a hypothesis predicts the outcome of an experiment based on existing knowledge. It is important to distinguish between a prediction and a hypothesis. A prediction is a simple statement about what will happen, while a hypothesis provides a reasoned explanation based on scientific principles.

A good hypothesis follows a three-step process:

1. Identify your variables and operationalise them:
   • Independent variable: What you change (e.g., concentration of acid).
   • Dependent variable: What you measure (e.g., volume of gas produced).
   • Operationalise: Define how you'll measure these variables. For example, "concentration of acid in mol/dm³" and "volume of gas in cm³".
2. Identify whether you are looking for a difference or a relationship:
   • Difference: Comparing two groups (e.g., high vs low concentration).
   • Relationship: How one variable affects another (e.g., concentration affects reaction rate).
3. Identify whether your hypothesis is directional or non-directional:
   • Directional: Predicts the direction of the effect (e.g., higher concentration increases the reaction rate).
   • Non-directional: States there will be an effect but not the direction (e.g., different concentrations will affect the reaction rate).

Planning Experiments

Planning an experiment involves designing a procedure to test your hypothesis. This includes identifying the variables, controls, and the steps needed to carry out the experiment.

A good method follows a three-step process:

1. Define the variables:
   • Independent variable: The variable you change or manipulate (e.g., temperature, concentration).
   • Dependent variable: The variable you measure (e.g., reaction rate, gas volume).
   • Control variables: Variables you keep constant to ensure a fair test (e.g., temperature, volume of reactants).
2. Write a step-by-step procedure:
   • Describe the exact steps to be taken in the experiment.
   • Include details such as apparatus, measurements, timings, and equipment settings.
   • Ensure the procedure is detailed enough to be repeatable by others.
3. Identify safety measures:
   • Include any safety precautions necessary to handle chemicals and equipment safely.
   • Make sure to wear appropriate personal protective equipment (PPE) such as gloves and safety goggles.

Choosing the right techniques (e.g. titration, distillation, calorimetry, etc.), apparatus, and materials is crucial for the success of an experiment. This involves understanding the properties of chemicals and the appropriate use of laboratory equipment. Ensure you know how to correctly and safely use all equipment. 

Carrying out experiments requires attention to detail and adherence to safety protocols. Proper manipulation of apparatus and accuracy in measurements are essential.