7.8.3 Variation in Colour of Crystals

Most thermometers require you to read a number from a scale. But imagine a thermometer that tells you the temperature simply by changing color. This is exactly what liquid crystal thermometers do. They use special materials called liquid crystals that change color in response to temperature changes. This property makes them useful for quick, visual temperature readings in applications ranging from medical thermometers to aquarium thermometers.

Definition

Liquid Crystals are substances that exhibit properties between those of conventional liquids and solid crystals. In the context of thermometry, liquid crystals are materials that change color with a change in temperature. This color change is used as a thermometric property in liquid crystal thermometers.

Main Idea

The main idea is that liquid crystals change color at specific temperatures due to changes in their molecular arrangement. When the temperature changes, the spacing between the liquid crystal molecules changes, which alters the wavelength of light they reflect. By calibrating the colors to specific temperatures, a liquid crystal thermometer can provide a visual indication of temperature without the need for a numerical scale. This makes them convenient for applications requiring quick, non-invasive temperature readings.

Core Concept

Liquid Crystals The text states: "Liquid crystals are those materials that change colour with a change in temperature. Liquid crystals are packed inside a plastic strip."

Liquid crystals are a unique state of matter that has properties between those of a liquid and a solid crystal. They can flow like a liquid but have some degree of order like a crystal. Certain liquid crystals change their molecular arrangement (and thus their optical properties) in response to temperature changes.

Liquid Crystal Thermometers The text states: "In these thermometers, a liquid crystal material is sealed in a plastic strip or patch. We touch it with the body whose temperature is to be measured. When its temperature changes, it also changes colour. By matching the colour to a temperature scale, we can determine the temperature of a body."

• Construction: A thin layer of liquid crystal material is sealed between plastic sheets.
• Operation: When the thermometer is placed in contact with an object, heat is transferred to the liquid crystals. The crystals change color in response to the temperature change.
• Reading: The color is matched to a calibrated scale printed on the strip to determine the temperature.

Applications The text states: "These thermometers are often used as fever thermometers and for aquariums and baby bottles."

• Fever thermometers: A strip placed on the forehead changes color to indicate body temperature. (Note: These are less accurate than digital thermometers and are not recommended for medical use by many health authorities, but they are convenient.)
• Aquarium thermometers: A strip attached to the outside of the aquarium changes color to indicate water temperature.
• Baby bottles: A strip on the bottle changes color to indicate if the milk is too hot or too cold.
• Room thermometers: Strips that indicate room temperature.

• Simple and easy to read: The color change provides a visual indication of temperature without the need for numbers.
• Inexpensive: They are cheap to manufacture.
• Non-invasive: They can be placed on the skin or on containers without direct contact with the liquid inside.
• Fast response: They respond quickly to temperature changes.
• No power required: They do not need batteries or electricity.

• Limited accuracy: They are less accurate than digital or liquid-in-glass thermometers (typically ±0.5°C to ±1°C).
• Limited range: They are only useful over a specific temperature range (e.g., 35°C to 40°C for fever thermometers).
• Single-use or limited lifespan: Some liquid crystal strips degrade over time and lose accuracy.
• Not suitable for precise scientific measurements: They are best for indicative readings, not for laboratory work.

Detailed Explanation

Liquid crystal thermometers are a fascinating application of the thermometric properties of liquid crystals.

The Physics of Liquid Crystals Liquid crystals are materials that exhibit a phase of matter between liquid and solid. In the nematic phase, the molecules are oriented in a preferred direction but can move freely. In the cholesteric (or chiral nematic) phase, the molecules are arranged in a helical structure. The pitch of the helix (the distance over which the molecules rotate 360°) changes with temperature. This change in pitch alters the wavelength of light that is reflected by the liquid crystal, causing a color change.

This property is called thermochromism—the ability of a substance to change color due to a change in temperature.

How Liquid Crystal Thermometers Work A liquid crystal thermometer consists of a thin layer of liquid crystal material sealed between two plastic sheets. The liquid crystals are formulated to change color at specific temperatures. When the thermometer is placed in contact with an object, heat is transferred to the liquid crystals, causing them to change color. The color is compared to a calibrated scale printed on the strip to determine the temperature.

Different formulations of liquid crystals are used for different temperature ranges. For example, a fever thermometer might use liquid crystals that change from brown to green to blue over the range of 35°C to 40°C.

• Medical: Fever strips are placed on the forehead to provide a quick indication of body temperature. They are convenient for screening but are not as accurate as digital thermometers.
• Aquariums: Liquid crystal strips attached to the outside of the aquarium glass change color to indicate the water temperature. This allows the aquarium owner to monitor the temperature without opening the tank.
• Baby bottles: Strips on baby bottles change color to indicate whether the milk is too hot, too cold, or just right.
• Industrial: Liquid crystal thermometers are used in some industrial applications for monitoring surface temperatures, such as in electronics or machinery.

Advantages and Limitations The main advantage of liquid crystal thermometers is their simplicity and ease of use. They provide a visual indication of temperature without the need for numbers or power. However, their accuracy is limited, and they are not suitable for applications requiring precise measurements. They are best used for indicative readings and for applications where a quick, visual check is sufficient.

Intermediate Level

In physics, liquid crystal thermometers use the thermochromic properties of liquid crystals. These materials change color in response to temperature changes due to changes in their molecular arrangement. The pitch of the helical structure of cholesteric liquid crystals changes with temperature, altering the wavelength of reflected light.

Liquid crystal thermometers consist of a thin layer of liquid crystals sealed in a plastic strip. They are inexpensive, easy to read, and require no power. However, they have limited accuracy (typically ±0.5°C to ±1°C) and a limited temperature range. They are used for applications such as fever thermometers, aquarium thermometers, and baby bottle thermometers, where a quick, visual indication of temperature is sufficient.

Important Points

• - Liquid Crystals: Materials that change color with temperature.
• - Thermochromism: The property of changing color with temperature.
• - Construction: Liquid crystals sealed in a plastic strip.
• - Operation: Color change indicates temperature (matched to a calibrated scale).
• - Applications: Fever thermometers, aquarium thermometers, baby bottles.
• - Advantages: Simple, inexpensive, easy to read, no power required.
• - Limitations: Limited accuracy and range; not suitable for precise measurements.

Remember

• - Liquid Crystals: Change color with temperature.
• - Thermochromism: Color change with temperature.
• - Construction: Plastic strip with liquid crystals.
• - Applications: Fever strips, aquarium strips, baby bottle strips.
• - Advantages: Simple, cheap, visual.
• - Limitations: Limited accuracy and range.

Common Mistakes

• - Thinking liquid crystal thermometers are very accurate: They are not as accurate as digital or liquid-in-glass thermometers. They are best for indicative readings.
• - Thinking liquid crystals are the same as liquid or solid crystals: Liquid crystals are a distinct state of matter with properties between those of liquids and solids.
• - Forgetting that liquid crystal thermometers have a limited range: They only work over a specific temperature range (e.g., 35°C to 40°C for fever thermometers). Outside this range, they may not give a valid reading.

Quick Revision

Liquid crystal thermometers use liquid crystals that change color in response to temperature changes. This color change is due to changes in the molecular arrangement of the crystals. The thermometers are simple, inexpensive, and easy to read, making them useful for fever thermometers, aquarium thermometers, and baby bottle thermometers. However, they have limited accuracy and a limited temperature range.

60-Second Revision

• Material: Liquid crystals that change color with temperature.
• Property: Thermochromism.
• Construction: Plastic strip with liquid crystals.
• Applications: Fever strips, aquarium strips, baby bottles.
• Advantages: Simple, cheap, visual.
• Limitations: Limited accuracy and range.

Frequently Asked Questions (FAQs)

Q1. What is a liquid crystal thermometer? Answer: A liquid crystal thermometer is a device that measures temperature by using liquid crystals that change color in response to temperature changes. The color is matched to a calibrated scale to determine the temperature.

Q2. How do liquid crystals change color with temperature? Answer: The molecular arrangement of certain liquid crystals changes with temperature. This changes the wavelength of light they reflect, causing a color change. This property is called thermochromism.

• Q3. What are the applications of liquid crystal thermometers?
• Answer: They are used as fever thermometers (forehead strips), aquarium thermometers, baby bottle thermometers, and room thermometers.

• Q4. What are the advantages of liquid crystal thermometers?
• Answer: They are simple, inexpensive, easy to read, require no power, and provide a quick visual indication of temperature.

• Q5. What are the limitations of liquid crystal thermometers?
• Answer: They have limited accuracy (typically ±0.5°C to ±1°C) and a limited temperature range. They are not suitable for precise scientific measurements.

• Q6. Are liquid crystal thermometers accurate?
• Answer: They are less accurate than digital or liquid-in-glass thermometers. They are best for indicative readings, not for precise measurements.

Q7. What is thermochromism? Answer: Thermochromism is the property of a substance to change color in response to a change in temperature. Liquid crystals used in thermometers exhibit this property.

Multiple Choice Questions (MCQs)

• Q1. Liquid crystal thermometers use a thermometric property based on:
• A. Volume expansion
• B. Color change
• C. Electrical resistance
• D. Pressure change

• Correct Answer: B
• Explanation: The text states that liquid crystals change color with a change in temperature.

• Q2. The property of changing color with temperature is called:
• A. Thermometric expansion
• B. Thermochromism
• C. Thermoemission
• D. Thermoelectricity

• Correct Answer: B
• Explanation: The property of changing color with temperature is called thermochromism.

• Q3. Liquid crystals are packed inside a:
• A. Glass tube
• B. Plastic strip
• C. Metal case
• D. Wooden frame

• Correct Answer: B
• Explanation: The text states that liquid crystals are packed inside a plastic strip.

• Q4. Which of the following is an application of liquid crystal thermometers?
• A. Laboratory experiments
• B. Fever thermometers
• C. Industrial furnaces
• D. Deep-freeze monitoring

• Correct Answer: B
• Explanation: The text states that these thermometers are often used as fever thermometers, as well as for aquariums and baby bottles.

• Q5. What is an advantage of liquid crystal thermometers?
• A. Very high accuracy
• B. Simple and easy to read
• C. Wide temperature range
• D. Requires power

• Correct Answer: B
• Explanation: Liquid crystal thermometers are simple and easy to read because the color change provides a visual indication of temperature.

• Q6. What is a limitation of liquid crystal thermometers?
• A. They are expensive
• B. They have limited accuracy
• C. They require batteries
• D. They are difficult to read

• Correct Answer: B
• Explanation: Liquid crystal thermometers have limited accuracy compared to digital or liquid-in-glass thermometers.

• Q7. Liquid crystal thermometers are used for aquariums because they:
• A. Are very accurate
• B. Can be attached to the outside of the glass
• C. Require electricity
• D. Are expensive

• Correct Answer: B
• Explanation: Liquid crystal strips can be attached to the outside of the aquarium glass and provide a visual indication of water temperature.

• Q8. The color change in liquid crystals is due to:
• A. A change in mass
• B. A change in molecular arrangement
• C. A change in volume
• D. A change in pressure

Correct Answer: B Explanation: The change in molecular arrangement of liquid crystals with temperature causes a change in the wavelength of reflected light, resulting in a color change.

• Q9. Liquid crystal thermometers are best used for:
• A. Precise scientific measurements
• B. Indicative readings
• C. High-temperature measurements
• D. Low-temperature measurements

• Correct Answer: B
• Explanation: Liquid crystal thermometers are best for indicative readings, not for precise measurements.

• Q10. A fever thermometer strip placed on the forehead is an example of:
• A. A liquid-in-glass thermometer
• B. A thermocouple
• C. A liquid crystal thermometer
• D. A gas thermometer

Correct Answer: C Explanation: The text states that liquid crystal thermometers are often used as fever thermometers.I am ready to proceed. Based on the source material, the next subtopic is:

7.8.4 Resistance Thermometers or Thermistors

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