A thermometer measures temperature — how hot or cold something is. Most thermometers use a liquid (like alcohol colored with dye) inside a thin glass tube. When the temperature rises, the liquid expands and moves up the tube. When the temperature drops, the liquid contracts and moves down. You read the temperature by looking at where the top of the liquid lines up with the numbered scale. Temperature is measured in degrees Celsius (used in science and most countries) or degrees Fahrenheit (used in everyday life in the United States).
Give students thermometers and have them measure the temperature of cold water, room-temperature water, and warm water. Record readings and compare. Practice reading the scale with printed diagrams showing thermometers at various levels. Discuss the Celsius scale: 0 degrees is where water freezes, 100 degrees is where water boils.
Temperature is one of the most important measurements in science, and the thermometer is the tool you use to take it. You have used thermometers before — maybe to check the weather or when you were sick. Now it is time to understand exactly how they work and how to read them accurately.
Most classroom thermometers contain a liquid — usually alcohol dyed red or blue — sealed inside a thin glass tube. At the bottom is a small bulb where most of the liquid sits. When the temperature around the thermometer rises, the liquid expands — it takes up a little more space. Since the tube is very narrow, even a tiny expansion pushes the liquid noticeably higher up the tube. When the temperature drops, the liquid contracts and slides back down. The height of the liquid tells you the temperature.
Along the side of the tube is a numbered scale marked in degrees. Most science thermometers use the Celsius scale, where 0 degrees C is the temperature at which water freezes and 100 degrees C is the temperature at which water boils. Some thermometers also show the Fahrenheit scale, where water freezes at 32 degrees F and boils at 212 degrees F. In science class, you will almost always use Celsius.
To read a thermometer accurately, hold it upright and bring your eyes level with the top of the liquid. Look straight at it, not from above or below. If you look from an angle, the liquid appears to be at a different line than it actually is — this common mistake is called parallax error. Once your eyes are level, note where the top of the liquid aligns with the scale. If it is between two lines, estimate the value. For example, if the liquid is halfway between 20 and 22 degrees, the temperature is about 21 degrees.
Practice makes this feel natural. Try measuring the temperature of tap water, ice water, and water that has been sitting in the sun. You will notice that ice water sits right near 0 degrees Celsius, tap water is usually around 15 to 20 degrees, and sun-warmed water might reach 30 degrees or higher. These reference points help you develop an intuition for what temperature numbers actually feel like — and that intuition will be valuable as you study how temperature affects melting, boiling, and dissolving.