Suspensions in Chemistry

Definition and Characteristics of Suspensions

Introduction

Key Characteristics

• Particle Size: Particles in suspensions are generally larger than 1 micrometre, allowing them to be seen with the naked eye or under a microscope.
• Turbidity: Suspensions display cloudiness or haziness due to the scattering of light.
• Settling Behaviour: Particles gradually settle when undisturbed, necessitating stirring to maintain uniform dispersion.

Diagram

Examples of Suspensions

• Muddy Water: Consists of soil particles suspended in water, resulting in a cloudy appearance.
• Paint: Contains pigments that need stirring before use to ensure even distribution.
• Orange Juice with Pulp: The pulp remains initially suspended, requiring agitation.

Key Properties of Suspensions

Heterogeneity and Physical Distinctness

• Suspension: A heterogeneous mixture where particles are visibly distinct and do not dissolve.

Separability

• Presence of larger particles enables separability through techniques like filtration.
  • Methods: Filtration, Decantation, Centrifugation

Instability and Agitation Need

• Naturally unstable, suspensions require agitation, such as shaking, to maintain particle dispersion.

Turbidity and Measurement

• Instruments such as Nephelometers and Spectrophotometers are used to measure turbidity.

Calculating Settling Velocity

• Stokes' Law describes the velocity of settling particles: $v = \frac{2}{9} \frac{r^2 g (\rho_p - \rho_m)}{\eta}$

Practical Examples and Everyday Life Applications

Natural Examples

River Water with Sediment

• Large, observable particles that settle over time when left undisturbed.

Household Examples

Orange Juice with Pulp

• The pulp is visible and settles, necessitating shaking.

Flour in Water

• Particles create a cloudy mixture that settles if not stirred.

Industrial Examples

Paints

• Pigments suspended in the medium require agitation for uniform application.

Medicines (Calamine Lotion)

• Shaking ensures even particle distribution before use.

Key Comparison Aspects

Particle Size

• Suspensions: Particles exceed 1 micrometre in size.

Stability

• Suspensions: Particles settle in the absence of stirring.

Light Interaction

• Suspensions: Particles scatter light, resulting in cloudiness.

Core Characteristics Embedded in Examples

• Suspensions: Like muddy water, in which particles settle when disturbed.

Distinguishing Features

Particle Size and Density Effects on Stability

• Particle Size: Larger particles settle more rapidly.
• Density: Higher particle density relative to the fluid enhances settling.

Medium Viscosity Impact

• Viscosity: A more viscous liquid slows down particle settlement.

Temperature's Role

• Temperature: Increasing temperature decreases viscosity, thus speeding up the settling process.

Agitation Techniques

• Mechanical Stirring: Keeps particles in motion to prevent settling.
• Vibration Methods: Effective for maintaining suspension integrity.

Additives for Stability Enhancement

• Emulsifiers: Employed to stabilise suspensions, reducing the need for constant agitation.

Suggested Practice Questions

1. How does introducing larger particles affect the stability in a low-density fluid like oil? Solution: Larger particles in oil will settle more quickly due to Stokes' Law. The settling velocity is proportional to the square of the particle radius, so doubling the particle size increases settling velocity by a factor of four, significantly reducing suspension stability.

2. Describe the effects of temperature on viscosity with an example related to cooking. Solution: Increasing temperature decreases viscosity. For example, honey flows more easily when warmed. In cooking, oil becomes less viscous when heated, making it easier to coat food items evenly during frying, but this also means that suspended spices will settle more quickly in hot oil than in cold oil.

3. Why are emulsifiers preferred over mechanical stirring in certain industrial applications? Solution: Emulsifiers provide long-term stability without continuous energy input, making them more cost-effective and practical for products that need long shelf lives. They work by modifying the surface properties of suspended particles, preventing them from aggregating and settling, whereas mechanical stirring only provides temporary dispersion and requires constant energy input.